Periodic Properties

Topics covered: Periodic laws, modern periodic table, blocks of elements, atomic size, ionisation potential, electronegativity, nature of oxides and hydroxides.

1) Development of Periodic Table

Prout’s Hypothesis

• Atomic weights of all elements are multiples of hydrogen.

Dobereiner’s Triads

• Mean atomic mass of first and third elements ≈ atomic mass of the middle element.
• Example: Li (7), Na (23), K (39)

Newlands’ Law of Octaves

• Every eighth element shows similar properties when arranged by atomic mass.

Lothar Meyer’s Atomic Volume Curve

• Alkali metals show maximum atomic volume.
• Transition metals have minimum atomic volume.

2) Modern Periodic Law

• Proposed by Moseley.
• Properties of elements are periodic functions of their atomic numbers.

√ν = a(Z − b)
Z = atomic number
  

3) Modern Periodic Table

• 7 periods and 18 groups.
• Periods are horizontal rows; groups are vertical columns.
• Lanthanides (58–71) and actinides (90–103) are placed separately.

4) Blocks of Elements

s-Block

• Last electron enters s-orbital.
• General configuration: ns¹ or ns²

p-Block

• Last electron enters p-orbital.
• General configuration: ns²np¹–⁶

d-Block (Transition Elements)

• General configuration: ns¹–² (n−1)d¹–¹⁰
• All are metals; Hg is liquid.

f-Block

• Lanthanides (58–71) and Actinides (90–103).
• Placed outside the main table.

5) Effective Nuclear Charge

Z* = Z − σ
Z* increases across a period
Screening order: s > p > d > f
  

6) Atomic and Ionic Radius

• Atomic radius decreases across a period.
• Atomic radius increases down a group.
• Cations are smaller than atoms.
• Anions are larger than atoms.

7) Ionisation Potential (IP)

• Energy required to remove the most loosely bound electron.
• IP increases across a period.
• IP decreases down a group.

Factors affecting IP:

• Atomic size (inverse relation)
• Effective nuclear charge
• Shielding effect
• Stability of half-filled and fully-filled orbitals
• Penetration power

8) Electronegativity

• Tendency of an atom to attract shared electrons.
• Increases across a period.
• Decreases down a group.
• Highest: Fluorine (F)
• Lowest: Cesium (Cs)

9) Nature of Oxides

• Higher electronegativity difference → basic oxide.
• Acidic character increases across a period.
• Basic character decreases across a period.

10) Nature of Hydroxides

• If electronegativity of element > 1.7 → acidic hydroxide.
• If electronegativity < 1.7 → basic hydroxide.

11) Important Facts (JEE Main)

• Highest electronegativity: F
• Lowest ionisation potential: Cs
• Liquid non-metal: Br
• Liquid metal: Hg
• Artificial element: Tc (43)
• Most abundant element in earth’s crust: O
• Noble gases belong to Group 18