What is an ion

An Ion is simply an atom with a charge. In order to explain what is an ion, we will briefly review the atom structure. Electron configuration teaches us that an atom is more stable when its outermost orbital has a whole set of electrons. Actually, the tendency of an atom to achieve a complete outer most orbital is very powerful that it will overcome the electrostatic force that hold the atom together.

Like the case of the Fluorine atom, it will accept an extra electron to achieve a full outmost orbital even
though it will end up having  one more electrons than protons. Basically, a neutral atom has the same number of protons and electrons.

Furthermore, an element like Sodium will give up an electron to attain a full outer orbital. This results in a difference and unbalance between electrons and protons in the same atom.
When in an atom protons (+ve) are more than electrons (-ve) = result is positive ion (cation).

Sodium atom loses one electron become Sodium positive ion cation

Some atoms give up more than one electron to have a total outermost orbital. While other atoms will acquire more than one electron in order to fill the outer orbital. Since electrons have a negative charge (negative ion) and protons have a positive charge (positive ion), a difference in the number of electrons and protons results a net charge. This is when an atom is therefore called an ion.

Depending upon whether it gains or loses an electron, an ion will have either a positive or a negative
charge. Cations are ions with a net positive charged and anions are ions with a net negative charge. We know that opposite charges are attracted while same charges are repelled. That means that cations and anions are attracted to one another. The strength of the attraction may make ions form a solid called a crystal lattice.