Explanation:
When a neutral atom tends to gain an electron then it acquires a negative charge. Hence, an anion is formed.
For example, atomic number of chlorine is 17 and its electronic distribution is 2, 8, 7.
So, in order to attain stability it tends to gain one electron from a donor atom. Therefore, a chlorine ion is formed as .
Since, it gains only one electron that is why, -1 charge occurs. Whereas if an atom gains two electrons then it acquires a -2 charge and so on.
Thus, we can conclude that chlorine (Cl) has an atomic number of 17. It often forms an ion by gaining 1 electron. Its charge be -1.