PRELIMS

Periodic Classification of Elements

Classification groups elements with similar properties together. Mendeleev is regarded as the father of the periodic table.

Historical Development

1. Lavoisier: Classified elements into metals and non-metals.
2. Dobereiner’s Triads (1829): Groups of three where the middle element's atomic mass was roughly the average of the first and third. (e.g., ).
3. Newland’s Law of Octaves (1866): Arranged by increasing atomic mass; every 8th element shared properties (like musical notes). Failed for elements heavier than Calcium.
4. Lother-Mayer’s Curve (1869): Plotted atomic volume against atomic mass.

Periodic Laws: Mendeleev vs. Modern

• Mendeleev’s Periodic Law: Properties are a periodic function of atomic masses.
  • Structure: 7 Periods, 9 Groups (I-VIII and Zero).
• Modern Periodic Law (Moseley): Properties are a periodic function of atomic numbers.
  • Structure: 7 Periods, 18 Groups.

Blocks in the Modern Periodic Table

• s-block: Groups 1 & 2 (Alkali and Alkaline Earth metals).
• p-block: Groups 13 to 18 (Includes Chalcogens, Pnictogens, Halogens, and Inert gases).
• d-block: Groups 3 to 12 (Transition elements).
• f-block: Inner transition elements (Lanthanides and Actinides).

Periodic Trends and Properties

1. Atomic Radii: Distance from nucleus to outermost shell.
   • Hierarchy: Van der Waals radii > Metallic radii > Covalent radii.
2. Ionic Radii:
   • Hierarchy: Anionic radii > Atomic radii > Cationic radii.
3. Ionization Potential (I.P.): Energy required to remove an electron from an isolated gaseous atom.
4. Electron Affinity ( ): Energy released when an extra electron is added.
   • Chlorine (Cl) has the highest electron affinity.
5. Electronegativity ( ): Tendency to attract shared electrons.
   • Fluorine (F) is the most electronegative atom.
   • Scale: (Ionic); (Polar Covalent); (Non-polar).
6. Solubility (Lattice vs. Hydration Energy):
   • If Hydration Energy > Lattice Energy Soluble in water.
   • If Hydration Energy < Lattice Energy Insoluble in water.

Summary of Trends Across a Period and Down a Group

• Across a Period (Left to Right): Atomic radius decreases; Ionization energy increases; Electronegativity increases; Metallic character decreases; Non-metallic character increases.
• Down a Group (Top to Bottom): Atomic radius increases; Ionization energy decreases; Electronegativity decreases; Metallic character increases; Non-metallic character decreases.
• Valency: Across a period, valency w.r.t. Hydrogen first rises (1 to 4) then falls (4 to 0/1) at the noble gas.

Confused Pairs

• Period vs. Group: A Period is a horizontal row (7 total; elements have increasing atomic number and different properties); a Group is a vertical column (18 total; elements share similar valence electron configuration and chemical properties).
• Ionization Energy vs. Electron Affinity: Ionization energy is the energy required to remove an electron from a neutral gaseous atom (always positive/endothermic); Electron affinity is the energy released on adding an electron (usually exothermic).

UPSC Relevance

Periodic trends are vital for conceptual S&T questions.

• Key Trends: Understanding why Fluorine is the most electronegative or why Chlorine has the highest affinity.
• Transition Elements: The significance of d-block elements in industrial catalysis and metallurgy.
• Solubility: The relationship between lattice/hydration energy explains the behavior of various salts in water.