Here's why:

* Octet Rule: Atoms tend to be most stable when they have eight electrons in their outermost shell (except for hydrogen and helium, which are stable with two electrons).

* Electronegativity: Atoms with seven valence electrons are highly electronegative, meaning they have a strong attraction for electrons.

* Ionic Bonding: By gaining one electron, the atom achieves a full outer shell and becomes a negatively charged ion (anion). This often occurs through ionic bonding, where the atom forms a bond with another atom that readily loses an electron (like a metal).

Example:

A chlorine atom (Cl) has seven valence electrons. It can gain one electron to become a chloride ion (Cl-), which has a complete outer shell of eight electrons. This is why chlorine often forms ionic bonds with metals like sodium (Na) to form sodium chloride (NaCl), commonly known as table salt.