The element chlorine (Cl) has 17 protons and 17 electrons. What happens to an atom of chlorine (Cl) if it gains an electron?

when an atom gain an electron the components of the electron will change as:

1 proton - 18

2. electron - 18

3. Atomic number - 18

4. mass number - 36

It becomes negatively charged

Explanation:

An atom of chlorine having equal number of protons and electrons is neutral. The protons are the positively charged particles located in the nucleus of an atom. Electrons are negatively charged particles located outside the nucleus.

When the number of electrons and protons are the same, such an atom is neutral.

If the number of protons is more, the atom becomes positively charged.

In this case, the atom adds or gains an electron making the number of electrons to go from 17 to 18. This is more than the number of protons which are 17 by 1. The charge on the atom is therefore - 1.