The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms. This outermost shell is known as the valence shell, and the electrons found in it are called valence electrons.

How does valence affect reactivity?

Within each group of metals, reactivity increases as you go down the group. The valence electrons are less tightly bound and easier to remove, because they are farther away from the nucleus of the atom. A nonmetal tends to attract additional valence electrons to attain a full valence shell.

How does the number of shells affect reactivity?

The number of electrons in the outermost shell of an atom determines its reactivity. Noble gases have low reactivity because they have full electron shells. Halogens are highly reactive because they readily gain an electron to fill their outermost shell.