Structure of Atom is a Chemistry topic but it appears in the Science section of AMU. 2-3 questions every year — mostly on atomic models, electronic configuration, isotopes/isobars, and valency. This is pure theory, no heavy maths. This expanded guide covers quantum numbers, energy level diagrams, 15+ PYQs, 15+ practice questions, common mistakes, and quick revision. Let's make it easy.
1) What is an Atom? — A Historical Journey
The word atom comes from the Greek word atomos, meaning "indivisible" or "uncuttable". An atom is the smallest unit of matter that retains the chemical properties of an element. For over 2000 years, atoms were thought to be indivisible spheres — until experiments in the late 19th and early 20th centuries revealed a whole world of subatomic particles inside.
1.1 Timeline of Atomic Theory
| Year | Scientist | Contribution |
|---|---|---|
| 400 BC | Democritus | First proposed the idea of "atomos" — indivisible particles |
| 1808 | John Dalton | Dalton's Atomic Theory (atoms are solid, indivisible spheres) |
| 1886 | E. Goldstein | Discovered proton (via canal rays / anode rays) |
| 1897 | J.J. Thomson | Discovered electron (via cathode ray tube experiment); proposed Plum Pudding Model |
| 1911 | Ernest Rutherford | α-particle scattering experiment → discovered nucleus |
| 1913 | Niels Bohr | Bohr's model — fixed energy levels / orbits for electrons |
| 1932 | James Chadwick | Discovered neutron |
🔥 AMU PYQ: Which of the following is the smallest unit of matter?
(A) Molecule (B) Atom (C) Electron (D) Cell
Ans: B (atom is the smallest unit of matter that retains chemical properties)
(A) Molecule (B) Atom (C) Electron (D) Cell
Ans: B (atom is the smallest unit of matter that retains chemical properties)
2) Dalton's Atomic Theory (1808)
John Dalton proposed his atomic theory based on experimental evidence from chemical reactions. It was the first scientific theory of the atom.
- Postulate 1: All matter is made of tiny, indivisible particles called atoms
- Postulate 2: Atoms of the same element are identical in mass, size, and chemical properties
- Postulate 3: Atoms of different elements have different masses and properties
- Postulate 4: Atoms combine in fixed whole-number ratios to form compounds
- Postulate 5: Atoms can neither be created nor destroyed in a chemical reaction (this is the law of conservation of mass)
🟡 Drawback of Dalton: He believed atoms were indivisible. Later discoveries (Thomson, Rutherford, Chadwick) proved that atoms contain subatomic particles — electrons, protons, and neutrons. Atoms CAN be divided, but then they become different elements or ions.
3) Discovery of Subatomic Particles
3.1 Discovery of Electron — Cathode Ray Tube Experiment (J.J. Thomson, 1897)
J.J. Thomson conducted experiments using a cathode ray tube (a sealed glass tube with two metal electrodes at low pressure). When a high voltage was applied, rays (called cathode rays) were produced from the negative electrode (cathode) toward the positive electrode (anode).
Properties of cathode rays (electrons):
1. They travel in straight lines (cast shadows)
2. They are negatively charged (deflected toward positive plate in electric/magnetic fields)
3. They cause fluorescence when they strike certain materials (ZnS coating)
4. They have both particle and wave nature
5. They can penetrate thin metal foils
Charge of electron: e⁻ =
Mass of electron: m_e =
Specific charge (e/m):
Mass compared to proton:
1. They travel in straight lines (cast shadows)
2. They are negatively charged (deflected toward positive plate in electric/magnetic fields)
3. They cause fluorescence when they strike certain materials (ZnS coating)
4. They have both particle and wave nature
5. They can penetrate thin metal foils
Charge of electron: e⁻ =
−1.6 × 10⁻¹⁹ CMass of electron: m_e =
9.1 × 10⁻³¹ kgSpecific charge (e/m):
1.76 × 10¹¹ C/kgMass compared to proton:
1/1836 amu (almost negligible!)
3.2 Discovery of Proton — Anode/Canal Rays (E. Goldstein, 1886)
Goldstein modified the cathode ray tube by using a perforated cathode (with holes). He observed rays traveling in the opposite direction of cathode rays — through the holes. These were called canal rays or anode rays.
Properties of canal rays (protons):
1. They travel from the anode toward the cathode (opposite to cathode rays)
2. They are positively charged (deflected toward negative plate)
3. Their mass depends on the gas used in the tube (for hydrogen, it's the smallest → proton)
Charge of proton: p⁺ =
Mass of proton: m_p =
Mass ratio: Proton is about 1836 times heavier than an electron
1. They travel from the anode toward the cathode (opposite to cathode rays)
2. They are positively charged (deflected toward negative plate)
3. Their mass depends on the gas used in the tube (for hydrogen, it's the smallest → proton)
Charge of proton: p⁺ =
+1.6 × 10⁻¹⁹ C (equal and opposite to electron)Mass of proton: m_p =
1.67 × 10⁻²⁷ kg ≈ 1 amuMass ratio: Proton is about 1836 times heavier than an electron
3.3 Discovery of Neutron (James Chadwick, 1932)
Chadwick bombarded a thin sheet of beryllium with α-particles. A highly penetrating radiation was emitted, which could knock protons out of paraffin wax. This radiation consisted of neutral particles — neutrons.
Properties of neutron:
1. It has no charge (neutral) — charge = 0
2. Its mass is approximately equal to that of a proton:
3. It is present in the nucleus of all atoms except ordinary hydrogen (¹H has no neutron)
4. Neutrons are stable inside the nucleus but unstable when free (free neutron decays in ~15 minutes)
1. It has no charge (neutral) — charge = 0
2. Its mass is approximately equal to that of a proton:
1.67 × 10⁻²⁷ kg ≈ 1 amu3. It is present in the nucleus of all atoms except ordinary hydrogen (¹H has no neutron)
4. Neutrons are stable inside the nucleus but unstable when free (free neutron decays in ~15 minutes)
| Particle | Discovered by | Year | Symbol | Charge | Mass | Location |
|---|---|---|---|---|---|---|
| Electron | J.J. Thomson | 1897 | e⁻ | −1.6 × 10⁻¹⁹ C | 9.1 × 10⁻³¹ kg (1/1836 amu) | Outside nucleus (shells) |
| Proton | E. Goldstein | 1886 | p⁺ | +1.6 × 10⁻¹⁹ C | 1.67 × 10⁻²⁷ kg (1 amu) | Nucleus |
| Neutron | James Chadwick | 1932 | n⁰ | 0 (neutral) | 1.67 × 10⁻²⁷ kg (1 amu) | Nucleus |
🔥 AMU PYQ: Who discovered the neutron?
(A) J.J. Thomson (B) Rutherford (C) Chadwick (D) Goldstein
Ans: C (James Chadwick, 1932)
🔥 AMU PYQ: The charge on an electron is:
(A) +1 (B) −1 (C) 0 (D) +2
Ans: B (electron is negatively charged)
🔥 AMU PYQ: Which atomic particle has no charge?
(A) Electron (B) Proton (C) Neutron (D) Alpha particle
Ans: C (neutron is neutral)
(A) J.J. Thomson (B) Rutherford (C) Chadwick (D) Goldstein
Ans: C (James Chadwick, 1932)
🔥 AMU PYQ: The charge on an electron is:
(A) +1 (B) −1 (C) 0 (D) +2
Ans: B (electron is negatively charged)
🔥 AMU PYQ: Which atomic particle has no charge?
(A) Electron (B) Proton (C) Neutron (D) Alpha particle
Ans: C (neutron is neutral)
4) Atomic Models — The Evolution ⭐
Understanding the order of atomic models is crucial for AMU. The sequence is: Dalton → Thomson → Rutherford → Bohr. Each model improved upon the previous one.
4.1 Thomson's Model — Plum Pudding Model (1898)
- Atom is a sphere of positive charge with negatively charged electrons embedded in it
- Also called "Plum Pudding Model" (plums = electrons, pudding = positive charge)
- Also called "Watermelon Model" (seeds = electrons, red part = positive charge)
- Limitation: Could not explain how the positive charge was arranged or how electrons were distributed. Also, it could not explain the results of Rutherford's experiment.
4.2 Rutherford's Model — Gold Foil Experiment (1911) ⭐ Most Important
Rutherford conducted the famous α-particle scattering experiment. He bombarded a very thin gold foil (few atoms thick) with a beam of α-particles (doubly charged helium ions, He²⁺, emitted by radioactive substances).
Setup:
A radioactive source → emits α-particles → narrow beam (lead shield) → strikes thin gold foil → scattered α-particles detected by a zinc sulphide (ZnS) screen → flash of light observed under microscope
A radioactive source → emits α-particles → narrow beam (lead shield) → strikes thin gold foil → scattered α-particles detected by a zinc sulphide (ZnS) screen → flash of light observed under microscope
4.2.1 Observations of Rutherford's Experiment
| Observation | Conclusion |
|---|---|
| Most α-particles (about 99.9%) passed straight through the foil without any deflection | Atom is mostly empty space |
| Some α-particles were deflected through small angles | There is a positively charged region inside the atom that repels α-particles |
| A very few (1 in 20,000) α-particles bounced back (scattered by more than 90°, some nearly 180°) | A tiny, dense, positively charged nucleus exists at the centre of the atom |
Rutherford's conclusions:
1. Atom has a small, dense, positively charged nucleus at its centre (contains most of the mass)
2. Electrons revolve around the nucleus in circular orbits
3. Most of the atom is empty space
4. The nucleus is about 10⁻¹⁵ m in diameter, while the atom is about 10⁻¹⁰ m — the nucleus is 10,000 to 100,000 times smaller than the atom!
1. Atom has a small, dense, positively charged nucleus at its centre (contains most of the mass)
2. Electrons revolve around the nucleus in circular orbits
3. Most of the atom is empty space
4. The nucleus is about 10⁻¹⁵ m in diameter, while the atom is about 10⁻¹⁰ m — the nucleus is 10,000 to 100,000 times smaller than the atom!
🟡 Drawback of Rutherford's Model:
According to Maxwell's electromagnetic theory, any charged particle in circular motion must continuously radiate energy. If an electron radiates energy, it would lose KE, slow down, and spiral into the nucleus. This would mean atoms are unstable and should collapse — but atoms ARE stable! Rutherford's model could not explain atomic stability. This was the key reason Bohr's model was needed.
According to Maxwell's electromagnetic theory, any charged particle in circular motion must continuously radiate energy. If an electron radiates energy, it would lose KE, slow down, and spiral into the nucleus. This would mean atoms are unstable and should collapse — but atoms ARE stable! Rutherford's model could not explain atomic stability. This was the key reason Bohr's model was needed.
4.3 Bohr's Model (1913) ⭐
Niels Bohr modified Rutherford's model by incorporating quantum theory (Max Planck's idea of quantized energy).
Bohr's Three Postulates:
Postulate 1 — Stationary Orbits:
Electrons revolve around the nucleus only in certain fixed, discrete, circular paths called orbits or energy levels (shells). While in these orbits, they do NOT radiate energy (they are "stationary states").
Postulate 2 — Quantized Angular Momentum:
The angular momentum of an electron in a stationary orbit is an integer multiple of h/2π:
Postulate 3 — Electron Jumps:
An electron can jump from one orbit to another by absorbing or emitting energy in the form of a photon. The energy difference between the two orbits equals the photon energy:
Postulate 1 — Stationary Orbits:
Electrons revolve around the nucleus only in certain fixed, discrete, circular paths called orbits or energy levels (shells). While in these orbits, they do NOT radiate energy (they are "stationary states").
Postulate 2 — Quantized Angular Momentum:
The angular momentum of an electron in a stationary orbit is an integer multiple of h/2π:
mvr = nh / 2π where n = 1, 2, 3, ... (principal quantum number)Postulate 3 — Electron Jumps:
An electron can jump from one orbit to another by absorbing or emitting energy in the form of a photon. The energy difference between the two orbits equals the photon energy:
ΔE = E₂ − E₁ = hν (where h = Planck's constant, ν = frequency of radiation)
4.3.1 Shell Notation (Energy Levels)
Shells are named from the nucleus outward:
Energy order: K < L < M < N (K has lowest energy, electrons fill K first)
Maximum electrons: 2n² per shell (K=2, L=8, M=18, N=32)
n = 1 → K shell (innermost, lowest energy)n = 2 → L shelln = 3 → M shelln = 4 → N shell (outermost for first 20 elements)Energy order: K < L < M < N (K has lowest energy, electrons fill K first)
Maximum electrons: 2n² per shell (K=2, L=8, M=18, N=32)
🟡 Limitations of Bohr's Model:
1. Could only explain the hydrogen atom spectrum (single electron)
2. Failed to explain the spectra of multi-electron atoms
3. Could not explain the splitting of spectral lines in magnetic fields (Zeeman effect)
4. Violated the Heisenberg Uncertainty Principle (both position and momentum cannot be precisely known)
5. Later replaced by the Quantum Mechanical Model (Schrödinger equation)
1. Could only explain the hydrogen atom spectrum (single electron)
2. Failed to explain the spectra of multi-electron atoms
3. Could not explain the splitting of spectral lines in magnetic fields (Zeeman effect)
4. Violated the Heisenberg Uncertainty Principle (both position and momentum cannot be precisely known)
5. Later replaced by the Quantum Mechanical Model (Schrödinger equation)
🔥 AMU PYQ: In Rutherford's experiment, most alpha particles passed through the gold foil because:
(A) Nucleus is heavy (B) Nucleus is positively charged (C) Atom is mostly empty space (D) Electrons attract them
Ans: C
🔥 AMU PYQ: Who proposed that electrons revolve in fixed energy levels?
(A) Thomson (B) Rutherford (C) Bohr (D) Chadwick
Ans: C (Bohr's model)
🔥 AMU PYQ: Which model is also called the "plum pudding model"?
(A) Rutherford (B) Thomson (C) Bohr (D) Dalton
Ans: B
(A) Nucleus is heavy (B) Nucleus is positively charged (C) Atom is mostly empty space (D) Electrons attract them
Ans: C
🔥 AMU PYQ: Who proposed that electrons revolve in fixed energy levels?
(A) Thomson (B) Rutherford (C) Bohr (D) Chadwick
Ans: C (Bohr's model)
🔥 AMU PYQ: Which model is also called the "plum pudding model"?
(A) Rutherford (B) Thomson (C) Bohr (D) Dalton
Ans: B
5) Atomic Number and Mass Number
These two numbers completely describe the composition of an atom.
Atomic Number (Z):
Mass Number (A):
Number of neutrons:
Z = Number of protons in the nucleusIn a neutral atom: Z = Number of protons = Number of electronsZ uniquely identifies the element (change Z → new element!)Mass Number (A):
A = Number of protons + Number of neutronsA is also called the "nucleon number" (protons + neutrons = nucleons)Number of neutrons:
n = A − Z
Standard notation for an element:
AZX (where X is the chemical symbol)
Examples:
2311Na → Z = 11 (sodium), A = 23, neutrons = 23 − 11 = 12
168O → Z = 8 (oxygen), A = 16, neutrons = 16 − 8 = 8
11H → Z = 1 (hydrogen), A = 1, neutrons = 0
AZX (where X is the chemical symbol)
Examples:
2311Na → Z = 11 (sodium), A = 23, neutrons = 23 − 11 = 12
168O → Z = 8 (oxygen), A = 16, neutrons = 16 − 8 = 8
11H → Z = 1 (hydrogen), A = 1, neutrons = 0
🔥 AMU PYQ: An atom has atomic number 8 and mass number 16. Number of neutrons is:
(A) 8 (B) 16 (C) 24 (D) 2
Ans: A (neutrons = 16 − 8 = 8)
🔥 AMU PYQ: The number of protons in an atom is equal to its:
(A) Mass number (B) Atomic number (C) Neutron number (D) Valency
Ans: B
🔥 AMU PYQ: What is the mass number of an atom with 12 protons and 12 neutrons?
(A) 12 (B) 24 (C) 0 (D) 144
Ans: B (A = 12 + 12 = 24)
🔥 AMU PYQ: The nucleus of an atom contains:
(A) Protons only (B) Protons and neutrons (C) Electrons and protons (D) Electrons only
Ans: B (nucleus = protons + neutrons)
(A) 8 (B) 16 (C) 24 (D) 2
Ans: A (neutrons = 16 − 8 = 8)
🔥 AMU PYQ: The number of protons in an atom is equal to its:
(A) Mass number (B) Atomic number (C) Neutron number (D) Valency
Ans: B
🔥 AMU PYQ: What is the mass number of an atom with 12 protons and 12 neutrons?
(A) 12 (B) 24 (C) 0 (D) 144
Ans: B (A = 12 + 12 = 24)
🔥 AMU PYQ: The nucleus of an atom contains:
(A) Protons only (B) Protons and neutrons (C) Electrons and protons (D) Electrons only
Ans: B (nucleus = protons + neutrons)
6) Isotopes, Isobars, and Isotones
These are terms used to describe relationships between different atoms. AMU frequently asks to identify or distinguish them.
6.1 Isotopes — Same Z, Different A
Atoms of the same element (same number of protons) that have different numbers of neutrons (hence different mass numbers).
Isotopes: Same Z, Different A
Same chemical properties (because chemical properties depend on protons/electrons)
Different physical properties (because mass differs)
Examples:
Hydrogen: 1H (protium, Z=1, n=0), 2H (deuterium, Z=1, n=1), 3H (tritium, Z=1, n=2 — radioactive)
Carbon: 12C (6p, 6n), 13C (6p, 7n), 14C (6p, 8n — radioactive, used for carbon dating)
Uranium: 235U (fissionable, nuclear fuel), 238U (non-fissionable, most abundant)
Same chemical properties (because chemical properties depend on protons/electrons)
Different physical properties (because mass differs)
Examples:
Hydrogen: 1H (protium, Z=1, n=0), 2H (deuterium, Z=1, n=1), 3H (tritium, Z=1, n=2 — radioactive)
Carbon: 12C (6p, 6n), 13C (6p, 7n), 14C (6p, 8n — radioactive, used for carbon dating)
Uranium: 235U (fissionable, nuclear fuel), 238U (non-fissionable, most abundant)
🟡 Applications of Isotopes:
• Uranium-235 — fuel for nuclear reactors and atomic bombs (nuclear fission)
• Cobalt-60 — used in cancer treatment (radiotherapy / gamma radiation)
• Iodine-131 — used to diagnose and treat thyroid disorders (concentrates in thyroid gland)
• Carbon-14 — used for radiocarbon dating of archaeological specimens (fossils, ancient artifacts)
• Deuterium (²H) — used in heavy water (D₂O) as a moderator in nuclear reactors
• Technetium-99m — used in medical imaging (bone scans, organ scans)
• Uranium-235 — fuel for nuclear reactors and atomic bombs (nuclear fission)
• Cobalt-60 — used in cancer treatment (radiotherapy / gamma radiation)
• Iodine-131 — used to diagnose and treat thyroid disorders (concentrates in thyroid gland)
• Carbon-14 — used for radiocarbon dating of archaeological specimens (fossils, ancient artifacts)
• Deuterium (²H) — used in heavy water (D₂O) as a moderator in nuclear reactors
• Technetium-99m — used in medical imaging (bone scans, organ scans)
6.2 Isobars — Same A, Different Z
Atoms of different elements that have the same mass number (A) but different atomic numbers (Z).
Isobars: Same A, Different Z
Different elements → different chemical properties
Different number of protons AND neutrons, but same total (p + n)
Examples:
4018Ar and 4020Ca (both have A = 40)
146C and 147N (both have A = 14)
3216S and 3215P (both have A = 32)
Different elements → different chemical properties
Different number of protons AND neutrons, but same total (p + n)
Examples:
4018Ar and 4020Ca (both have A = 40)
146C and 147N (both have A = 14)
3216S and 3215P (both have A = 32)
6.3 Isotones — Same Number of Neutrons
Atoms of different elements that have the same number of neutrons but different atomic numbers.
Isotones: Same N (neutron number), Different Z
Examples:
146C (6p, 8n) and 168O (8p, 8n) — both have 8 neutrons
136C (6p, 7n) and 147N (7p, 7n) — both have 7 neutrons
Examples:
146C (6p, 8n) and 168O (8p, 8n) — both have 8 neutrons
136C (6p, 7n) and 147N (7p, 7n) — both have 7 neutrons
| Property | Isotopes | Isobars | Isotones |
|---|---|---|---|
| Definition | Same Z, different A | Same A, different Z | Same neutrons (N), different Z |
| Same element? | ✅ Yes (same element) | ❌ No (different elements) | ❌ No (different elements) |
| Same protons? | ✅ Yes | ❌ No | ❌ No |
| Same neutrons? | ❌ No | ❌ No | ✅ Yes |
| Chemical properties | Same | Different | Different |
| Memory trick | "IsoTOPE" — same place in periodic table (same Z) | "IsoBAR" — same weight (same A) | "IsoTONE" — same tone (same neutron count) |
🔥 AMU PYQ: Isotopes of an element have the same number of:
(A) Neutrons (B) Protons (C) Nucleons (D) Mass number
Ans: B (same Z → same protons)
🔥 AMU PYQ: Cobalt-60 is used in:
(A) Nuclear fuel (B) Cancer treatment (C) Archaeology (D) Dating fossils
Ans: B
🔥 AMU PYQ: Which of the following is an isobar of ¹⁴C?
(A) ¹²C (B) ¹⁴N (C) ¹³C (D) ¹⁶O
Ans: B (same mass number 14)
(A) Neutrons (B) Protons (C) Nucleons (D) Mass number
Ans: B (same Z → same protons)
🔥 AMU PYQ: Cobalt-60 is used in:
(A) Nuclear fuel (B) Cancer treatment (C) Archaeology (D) Dating fossils
Ans: B
🔥 AMU PYQ: Which of the following is an isobar of ¹⁴C?
(A) ¹²C (B) ¹⁴N (C) ¹³C (D) ¹⁶O
Ans: B (same mass number 14)
7) Electronic Configuration ⭐
Electronic configuration tells us how electrons are arranged in the various shells (energy levels) around the nucleus. It determines the chemical properties of an element.
Maximum number of electrons in a shell = 2n²
K shell (n = 1) → 2 × 1² = 2 electronsL shell (n = 2) → 2 × 2² = 8 electronsM shell (n = 3) → 2 × 3² = 18 electronsN shell (n = 4) → 2 × 4² = 32 electrons
7.1 Rules for Filling Electrons in Shells
- Fill inner shells first (building-up principle): Electrons first occupy the K shell (lowest energy), then L, then M, then N
- The outermost shell (valence shell) cannot have more than 8 electrons — this is the octet rule
- The second-last shell cannot have more than 18 electrons (for the first 20 elements, it's limited to 8)
- After filling K (2) and L (8), the remaining electrons go to M (up to 8 for first 20 elements, then 18 for higher Z)
7.2 Electronic Configuration of First 20 Elements
Memorize this table — AMU often asks for the configuration of a specific element!
| Element | Symbol | Z | Electronic Config | Valence Electrons |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Helium | He | 2 | 2 | 2 |
| Lithium | Li | 3 | 2, 1 | 1 |
| Beryllium | Be | 4 | 2, 2 | 2 |
| Boron | B | 5 | 2, 3 | 3 |
| Carbon | C | 6 | 2, 4 | 4 |
| Nitrogen | N | 7 | 2, 5 | 5 |
| Oxygen | O | 8 | 2, 6 | 6 |
| Fluorine | F | 9 | 2, 7 | 7 |
| Neon | Ne | 10 | 2, 8 | 8 (Full → inert) |
| Sodium | Na | 11 | 2, 8, 1 | 1 |
| Magnesium | Mg | 12 | 2, 8, 2 | 2 |
| Aluminium | Al | 13 | 2, 8, 3 | 3 |
| Silicon | Si | 14 | 2, 8, 4 | 4 |
| Phosphorus | P | 15 | 2, 8, 5 | 5 |
| Sulfur | S | 16 | 2, 8, 6 | 6 |
| Chlorine | Cl | 17 | 2, 8, 7 | 7 |
| Argon | Ar | 18 | 2, 8, 8 | 8 (Full → inert) |
| Potassium | K | 19 | 2, 8, 8, 1 | 1 |
| Calcium | Ca | 20 | 2, 8, 8, 2 | 2 |
🟡 Important notes:
• Potassium (K, Z=19) has configuration 2, 8, 8, 1 (NOT 2, 8, 9). Why? Because the outermost shell cannot have more than 8 electrons. The 19th electron goes to the N shell, not the M shell.
• Similarly, Calcium (Ca, Z=20) is 2, 8, 8, 2, not 2, 8, 10.
• This is because after 8 electrons in the M shell, the next electrons go to the N shell (energy order is 3p < 4s < 3d for these elements).
• Potassium (K, Z=19) has configuration 2, 8, 8, 1 (NOT 2, 8, 9). Why? Because the outermost shell cannot have more than 8 electrons. The 19th electron goes to the N shell, not the M shell.
• Similarly, Calcium (Ca, Z=20) is 2, 8, 8, 2, not 2, 8, 10.
• This is because after 8 electrons in the M shell, the next electrons go to the N shell (energy order is 3p < 4s < 3d for these elements).
🔥 AMU PYQ: The electronic configuration of chlorine (Z = 17) is:
(A) 2, 8, 7 (B) 2, 8, 8 (C) 2, 7, 8 (D) 2, 8, 6
Ans: A (2, 8, 7)
🔥 AMU PYQ: The maximum number of electrons in the L-shell is:
(A) 2 (B) 8 (C) 18 (D) 32
Ans: B (2n² = 2 × 2² = 8)
🔥 AMU PYQ: An element has electronic configuration 2, 8, 2. Identify the element and its group.
Ans: Z = 12 → Magnesium (Mg), Group 2 (alkaline earth metal)
🔥 AMU PYQ: The noble gas with electronic configuration 2, 8 is:
(A) He (B) Ne (C) Ar (D) Kr
Ans: B (Neon, Z=10)
(A) 2, 8, 7 (B) 2, 8, 8 (C) 2, 7, 8 (D) 2, 8, 6
Ans: A (2, 8, 7)
🔥 AMU PYQ: The maximum number of electrons in the L-shell is:
(A) 2 (B) 8 (C) 18 (D) 32
Ans: B (2n² = 2 × 2² = 8)
🔥 AMU PYQ: An element has electronic configuration 2, 8, 2. Identify the element and its group.
Ans: Z = 12 → Magnesium (Mg), Group 2 (alkaline earth metal)
🔥 AMU PYQ: The noble gas with electronic configuration 2, 8 is:
(A) He (B) Ne (C) Ar (D) Kr
Ans: B (Neon, Z=10)
8) Valence Electrons and Valency
Valence electrons are the electrons present in the outermost shell of an atom. These electrons determine the chemical properties and combining capacity (valency) of the element.
8.1 How to Find Valence Electrons
Simply look at the last number in the electronic configuration:
- Na (2, 8, 1) → 1 valence electron
- Mg (2, 8, 2) → 2 valence electrons
- Al (2, 8, 3) → 3 valence electrons
- C (2, 4) → 4 valence electrons
- O (2, 6) → 6 valence electrons
- Cl (2, 8, 7) → 7 valence electrons
- Ne (2, 8) → 8 valence electrons (fully filled → stable, inert)
8.2 How to Find Valency
Valency Rules:
• If valence electrons = 1, 2, or 3 → Valency = number of valence electrons
(These atoms lose electrons to achieve stable configuration)
Example: Na (1) → valency 1, Mg (2) → valency 2, Al (3) → valency 3
• If valence electrons = 5, 6, or 7 → Valency = 8 − valence electrons
(These atoms gain electrons to achieve octet)
Example: N (5) → 8−5=3, O (6) → 8−6=2, Cl (7) → 8−7=1
• If valence electrons = 4 → Valency = 4 (can either lose or gain 4 electrons)
Example: C (4) → valency 4, Si (4) → valency 4
• If valence electrons = 8 (He, Ne, Ar) → Valency = 0 (inert/noble gases — they don't combine)
• If valence electrons = 1, 2, or 3 → Valency = number of valence electrons
(These atoms lose electrons to achieve stable configuration)
Example: Na (1) → valency 1, Mg (2) → valency 2, Al (3) → valency 3
• If valence electrons = 5, 6, or 7 → Valency = 8 − valence electrons
(These atoms gain electrons to achieve octet)
Example: N (5) → 8−5=3, O (6) → 8−6=2, Cl (7) → 8−7=1
• If valence electrons = 4 → Valency = 4 (can either lose or gain 4 electrons)
Example: C (4) → valency 4, Si (4) → valency 4
• If valence electrons = 8 (He, Ne, Ar) → Valency = 0 (inert/noble gases — they don't combine)
8.3 Variable Valency
Some elements can exhibit more than one valency because they can lose different numbers of electrons depending on the chemical conditions:
| Element | Valencies | Examples |
|---|---|---|
| Iron (Fe) | 2 and 3 | FeCl₂ (ferrous), FeCl₃ (ferric) |
| Copper (Cu) | 1 and 2 | CuCl (cuprous), CuCl₂ (cupric) |
| Mercury (Hg) | 1 and 2 | Hg₂Cl₂ (mercurous), HgCl₂ (mercuric) |
| Tin (Sn) | 2 and 4 | SnCl₂ (stannous), SnCl₄ (stannic) |
| Lead (Pb) | 2 and 4 | PbO (plumbous), PbO₂ (plumbic) |
🔥 AMU PYQ: Valency of oxygen is:
(A) 1 (B) 2 (C) 6 (D) 8
Ans: B (O has 6 valence electrons → 8−6=2)
🔥 AMU PYQ: An element has electronic configuration 2, 8, 2. Its valency is:
(A) 1 (B) 2 (C) 3 (D) 4
Ans: B (2 valence electrons → valency = 2)
🔥 AMU PYQ: Variable valency is shown by which element?
(A) Sodium (B) Iron (C) Magnesium (D) Aluminium
Ans: B (Fe shows +2 and +3)
(A) 1 (B) 2 (C) 6 (D) 8
Ans: B (O has 6 valence electrons → 8−6=2)
🔥 AMU PYQ: An element has electronic configuration 2, 8, 2. Its valency is:
(A) 1 (B) 2 (C) 3 (D) 4
Ans: B (2 valence electrons → valency = 2)
🔥 AMU PYQ: Variable valency is shown by which element?
(A) Sodium (B) Iron (C) Magnesium (D) Aluminium
Ans: B (Fe shows +2 and +3)
9) Quantum Numbers — Basic Concept
In the quantum mechanical model of the atom, electrons are described by four quantum numbers that give their energy, shape, orientation, and spin. At the Class 10/AMU 11 level, you need a basic understanding.
The Four Quantum Numbers:
1. Principal Quantum Number (n)
n = 1, 2, 3, 4, ...
Determines size and energy of the orbital
Corresponds to shells: K (n=1), L (n=2), M (n=3), N (n=4)
Larger n → larger orbital → higher energy
2. Azimuthal Quantum Number (l)
l = 0 to (n−1)
Determines the shape of the orbital (subshell)
l = 0 → s orbital (spherical)
l = 1 → p orbital (dumbbell-shaped)
l = 2 → d orbital (clover-shaped)
l = 3 → f orbital (complex)
3. Magnetic Quantum Number (mₗ)
mₗ = −l to 0 to +l
Determines the orientation of the orbital in space
For l = 0 (s): mₗ = 0 → one orientation
For l = 1 (p): mₗ = −1, 0, +1 → three orientations (pₓ, pᵧ, p_z)
4. Spin Quantum Number (mₛ)
mₛ = +½ or −½
Describes the spin of the electron (clockwise or anticlockwise)
Two electrons in the same orbital must have opposite spins (Pauli exclusion principle)
1. Principal Quantum Number (n)
n = 1, 2, 3, 4, ...
Determines size and energy of the orbital
Corresponds to shells: K (n=1), L (n=2), M (n=3), N (n=4)
Larger n → larger orbital → higher energy
2. Azimuthal Quantum Number (l)
l = 0 to (n−1)
Determines the shape of the orbital (subshell)
l = 0 → s orbital (spherical)
l = 1 → p orbital (dumbbell-shaped)
l = 2 → d orbital (clover-shaped)
l = 3 → f orbital (complex)
3. Magnetic Quantum Number (mₗ)
mₗ = −l to 0 to +l
Determines the orientation of the orbital in space
For l = 0 (s): mₗ = 0 → one orientation
For l = 1 (p): mₗ = −1, 0, +1 → three orientations (pₓ, pᵧ, p_z)
4. Spin Quantum Number (mₛ)
mₛ = +½ or −½
Describes the spin of the electron (clockwise or anticlockwise)
Two electrons in the same orbital must have opposite spins (Pauli exclusion principle)
10) Energy Level Diagram and Order of Filling
The order in which subshells are filled is not simply 1s → 2s → 2p → 3s → 3p → 3d... The actual order (based on increasing energy) follows the Aufbau principle. The energy of orbitals increases as (n + l) increases. For same (n + l), lower n has lower energy.
Order of filling (Aufbau order):
For first 20 elements (AMU level):
1s² → 2s² → 2p⁶ → 3s² → 3p⁶ → 4s² → 3d⁰ (K and Ca fill 4s before 3d!)
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s...For first 20 elements (AMU level):
1s² → 2s² → 2p⁶ → 3s² → 3p⁶ → 4s² → 3d⁰ (K and Ca fill 4s before 3d!)
🟡 Important rule: The 4s orbital has lower energy than 3d. So for K (Z=19) and Ca (Z=20), the last electrons go into the 4s orbital, not 3d. This is why their configurations end with 4s¹ and 4s², respectively. After Ca, the 3d orbital starts filling (Sc, Ti, etc.).
11) PYQ Battle — AMU Past Questions
Q1. Which model is also called the "plum pudding model"?
(A) Rutherford (B) Thomson (C) Bohr (D) Dalton
Ans: B
(A) Rutherford (B) Thomson (C) Bohr (D) Dalton
Ans: B
Q2. The maximum number of electrons in the L-shell is:
(A) 2 (B) 8 (C) 18 (D) 32
Ans: B (2n² = 2 × 2² = 8)
(A) 2 (B) 8 (C) 18 (D) 32
Ans: B (2n² = 2 × 2² = 8)
Q3. An element has electronic configuration 2, 8, 2. Its valency is:
(A) 1 (B) 2 (C) 3 (D) 4
Ans: B (2 valence electrons → valency = 2)
(A) 1 (B) 2 (C) 3 (D) 4
Ans: B (2 valence electrons → valency = 2)
Q4. The nucleus of an atom contains:
(A) Protons only (B) Protons and neutrons (C) Electrons and protons (D) Electrons only
Ans: B (nucleus = protons + neutrons)
(A) Protons only (B) Protons and neutrons (C) Electrons and protons (D) Electrons only
Ans: B (nucleus = protons + neutrons)
Q5. What is the mass number of an atom with 12 protons and 12 neutrons?
(A) 12 (B) 24 (C) 0 (D) 144
Ans: B (12 + 12 = 24)
(A) 12 (B) 24 (C) 0 (D) 144
Ans: B (12 + 12 = 24)
Q6. Isotopes have the same number of protons but different number of:
(A) Electrons (B) Neutrons (C) Protons (D) Orbits
Ans: B
(A) Electrons (B) Neutrons (C) Protons (D) Orbits
Ans: B
Q7. Which of the following is an isobar of ¹⁴C?
(A) ¹²C (B) ¹⁴N (C) ¹³C (D) ¹⁶O
Ans: B (same mass number 14)
(A) ¹²C (B) ¹⁴N (C) ¹³C (D) ¹⁶O
Ans: B (same mass number 14)
Q8. The noble gas with electronic configuration 2, 8 is:
(A) He (B) Ne (C) Ar (D) Kr
Ans: B (Neon, Z=10)
(A) He (B) Ne (C) Ar (D) Kr
Ans: B (Neon, Z=10)
Q9. Which atomic particle has no charge?
(A) Electron (B) Proton (C) Neutron (D) Alpha particle
Ans: C
(A) Electron (B) Proton (C) Neutron (D) Alpha particle
Ans: C
Q10. Rutherford's gold foil experiment proved that:
(A) Atom contains a nucleus (B) Atom is indivisible (C) Electrons have fixed orbits (D) Atoms combine
Ans: A
(A) Atom contains a nucleus (B) Atom is indivisible (C) Electrons have fixed orbits (D) Atoms combine
Ans: A
Q11. In Bohr's model, electrons revolve in:
(A) Any random orbit (B) Fixed energy levels (C) Elliptical paths (D) Straight lines
Ans: B
(A) Any random orbit (B) Fixed energy levels (C) Elliptical paths (D) Straight lines
Ans: B
Q12. The atomic number of an element with 13 protons and 14 neutrons is:
(A) 13 (B) 14 (C) 27 (D) 1
Ans: A (Z = number of protons = 13)
(A) 13 (B) 14 (C) 27 (D) 1
Ans: A (Z = number of protons = 13)
Q13. Which of the following is used for carbon dating?
(A) ¹²C (B) ¹³C (C) ¹⁴C (D) ¹⁵C
Ans: C (Carbon-14 is radioactive, used for dating fossils)
(A) ¹²C (B) ¹³C (C) ¹⁴C (D) ¹⁵C
Ans: C (Carbon-14 is radioactive, used for dating fossils)
Q14. The number of electrons in the outermost shell of chlorine is:
(A) 5 (B) 6 (C) 7 (D) 8
Ans: C (Cl config = 2, 8, 7 → 7 valence electrons)
(A) 5 (B) 6 (C) 7 (D) 8
Ans: C (Cl config = 2, 8, 7 → 7 valence electrons)
Q15. Who discovered the electron?
(A) Rutherford (B) Goldstein (C) Thomson (D) Chadwick
Ans: C (J.J. Thomson, 1897)
(A) Rutherford (B) Goldstein (C) Thomson (D) Chadwick
Ans: C (J.J. Thomson, 1897)
Q16. The principal quantum number (n) represents:
(A) Shape of orbital (B) Size and energy level (C) Orientation (D) Spin
Ans: B (n determines shell/size/energy)
(A) Shape of orbital (B) Size and energy level (C) Orientation (D) Spin
Ans: B (n determines shell/size/energy)
12) Practice Problems — 15+ Questions with Solutions
Practice 1: Find the number of protons, neutrons, and electrons in 3115P.
Solution: Z = 15 → 15 protons, 15 electrons (neutral). Neutrons = A − Z = 31 − 15 = 16 neutrons
Solution: Z = 15 → 15 protons, 15 electrons (neutral). Neutrons = A − Z = 31 − 15 = 16 neutrons
Practice 2: An element has Z = 19, A = 39. Find neutrons and electronic configuration.
Solution: Neutrons = 39 − 19 = 20. Z=19 → K (potassium). Config = 2, 8, 8, 1
Solution: Neutrons = 39 − 19 = 20. Z=19 → K (potassium). Config = 2, 8, 8, 1
Practice 3: Write the electronic configuration of aluminium (Z = 13). Find its valency.
Solution: Config = 2, 8, 3. Valence electrons = 3 → valency = 3
Solution: Config = 2, 8, 3. Valence electrons = 3 → valency = 3
Practice 4: How many neutrons are in 37Cl?
Solution: Cl has Z = 17. Neutrons = 37 − 17 = 20
Solution: Cl has Z = 17. Neutrons = 37 − 17 = 20
Practice 5: Identify the element with configuration 2, 8, 18, 2. What is its valency?
Solution: Z = 30 → Zinc (Zn). Valence = 2 → valency = 2
Solution: Z = 30 → Zinc (Zn). Valence = 2 → valency = 2
Practice 6: 16O and 17O are which type of atoms?
Solution: Same Z (8), different A → they are isotopes of oxygen
Solution: Same Z (8), different A → they are isotopes of oxygen
Practice 7: 40Ar and 40Ca are which type of atoms?
Solution: Same A (40), different Z → they are isobars
Solution: Same A (40), different Z → they are isobars
Practice 8: What is the valency of calcium (Z = 20)?
Solution: Config = 2, 8, 8, 2. Valence = 2 → valency = 2
Solution: Config = 2, 8, 8, 2. Valence = 2 → valency = 2
Practice 9: An atom has electronic configuration 2, 8, 5. Find its atomic number and valency.
Solution: Z = 15 → Phosphorus. Valence = 5 → Valency = 8−5 = 3
Solution: Z = 15 → Phosphorus. Valence = 5 → Valency = 8−5 = 3
Practice 10: How many electrons can the M shell hold maximally?
Solution: M shell (n=3): 2 × 3² = 18 electrons
Solution: M shell (n=3): 2 × 3² = 18 electrons
Practice 11: What is the mass number of an isotope of carbon with 8 neutrons?
Solution: C has Z = 6. A = 6 + 8 = 14 (¹⁴C)
Solution: C has Z = 6. A = 6 + 8 = 14 (¹⁴C)
Practice 12: The electronic configuration of an element is 2, 8, 8, 2. Identify it.
Solution: Z = 20 → Calcium (Ca)
Solution: Z = 20 → Calcium (Ca)
Practice 13: Write the charge and mass of an electron.
Solution: Charge = −1.6 × 10⁻¹⁹ C, Mass = 9.1 × 10⁻³¹ kg (or 1/1836 amu)
Solution: Charge = −1.6 × 10⁻¹⁹ C, Mass = 9.1 × 10⁻³¹ kg (or 1/1836 amu)
Practice 14: Who proposed the neutron and when?
Solution: James Chadwick in 1932
Solution: James Chadwick in 1932
Practice 15: What is the drawback of Rutherford's model that Bohr fixed?
Solution: Rutherford's model could not explain atomic stability (electrons should spiral into nucleus due to energy loss). Bohr proposed fixed stationary orbits where electrons do not radiate energy.
Solution: Rutherford's model could not explain atomic stability (electrons should spiral into nucleus due to energy loss). Bohr proposed fixed stationary orbits where electrons do not radiate energy.
Practice 16: An atom of nitrogen has Z = 7 and A = 14. Find p⁺, e⁻, n⁰.
Solution: p⁺ = 7, e⁻ = 7, n⁰ = 14 − 7 = 7
Solution: p⁺ = 7, e⁻ = 7, n⁰ = 14 − 7 = 7
13) Common Mistakes & Tricks
⚠️ Top common mistakes in AMU exam:
Mistake 1: Confusing atomic number with mass number
→ Z = number of protons (defines the element). A = protons + neutrons. Change Z → new element. Change A → only an isotope.
Mistake 2: Wrong electronic configuration of K and Ca
→ K (Z=19): 2, 8, 8, 1 (NOT 2, 8, 9). Ca (Z=20): 2, 8, 8, 2 (NOT 2, 8, 10). The outermost shell cannot exceed 8 electrons!
Mistake 3: Thinking neutrons = electrons
→ Neutrons = A − Z. NOT equal to electrons (except by coincidence).
Mistake 4: Isobars vs Isotopes
→ Isobars = same mass number (A). Isotopes = same atomic number (Z). "Bar" sounds like "mass" → Isobars = same mass.
Mistake 5: Scientists and their discoveries
→ Thomson → electron. Goldstein → proton. Chadwick → neutron. Rutherford → nucleus. Bohr → fixed orbits. Don't mix these up!
Mistake 6: Valence electrons vs Valency
→ For metals, valency = number of valence electrons (they lose electrons). For non-metals (5-7 valence), valency = 8 − valence (they gain electrons).
Mistake 7: Writing 2n electrons instead of 2n²
→ Maximum electrons in a shell = 2n², NOT 2n. For L shell (n=2): 2×4=8, not 4.
Mistake 1: Confusing atomic number with mass number
→ Z = number of protons (defines the element). A = protons + neutrons. Change Z → new element. Change A → only an isotope.
Mistake 2: Wrong electronic configuration of K and Ca
→ K (Z=19): 2, 8, 8, 1 (NOT 2, 8, 9). Ca (Z=20): 2, 8, 8, 2 (NOT 2, 8, 10). The outermost shell cannot exceed 8 electrons!
Mistake 3: Thinking neutrons = electrons
→ Neutrons = A − Z. NOT equal to electrons (except by coincidence).
Mistake 4: Isobars vs Isotopes
→ Isobars = same mass number (A). Isotopes = same atomic number (Z). "Bar" sounds like "mass" → Isobars = same mass.
Mistake 5: Scientists and their discoveries
→ Thomson → electron. Goldstein → proton. Chadwick → neutron. Rutherford → nucleus. Bohr → fixed orbits. Don't mix these up!
Mistake 6: Valence electrons vs Valency
→ For metals, valency = number of valence electrons (they lose electrons). For non-metals (5-7 valence), valency = 8 − valence (they gain electrons).
Mistake 7: Writing 2n electrons instead of 2n²
→ Maximum electrons in a shell = 2n², NOT 2n. For L shell (n=2): 2×4=8, not 4.
✅ Tricks to score full marks:
• Atomic number Z = number of protons. In neutral atom: Z = p⁺ = e⁻
• Mass number A = p⁺ + n⁰. So n⁰ = A − Z
• Memory trick: "Z is at the bottom" → Z is the lower number, A is the upper number in notation ᴬ₂X
• "Isotopes are buddies" → same Z (same element). "Isobars are at the bar" → same A (same weight at the bar)
• Shell capacity: K=2, L=8, M=18, N=32 — at least memorize K, L, M
• For valency: 1,2,3 → lose; 5,6,7 → 8−n; 4 → both; 8 → 0
• Noble gases (He, Ne, Ar) have valency = 0 → they don't react
• Rutherford → nucleus (gold foil). Bohr → fixed orbits (energy levels). Thomson → electron (cathode rays). Chadwick → neutron (beryllium experiment)
• Atomic number Z = number of protons. In neutral atom: Z = p⁺ = e⁻
• Mass number A = p⁺ + n⁰. So n⁰ = A − Z
• Memory trick: "Z is at the bottom" → Z is the lower number, A is the upper number in notation ᴬ₂X
• "Isotopes are buddies" → same Z (same element). "Isobars are at the bar" → same A (same weight at the bar)
• Shell capacity: K=2, L=8, M=18, N=32 — at least memorize K, L, M
• For valency: 1,2,3 → lose; 5,6,7 → 8−n; 4 → both; 8 → 0
• Noble gases (He, Ne, Ar) have valency = 0 → they don't react
• Rutherford → nucleus (gold foil). Bohr → fixed orbits (energy levels). Thomson → electron (cathode rays). Chadwick → neutron (beryllium experiment)
14) Important Formulae and Facts — At a Glance
| Concept | Formula / Fact |
|---|---|
| Atomic Number | Z = Number of protons |
| Mass Number | A = Number of protons + Number of neutrons |
| Neutrons | n = A − Z |
| Neutral atom | Number of electrons = Z |
| Max electrons in shell | 2n² (K=2, L=8, M=18, N=32) |
| Electron charge | −1.6 × 10⁻¹⁹ C |
| Electron mass | 9.1 × 10⁻³¹ kg (1/1836 amu) |
| Proton charge | +1.6 × 10⁻¹⁹ C |
| Proton mass | 1.67 × 10⁻²⁷ kg (≈ 1 amu) |
| Neutron mass | 1.67 × 10⁻²⁷ kg (≈ 1 amu, neutral) |
| Specific charge (e/m) of e⁻ | 1.76 × 10¹¹ C/kg |
| Nucleus size | ≈ 10⁻¹⁵ m |
| Atom size | ≈ 10⁻¹⁰ m |
| Isotopes | Same Z, different A |
| Isobars | Same A, different Z |
| Isotones | Same N (neutrons), different Z |
| Valency (1-3 valence e⁻) | Valency = Number of valence electrons |
| Valency (5-7 valence e⁻) | Valency = 8 − Number of valence electrons |
| Principal quantum number | n = 1, 2, 3... (shell size/energy) |
| Azimuthal quantum number | l = 0 (s), 1 (p), 2 (d), 3 (f) |
| Shell order (Aufbau) | 1s<2s<2p<3s<3p<4s<3d<4p... |
15) Quick Revision — Power-packed One-liners
- Atom = smallest particle of matter. Dalton said indivisible → later split into e⁻, p⁺, n⁰
- Thomson → Plum Pudding Model (positive sphere with embedded electrons)
- Rutherford → α-particle scattering → discovered nucleus (small, dense, positive), atom is mostly empty space
- Bohr → fixed energy levels (shells K, L, M, N). Electrons do NOT radiate energy in stationary orbits
- Sequence of models: Dalton → Thomson → Rutherford → Bohr
- Electron: discovered by J.J. Thomson (cathode rays). Charge = −1.6 × 10⁻¹⁹ C. Mass negligible.
- Proton: discovered by Goldstein (anode/canal rays). Charge = +1.6 × 10⁻¹⁹ C. Mass ≈ 1 amu.
- Neutron: discovered by Chadwick. No charge, mass ≈ 1 amu. Present in all nuclei except ¹H.
- Z = Atomic number = number of protons. Defines the element.
- A = Mass number = p⁺ + n⁰. Neutrons = A − Z.
- Isotopes = same Z, different A (same element, different neutron count)
- Isobars = same A, different Z (different elements, same mass number)
- Isotones = same neutrons, different Z
- Max e⁻ in a shell = 2n². Outermost shell max = 8 electrons.
- Valence electrons = outermost shell electrons. Determine chemical properties.
- Valency: 1-3 e⁻ → valency = number; 5-7 e⁻ → valency = 8 − n; 4 → valency = 4; 8 → valency = 0
- Noble gases (He, Ne, Ar): full outer shell → valency = 0 (inert)
- Variable valency: Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺, Hg⁺/Hg²⁺ (because of different electron loss)
- Quantum numbers: n (shell/size), l (shape), mₗ (orientation), mₛ (spin)
- Order of filling: 1s<2s<2p<3s<3p<4s<3d... (K and Ca fill 4s before 3d)
✅ Structure of Atom — COMPLETE. Remember the sequence: Dalton → Thomson → Rutherford → Bohr. And never forget: Z = protons, A = protons + neutrons. These two formulas alone will get you through most AMU questions from this chapter!