Here's why:

* Atoms are normally neutral: They have an equal number of protons (positively charged) and electrons (negatively charged).

* Gaining electrons creates a negative charge: When an atom gains extra electrons, it becomes negatively charged.

* Anion: The term "anion" specifically refers to a negatively charged ion.

Example: A chlorine atom (Cl) normally has 17 electrons. When it gains an extra electron, it becomes a chloride ion (Cl-) with 18 electrons and a net negative charge.