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 $O _{2}^{2-}$

$O _{2}^{2-}(18)=\sigma 1 s^{2}, \sigma^{\star} 1 s^{2}, \sigma 2 s^{2}, \sigma^{\star} 2 s^{2}, \sigma 2 p_{z}^{2}$

$\pi 2 p_{x}^{2} \approx \pi 2 p_{y}^{2}, \pi^{\star} 2 p_{x}^{2} \approx \pi^{\star} 2 p_{y}^{2}$

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.