What are Valence Electrons?

In short

Valence electrons are the electrons in the outermost shell of an atom. They decide how the atom bonds and how it reacts.

In simple words

For a main-group element, the number of valence electrons is the group number (for groups 1 and 2) or the group number minus $10$ (for groups 13 to 18). Atoms tend to gain, lose or share electrons until they have a full outer shell, usually 8 electrons, the octet rule. Helium is the exception, with 2. The outer shell is also called the valence shell.

Example

Chlorine has the configuration $2, 8, 7$ , so it has 7 valence electrons. It needs 1 more to reach an octet, so it forms the $\text{Cl}^-$ ion or shares one pair of electrons.

Also searched as: outer valence shell, valence shell numbers, shells atoms.