What is an ion? , give an example.

An atom or molecule with a net electric charge due to the loss or gain of one or two electrons is known as ion . 
ions are of two types 
1. cations ( positive ion )
2. anion (negative ion )
for e.g Al³+

An Atom is electrically neutral with a charge equal to 0, because it have the same number of protons and electrons.
In ion is an atom who lose or gain electrons. When it lose an electron, the number of protons will be higher than then number of electrons in an normal atom, and will be called positive ion or cation. 
Example: The Aluminium lose 3 electrons. So the ion of Al will be , and its charge is 3+
Ionization equation:
And When an ion gain electrons, the number of electrons will be higher than the number of protons in an normal atom, and will be called negative ion, or Anion..
Example: the Chloride gain 1 electron. So the ion of Cl is , and its charge is 1-
Ionization equation: 

HOPE THIS HELPS :D

The astronomical unit or also know as AU. Scientists use this because it’s convenient and easier to understand! Hope this helps ;)