Give an example of an ion which is monoatomic.

Hint: When an atom loses or gains a pair of electrons then the formation of electrically charged atoms takes place which are known as ions. When electrons are lost by an atom, then a positively charged ion is formed known as cation and when an atom gains electrons, then negatively charged species are formed known as anions.

Complete answer:
An ion can be categorized on the basis of the number of atoms present in the formula as mono-atomic, diatomic and polyatomic ions.
Mono-atomic ions can be explained as follows:
Ions which are composed of a single atom are known as mono-atomic ions and the charge on the ion represents the difference between the number of electrons and protons for that atom. If the number of protons exceeds the number of electrons, then the ion will acquire a positive charge whereas if the number of electrons is greater than the number of electrons, then the ion will acquire a negative charge. Usually, metals form cations and nonmetals form anions.
Some examples of mono-atomic ions are: $ {K^ + } $ , $ N{a^ + } $ , $ {F^ - } $ , $ C{l^ - } $ , etc.

Additional Information:
Diatomic ions: The ions which are composed of two atoms are known as diatomic ions. Examples of diatomic ions are: $ O{H^ - } $ , $ C{N^ - } $ , etc.
Polyatomic ions: The ions which are composed of more than two atoms are known as polyatomic ions. Examples of polyatomic ions are: $ NH_4^ + $ , $ NO_3^ - $ , etc.

Note:
It is important to note that mono-atomic ions and mono-atomic atoms are not the same terms. Technically, a mono-atomic ion is a form of mono-atomic atom. However, the term mono-atomic atom refers to a neutral atom whereas mono-atomic ion refers to the electrically charged species.