Question

Peroxide ion ___________
a) is diamagnetic
b) has five completely filled antibonding molecular orbitals
c) is isoelectronic with neon
d) has bond order one.
Which one of these is correct?

• A. a), b) and d)
• B. d) and c)
• C. a) and d)
• D. a), b) and c)

Answer 1

Answer: (C)

Peroxide ion is $O_2^{2-}$

$O_2^{2-}(18)=\sigma 1s^2,{\sigma }^{\star }1s^2,\sigma 2s^2,{\sigma }^{\star }2s^2,\sigma 2p_z^2$

$\pi 2p_x^2\approx \pi 2p_y^2,{\pi }^{\star }2p_x^2\approx {\pi }^{\star }2p_y^2$

Bond order $=\frac{N_b-N_a}{2}=\frac{10-8}{2}=1$

It contains four completely filled antibonding molecular orbitals. Since, all the electrons are paired, $O_2^{2-}$ is diamagnetic. Peroxide ion is isoelectronic with argon, not with neon.