Q.Draw the molecular orbital diagram for O2 molecule. Write its molecular orbital electronic configuration and predict its magnetic behaviour.

CO is the only one of the four species that is NOT paramagnetic — it is diamagnetic.

Using molecular orbital theory:

• N2+ (13 electrons, isoelectronic-type with an odd electron count) has 1 unpaired electron — paramagnetic.
• CO (14 electrons, isoelectronic with N2) has a fully-paired MO configuration — diamagnetic. …

O2 has 2 unpaired electrons (in the π2px and π2py antibonding orbitals), explaining its paramagnetism (visible in liquid O2 being attracted to a magnet).

By molecular orbital theory, the electronic configuration of O2 is:

$\sigma_{1s}^2 \sigma_{1s}^{*2} \sigma_{2s}^2 \sigma_{2s}^{*2} \sigma_{2p_z}^2 \pi_{2p_x}^2 \pi_{2p_y}^2 \pi_{2p_x}^{*1} \pi_{2p_y}^{*1}$

…

Oxygen is genuinely paramagnetic (assertion true), but its two unpaired electrons sit in the antibonding pi*2p molecular orbitals, not the bonding ones -- so the reason, as worded, is false.

By molecular orbital theory, the electronic configuration of O2 (16 electrons total) is:

sigma1s2 sigma1s2 sigma2s2 sigma2s2 sigma2pz2 pi2px2 pi2py2 pi2px1 pi2py1

The last two electrons singly occupy the two degenerate antibonding pi* molecular orbitals (pi2px and pi2py), each with one unpaired electron (following Hund's rule). This gives O2 a net of two unpaired electrons, which is exactly why it is paramagnetic (attracted into a magnetic field) -- a result MO theory correctly predicts, unlike simple Lewis/valence-bond structures which show all electrons paired …

Molecular orbital theory shows O2 and O2+ both have unpaired electrons in their pi* antibonding orbitals (paramagnetic), while O2^2- (peroxide, 18 valence-region electrons) has those pi* orbitals completely filled, leaving no unpaired electrons — so it is diamagnetic.

Building the molecular orbital electron configuration for the O2 family (order: sigma2s, sigma2s, sigma2pz, pi2px = pi2py, pi2px = pi2py, sigma2pz):

O2 (16 electrons): (sigma2s)2 (sigma2s)2 (sigma2pz)2 (pi2px)2(pi2py)2 (pi2px)1(pi2py)1 (sigma2pz)0

→ 2 unpaired electrons in the two pi* orbitals → PARAMAGNETIC.

O2+ (15 electrons, one less than O2): removes one electron from a pi* orbital → (pi2px)1(pi2py)0

→ 1 unpaired electron → PARAMAGNETIC.

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