An ion is fundamentally a group of charged atoms ( electrically charged ) which is formed either by the gain of electrons or by the loss of electrons. The charge of an electron is negative whereas the charge on a proton is positive. When a certain number of electrons are added or eliminated, the stable atom that exists becomes an ion.

Cation: When the electrons are eliminated from an atom it becomes positively charged and the net charge of ion turns to positive such ions are known as cations. Some of the examples of cations are:

• Calcium           Ca2+
• Sodium            Na+1
• Mercury (I)       Hg2+2
• Mercury (II)      Hg+2
• Aluminum        Al+3

The ions list above are very common. The table salt that we use in our food is also known as Sodium. The device thermometers used in clinics has mercury in it. Most of the metallic things that exists today are made up or has some amount of Aluminium in it. Hence, ions are present everywhere.

Anion: When the electrons are added to an atom it becomes negatively charged and the net charge of ion turns to negative such ions are known as an anion. For example, if you will add an electron to a neutral atom it becomes an anion. Examples of anion

• Iodide               I−
  Nitride              N3−
• Bromide          Br−
  Chloride           Cl−
  Fluoride           F−

Like cations, we must have encountered  some of these anions in our daily lives. Sometimes the water communities add Fluoride to the water. Another common use is, dentist may give you a flouride treatment.

If we talk about Hydrogen which has unusual behaviour, both cations and anions can be formed. Mostly elements much prefer to form only one or the other.

The cation and anion attract each other because of the opposite charges they carry and easily forms the ionic compounds. The ions that comprise of the single atom are known as the monatomic ion, whereas the ion with more than one atom is called as polyatomic ions. Example of monatomic ion is calcium carbonate Ca²⁺ whereas Hydroxide ion is an example of polyatomic ion; it consists of one hydroxide atom and one oxide atom, hydroxide has a charge of −1, the chemical formula of hydroxide ion is OH⁻.

Let us compare the  polyatomic ions to monatomic ions. As stated above an atom that has been ionized by losing or gaining electrons is a monatomic ion . Since the total number of electrons is not balanced by the total number of protons in the nucleus, the ion has net charge. When we compare the neutral atom, we have extra electron – in the case of a negatively charged anion – or not enough electrons – in the case of a positively charged cation. For example, 17 is the atomic number of a chlorine atom that is neutral, which implies that it the protons and the electrons count is equal. A chloride anion is formed when the neutral atom gain an extra electron. An electron gain makes chloride anion configuration to 17 protons and 18 electrons. As there is an extra electron compared to the number of protons, the ion has a net charge of 1-. In a similar fashion, we can think of a polyatomic ion as a molecule that has been ionized by gaining or losing electrons.

Do not get confused between ion and atom.Talking about the atom, it is the basic unit of an element, an atom is considered stable when it comprises the same number of protons and electrons in its nucleus. Whereas an ion is the cluster of particles with some net negative or positive charge.