Periodic Classification of Elements
Why This Chapter Matters
Periodic table questions appear in every board exam — 4-5 marks. Trends in periodic properties, Mendeleev vs Modern periodic law, and classification of elements are all tested.
Prerequisites
Core Concepts
1. Historical Development
Dobereiner's Triads (1817):
Groups of 3 elements with similar properties. Atomic mass of middle element = average of other two.
Li (7), Na (23), K (39): Na = (7+39)/2 = 23 ✓
Ca (40), Sr (88), Ba (137): Sr = (40+137)/2 ≈ 88 ✓
Limitation: Only a few triads found. Could not cover all known elements.
Newlands' Law of Octaves (1866):
When elements arranged by increasing atomic mass, every 8th element had properties similar to the 1st.
Like musical octaves — Li, Be, B, C, N, O, F | Na similar to Li
Limitation: Only worked up to Ca. Failed when more elements discovered. Noble gases didn't fit.
Mendeleev's Periodic Law (1869):
"Properties of elements are a periodic function of their ATOMIC MASSES."
Arranged elements in rows (periods) and columns (groups).
Achievements: Left gaps for undiscovered elements and predicted their properties (Eka-aluminium = Gallium found later).
Limitations: Couldn't explain position of isotopes, hydrogen placed in Group IA and VIIA both.
2. Modern Periodic Law
Statement: "Properties of elements are a periodic function of their ATOMIC NUMBERS (Z)."
Proposed by Moseley (1913) based on X-ray experiments.
Modern Periodic Table:
| Period | Elements | Shells Filled |
|---|
|---|---|---|
| 1 | H, He | Shell 1 only (2 elements) |
|---|---|---|
| 2 | Li to Ne | Shells 1, 2 (8 elements) |
| 3 | Na to Ar | Shells 1, 2, 3 (8 elements) |
| 4, 5 | K to Kr, Rb to Xe | 18 elements each |
| 6, 7 | Cs to Rn, Fr to Og | 32 elements each |
3. Periodic Trends (Most Important for Board!)
#### Valency
Valency = number of electrons in outermost shell that participate in bonding.
#### Atomic Size (Atomic Radius)
Reason: More protons → increased nuclear attraction → electrons pulled closer
Reason: New shell added → electrons farther from nucleus
#### Metallic/Non-metallic Character
Na(metal) → Mg → Al → Si(metalloid) → P → S → Cl → Ar(non-metal/noble gas)
Reason: Easier to lose outermost electrons (more shells, less nuclear attraction)
#### Electronegativity
4. Electronic Configuration and Group/Period
Group number (main group elements):
Period number = number of electron shells
Examples:
PYQs
2023
Q: State Modern Periodic Law. How does atomic radius vary down a group?
Modern Periodic Law: Properties are periodic function of atomic number.
Down a group: Atomic radius INCREASES (more shells added, outer electrons farther from nucleus).
2022
Q: An element has atomic number 16. In which period and group is it placed?
Electronic configuration: 2, 8, 6
Period: 3 (three shells), Group: 16 (6 valence electrons in p-block), Valency: 2
2021
Q: Give one example of Dobereiner's triad. What was the limitation of this classification?
Triad: Li (7), Na (23), K (39) — Na's atomic mass = average of Li and K.
Limitation: Only a few triads could be identified from then-known elements.