Question

Question 3.20 Which of the following pairs of elements would have a more negative electron gain enthalpy?

(i) O or F

(ii) F or Cl

Class XI Classification of Elements and Periodicity in Properties Page 93

Answer 1

(i) O or F
if you noticed Oxygen and fluorine both belongs to 2nd period . we know, electron gain enthalpy generally becomes more negative across period when we move from left to right.

on moving oxygen to fluorine , the effective nuclear charge increases and atomic size decreases with increase atomic number. due to this , force of attraction between nucleus and incoming electron increases.

that's why electron gain enthalpy of fluorine ( F) is higher than oxygen ( O ) .

(ii) F and Cl
In group , electron gain enthalpy becomes less negative when we move down the group.But electron gain enthalpy of Cl is higher than F . This is because , size of F is small due to this , the electron - electron repulsion in relatively compact 2p- orbital is greater than in the larger 3p-orbital and hence, incoming electron feels greater repulsion in fluorine than in chlorine.

That's why electron gain enthalpy of Cl is higher than F.

Answer 2

Explanation:

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