Here's why:

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

* Gaining electrons creates a negative charge: When an atom gains one or more electrons, it has more negative charges than positive charges, making it overall negatively charged. This is called an anion.

Example: A chlorine atom normally has 17 protons and 17 electrons. If it gains one electron, it becomes a chloride ion (Cl-) with 17 protons and 18 electrons, carrying a negative charge.