The ability of an atom to borrow or lend electrons is called 'valence' which is also known as 'valency' or 'valence number'.
Electrons are an essential part of an atom. Unlike protons and neutrons, valence electrons take part in the excitement of a chemical reaction.
Valence electrons are the electrons located at the outermost shell of an atom. When two atoms interact, the electrons in the outermost shells are the first ones to come into contact with each other and are the ones that determine how an atom will react in a chemical reaction.
The shells of an atom can only hold so many electrons. Each shell has a certain amount of subshells (s, p, d, etc) that have a certain amount of orbitals. Each orbital can hold 2 electrons. The first shell has one subshell, s, which has one orbital, so it can hold 2 electrons. The total number of electrons that each shell can hold is:
Shell 1 - has subshell s, which has one orbital. It can therefore hold 2 electrons.
Shell 2 - has subshells s and p. p has 3 orbitals, so can hold 6 electrons. Add the two that subshell s can hold, and we know that shell 2 can hold 8 total electrons
Shell 3 - has subshells s, p, and d. d has 5 orbitals, so can hold 10 electrons. Shell 3 can hold a total of 18 electrons.
Learn how to find the number of valence electrons of any element at Study.com.