Question

Give reasons for the following :
(i) Though nitrogen exhibits +5 oxidation state, it does not form pentahalide.
(ii) Electron gain enthalpy with negative sign of fluorine is less than that of chlorine.
(iii) The two oxygen-oxygen bond lengths in ozone molecule are identical.

Answer 1

(i)Electronic configuration of N is $1s^{2}2s^{2}2p_x^1,2p_y^{1}2p_z^1$. It has no vacant d-orbitals. If one 2s electron is excited to 3s orbital five half-filled orbitals needed to form pentahalide can still be obtained. But this is thermodynamically not favourable since the energy needed for excitation is more than the energy expected to be generated during the formation of two additional bonds. Therefore, nitrogen does not form pentahalide.

(ii) Due to small size, the electron-electron repulsion in the relatively compact (small) 2p subshell of F are comparatively large and hence the incoming electron is not accepted with the same ease as in case of Cl where 3p subshell is comparatively bigger. Consequetly, the energy released during formation of $F_{\left(g\right)}^{-}$ forms $F_{\left(g\right)}$.
(iii) Ozone molecule is a reasonance hybrid of following contributing structures:
Due to resonance, both the oxygen-oxygen bonds have partial double bond character and their length is equal (128 pm).