Question

How to decide if a substance is paramagnetic or diamagnetic?

Answer 1

To solve this question we should know about:
Paramagnetic substance: When placed in a magnetic field, paramagnetic materials tend to become weakly magnetized in the direction of the magnetizing field. A permanent dipole moment or permanent magnetic moment exists in paramagnetic materials.
Diamagnetic substance: A diamagnetic material has no permanent magnetic dipole moment in its atoms.
So, we will read this definition and we will go for its electronic configuration. Then we will draw some conclusions.

Complete answer:
The behavior of an electron is similar to that of a spinning axis. A charge is carried by an electron, and a charge in motion is an electric current.
In physics, we learn that an electric current produces a magnetic field. As a result, an electron acts like a tiny magnet.
An electron's spin magnetic moment can line up with or against a magnetic field applied to it. The lower energy state is aligning with the field. It has the value ${m_s} = + \dfrac{1}{2}$
If all electrons in an atom are paired, the number of electrons with ${m_s} = + \dfrac{1}{2}$ is equal to the number of electrons with ${m_s} = - \dfrac{1}{2}$ . The magnetic moments cancel, resulting in a diamagnetic material.
Thus, $NaCl$ is diamagnetic.
The magnetic dipoles of unpaired electrons will line up with an applied magnetic field if an atom has one or more unpaired electrons. It will be a paramagnetic material.
Nickel, for example, has the electron configuration$[Ar]4{s^2}3{d^8}$. Nickel is paramagnetic because it possesses two unpaired d electrons.

Note:
To solve such a question we should know about electronic configuration. The arrangement of electrons in energy levels surrounding an atomic nucleus is known as electronic configuration, or electronic structure. Electrons occupy multiple levels in the older shell atomic model, from the first shell, K, closest to the nucleus, to the seventh shell, Q, farthest from the nucleus. The K–Q shells are subdivided into a set of orbitals (see orbital) that can each be inhabited by no more than a pair of electrons in a more sophisticated quantum-mechanical model.