If an atom gains one electron, it becomes a negatively charged ion, specifically called an anion.

Here's why:

* Atoms are neutral because they have an equal number of protons (positively charged) and electrons (negatively charged).

* Gaining an electron means the atom now has more electrons than protons, creating a net negative charge.

* Anions are negatively charged ions.

For example, a chlorine atom (Cl) has 17 protons and 17 electrons. When it gains one electron, it becomes a chloride ion (Cl-), with 17 protons and 18 electrons, resulting in a -1 charge.