📊 Periodic Classification of Elements – Full Breakdown

AMU Class 11 Science Chemistry June 2026

        This chapter traces the history of how scientists organised elements — from Döbereiner's triads to Mendeleev's periodic table to the Modern Periodic Table. AMU focuses on Mendeleev's contributions, the Modern Periodic Law, periods vs groups, and trends (valency, atomic size, metallic character). The "periods and groups" identification questions are guaranteed.
    

1) Early Attempts at Classification

Döbereiner's Triads (1817)

Groups of 3 elements with similar properties — atomic mass of middle element ≈ average of the other two.

Example: Li (7), Na (23), K (39) → Avg of Li+K = (7+39)/2 = 23 = Na ✓

Ca (40), Sr (88), Ba (137) → (40+137)/2 = 88.5 ≈ Sr ✓

Limitation: Only a few elements could be grouped into triads.

Newlands' Law of Octaves (1864)

When arranged by atomic mass, every 8th element has similar properties (like musical octaves).

Problem: Only worked up to Ca. He adjusted elements (put Co and Ni in same slot). Failed for noble gases (discovered later).

⚠️ AMU: "Who proposed the Law of Triads?" → Döbereiner. "Who proposed the Law of Octaves?" → Newlands. "Which law was compared to the musical scale?" → Newlands' Law of Octaves.

2) Mendeleev's Periodic Table (1869) — VERY Important for AMU

Mendeleev's Periodic Law: The properties of elements are a periodic function of their atomic masses.

Key Features

Arranged 63 known elements by atomic mass
Left gaps for undiscovered elements — predicted their properties (e.g., Eka-aluminium → Germanium, Eka-silicon → Gallium)
Reversed order wherever properties didn't match atomic mass (e.g., Co (58.9) placed before Ni (58.7) despite higher mass)
Elements with similar properties in same group

Achievements

Systematic study of elements became possible
Predicted properties of undiscovered elements (Eka-boron, Eka-aluminium, Eka-silicon)
Could accommodate noble gases later (without disturbing the table)

Limitations

Couldn't explain position of hydrogen (placed in group 1 but resembles group 17 too)
Position of isotopes not explained (different masses → different positions)
Some elements with higher mass placed before lower mass (Co before Ni)
No place for lanthanides and actinides

⚠️ AMU Gold: "What was the basis of Mendeleev's classification?" → Atomic mass. "What was Mendeleev's periodic law?" → Properties are periodic function of atomic masses. "Which element was predicted as Eka-aluminium?" → Gallium (Ga). "Which was Eka-silicon?" → Germanium (Ge).

3) Modern Periodic Table (1913 — Moseley)

Modern Periodic Law: Properties of elements are a periodic function of their atomic number (NOT atomic mass).
— Henry Moseley

Structure of the Modern Periodic Table (18 Groups × 7 Periods)

Feature	Details
Groups (columns)	18 groups. Same group = same valence electrons = similar chemical properties.
Periods (rows)	7 periods. Same period = same number of shells.
Period 1	Shortest — 2 elements (H, He)
Periods 2 & 3	8 elements each
Periods 4 & 5	18 elements each
Period 6	32 elements (includes lanthanides)
Period 7	Incomplete (includes actinides)

Classification into Blocks

Block	Filled by e⁻ in	Groups
s-block	s-orbital	1 & 2 (alkali & alkaline earth metals)
p-block	p-orbital	13 to 18
d-block	d-orbital	3 to 12 (transition metals)
f-block	f-orbital	Lanthanides & Actinides

🧠 Trick — Groups and their names:
Group 1: Alkali metals (Li, Na, K, Rb, Cs, Fr) — "Alkali" = ash (they form alkaline oxides)
Group 2: Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra)
Group 17: Halogens (F, Cl, Br, I, At) — "Salt-producers"
Group 18: Noble gases (He, Ne, Ar, Kr, Xe, Rn) — "Inert / Zero valency"

4) Trends in the Modern Periodic Table

1. Valency

Across a period: Changes from 1 → 8 (Group 1 = +1, Group 2 = +2, Group 13 = +3, etc.)
Down a group: Same valency (same number of valence electrons).

2. Atomic Size (Atomic Radius)

Across a period → Decreases (nuclear charge ↑ pulls electrons closer)
Down a group → Increases (new shells added)

3. Metallic Character

Across a period → Decreases (metals → non-metals)
Down a group → Increases (more metallic)

4. Non-Metallic Character

Opposite of metallic character. Increases across a period, decreases down a group.

5. Electronegativity

Tendency to attract shared electrons.
Across a period → Increases (F is most electronegative)
Down a group → Decreases

Periodic Trends Summary:

Property Across Period → Down Group ↓
Atomic size DECREASES INCREASES
Metallic char. DECREASES INCREASES
Non-metallic ch. INCREASES DECREASES
Electronegativity INCREASES DECREASES
Valency 1 → 8 Constant

⚠️ AMU Favourite: "Which element has the largest atomic radius?" → Francium (Fr) — down and left. "Which has the smallest?" → Helium (He) — top right (but among non-noble gases, Fluorine). "Which is the most electronegative element?" → Fluorine (F). "Which is most electropositive?" → Francium (Fr).

5) Common AMU-Style MCQs

Q1:

The modern periodic law is based on:

(a) Atomic mass (b) Atomic number (c) Mass number (d) Number of neutrons

Answer: (b) Atomic number (Moseley, 1913)

Q2:

How many groups and periods are there in the modern periodic table?

(a) 8 groups, 7 periods (b) 18 groups, 7 periods (c) 16 groups, 7 periods (d) 18 groups, 8 periods

Answer: (b) 18 groups, 7 periods

Q3:

Which of the following is the most electronegative element?

(a) Cl (b) O (c) F (d) N

Answer: (c) F

Q4:

Atomic radius decreases across a period because:

(a) Nuclear charge increases (b) Nuclear charge decreases (c) Shells increase (d) Electrons increase repulsion

Answer: (a) Nuclear charge increases — pulls electrons closer.

Q5:

Mendeleev's periodic table was based on:

(a) Atomic number (b) Atomic mass (c) Density (d) Neutrons

Answer: (b) Atomic mass

Q6:

Which element was predicted by Mendeleev as 'Eka-aluminium'?

(a) Gallium (b) Germanium (c) Scandium (d) Silicon

Answer: (a) Gallium

Q7:

Halogens belong to group:

(a) 1 (b) 2 (c) 17 (d) 18

Answer: (c) 17

Q8:

Down a group, the metallic character:

(a) Increases (b) Decreases (c) Remains same (d) First increases then decreases

Answer: (a) Increases

6) Quick Revision

Döbereiner: Triads (middle = avg of other two).
Newlands: Octaves (every 8th element similar). Failed after Ca.
Mendeleev: Atomic mass basis. Left gaps. Predicted Ga, Ge, Sc.
Moseley: Modern law — atomic number basis (fixed all issues).
18 groups (columns), 7 periods (rows).
s-block (1,2), p-block (13-18), d-block (3-12 transition), f-block (inner transition).
Atomic size: ↓ across period, ↑ down group.
Metallic character: ↓ across, ↑ down.
Electronegativity: ↑ across, ↓ down. F = most electronegative.
Group 1: Alkali metals. Group 2: Alkaline earth metals. Group 17: Halogens. Group 18: Noble gases.

🧠 Final tip: For AMU, focus on: (1) Mendeleev's vs Modern periodic table differences (2) Periodic trends (direction of atomic size, metallic character, electronegativity) (3) Group names — alkali metals, halogens, noble gases. These are the highest yield topics. The question "property increases/decreases across period or down group" is practically guaranteed (2-3 MCQs).