Nonmetals form negative ions, also known as anions, through a process called electron gain. When a nonmetal atom encounters an environment where it can accept electrons, it tends to do so because nonmetals typically have high electronegativity. This means they have a strong tendency to attract electrons towards themselves.
For example, take a chlorine atom. It has seven electrons in its outermost shell, and it needs one more electron to achieve a full outer shell, which is typically a more stable configuration. When a chlorine atom gains an electron, it now has a total of eight electrons in its outer shell. As a result of this gain, the chlorine atom becomes negatively charged, transforming into a chloride ion (Cl-).
This tendency for nonmetals to gain electrons and form anions is a crucial part of chemical bonding, particularly in ionic compounds. In these compounds, nonmetals, which are often electronegative elements, combine with metals that easily lose electrons. The transfer of electrons leads to the formation of oppositely charged ions, which are held together by electrostatic attractions, creating stable compounds.