Cation vs Anion

In an atom, there is a nucleus containing neutrons and protons. The protons have a positive charge while neutrons are neutral with no charge. That makes the charge of the whole nucleus positive. The electrons which revolve around the nucleus in their specific orbital have a negative charge. An atom contains an equal number of protons and electrons to stabilize both the charges and make it a stable and neutral entity.

Ion
An ion is a charged particle formed from an atom or a molecule. It is a particle in which the total number of electrons is not equal to the number of protons which, in turn, leads to a charge being generated. If the ion is formed by a loss of electrons, a positive charge is developed, and if it is formed by a gain of electrons, then a negative charge is imparted to the particle. The process of generation of ions is called ionization.

Cation
A cation is an ion with a net positive charge on it. This positive charge is developed because the number of electrons is less in it for any reason of bonding. If one electron is lost from the last shell, also called the valance shell in ionic bonding, the number of electrons decreases resulting in the charge of positive protons to dominate. This makes the atom a cation.
Taking the example of Sodium, Na, which has 11 electrons and 11 protons in a natural state, the outermost orbit has 1 electron. The details are given in the diagram in accordance with the Bohr’s model and Aufbau principle. In order to stabilize its octet, the atom of Na loses its outermost electron. This causes a net positive charge to develop on the atom now called a cation.
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Examples of cations are: Na+, Ca2+, Al3+ , H3O+ (Hydronium ion), NH4+ (Ammonium ion) etc.
Usually all metals form cations. The resulting reaction is endothermic.

Anion
An anion is just the opposite to a cation. While a cation holds a net positive charge, an anion is an ion that holds a net negative charge. This is due to the fact that, in an anion, there is an addition of electrons in the valance shell. So the number of electrons which carry a negative charge becomes greater than the number of protons. As a result, there is a net negative charge imparted to the structure making an atom a negative charge-carrying-ion known as an anion.
A chlorine atom becomes an anion after acquiring an electron to stabilize its octet according to the following illustration.
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For examples of anions are: F−, SO42- ,O2− , PO42- , NO3- , etc.
Usually non-metals form anions and the resulting reaction is exothermic.

Summary:

1.Anions are the negatively charged atoms which are formed by the addition of electrons to its valance shell while cations are formed when any electron is lost from the valance shell causing the atom’s net charge to become positive.
2.Metals form cations mostly while anions mostly are formed by non-metals.
3.The process of ionization in a cation is endothermic while in the case of an anion is exothermic.

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