Question

Why is the electron gain enthalpy of O or F less than that of S or CI?

• A. O and F are more electronegative than S and Cl.
• B. When an electron is added to O or F, it goes to a smaller (n = 2) level and suffers more repulsion than the electron in S or Cl in larger level (n = 3).
• C. Adding an electron to 3p-orbital leads to more repulsion than 2p-orbital.
• D. Electron gain enthalpy depends upon the electron affinity of the atom.

Answer 1

The correct option is B When an electron is added to O or F, it goes to a smaller (n = 2) level and suffers more repulsion than the electron in S or Cl in larger level (n = 3).

The order of electron gain enthalpies (kJ/mol) is Cl (-349)< F (-328) < S (-200) < O (-141). But if we neglect negative sign and consider magnitude, then Cl>F>S>O.

Electron gain enthalpy becomes more negative across a period while it becomes less negative in a group. However, electron gain enthalpy of O or F is less negative than that of S or Cl. When the electron is added to n = 2 (in case of O or F), the repulsion is more than when it is added to n = 3 (in case of S or CI). an n=2 level is smaller than the n=3 level.