Question

Which of the following species is paramagnetic in nature?
a) Carbanion
b) Carbonium ion
c) Free Radical
d) None of these

Answer 1

A molecule having one or more unpaired electrons in their valence shell is paramagnetic or having an odd electron system is paramagnetic. We have to know about molecular orbital theory in detail for further clarification.

Complete answer:
Molecular orbital (MO) theory describes the behavior of electrons in an exceedingly large molecule in terms of combinations of the atomic wave functions. Materials with unpaired electrons or odd numbered electrons are paramagnetic and drawn to a magnetic field, while those with all-paired electrons that's even no of electrons are diamagnetic and repelled by a magnetic force field.
According to the options –
a) Carbanion has a negative charge on the carbon, which means one pair of electrons is not bonded and the other six electrons are bonded to other atoms. So, it contains eight electrons. It is diamagnetic.
b) Carbonium ions have a positive charge on the carbon and it has 4 electrons. It is also diamagnetic.
c) Free Radical contains any molecule of a species capable of independent existence that contains an unpaired electron in an atomic orbital. Therefore, it is paramagnetic in nature.
Option c) Free Radical is the correct option.

Additional Information-
During chemical bonding two types of forces come into nature one is attractive and second is repulsive. The force of attraction is between electrons and nuclei and force of repulsion is between nuclei of atoms. Due to these two forces two types of orbitals are formed during chemical bonding one is bonding orbital and second is anti-bonding orbital. In these orbitals electrons are filled in increasing order of energy level in these bonding and antibonding molecular orbitals.
The increasing energy order of orbitals is as follows:
$\;\;\sigma 1s < \sigma 1s * < \sigma 2s < \sigma 2s * < \sigma 2px < \pi 2py = \pi 2pz < \pi 2py * = \pi 2pz * < \sigma 2p *$
(For total numbers of electron greater than $14$)
$\;\sigma 1s < \sigma 1s * < \sigma 2s < \sigma 2s * < \sigma 2px < \pi 2py = \pi 2pz < \sigma 2p * x < \pi 2py * = \pi 2pz *$
(For total numbers of electron less than or equal to $14$ )
It is used for denoting anti bonding molecular orbital.
While filling these orbitals we should follow Hund’s rule, Aufbau’s principle and Pauli’s exclusion principle. Now after filling electrons in these molecular orbitals in increasing energy order if there is any unpaired electron then the compound is called paramagnetic.

Note:
A simple rule of thumb is employed in chemistry to work out whether a particle (atom, ion, or molecule) is paramagnetic or diamagnetic in nature. If all the electrons within the particle are paired, then the substance product of this particle is diamagnetic, if it's unpaired electrons, then the substance is paramagnetic in nature.