Classification of Elements and Periodicity in Properties

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R : In the light of the above statements, choose the most appropriate from the options given below :

1. A is false but R is true
2. Both A and R are true but R is NOT the correct explanation of A
3. Both A and R are true and R is the correct explanation of A
4. A is true but R is false

Answer

(1) A is false but R is true

Solution

Across a period: Z increases by 1 at each step, but the new electron enters the same shell, so shielding increases only a little.

⇒ Zeff increases ⇒ the outer electron is held more tightly

∴ First ionisation enthalpy generally INCREASES across a period (e.g. Li 520 → Ne 2080 kJ mol-1) ⇒ A is false.

∵ Increasing nuclear charge outweighs the shielding ⇒ R is true.

Answer: (1)