Understand that atoms are made up of protons, neutrons, and electrons. Protons carry a positive charge, electrons carry a negative charge, and neutrons are neutral.
Recall that the overall charge of an atom depends on the balance between the number of protons and electrons.
Anions are ions that carry a negative charge, which means they have more electrons than protons.
This negative charge arises because the atom has gained one or more electrons, increasing the number of negatively charged particles.
Therefore, anions carry a negative charge due to the gain of electrons, not because of changes in protons or neutrons.