What does an atom become when it gains an electron and why does it happen?

An atom that gains a negative electron, it becomes a negative ion. If it loses an electron it becomes a positive ion. Some atoms lose electrons and become positive ions. Others gain electrons and become negative ions.
The atom then loses or gains a "negative" charge. These atoms are then called ions. Positive Ion - Occurs when an atom loses an electron (negative charge) it has more protons than electrons.

This happens because when an atom accepts or gives up an electron to form an ion, it generally assumes a noble gas configuration. For Na, when it loses an electron, its outer shell will be removed thus having a smaller ionic radius. ... When an atom becomes an anion, it gains electrons, which means it gains an outer shell.

Ions are formed when atoms lose or gain electrons in order to fulfill the octet rule and have full outer valence electron shells. ... When they gain electrons, they are negatively charged and are named anions. You can predict the charge of an ion by looking at its group number on the periodic table.

When an atom gains an electron, but nothing else, it becomes negatively charged. This is because the electron is the negatively charged particle in an atom. Protons have a positive charge, while neutrons have no charge; they’re neutral. That’s really all there is to it. However, there is a way to help you remember which charge a specific part of the atom has.

Protons and positive both start with “p”, so you can use that to remember that protons are the positive particle. Neutrons and neutral literally have the same four letters in the beginning of the words: “neut-”. This will help you remember that neutrons are neutral. Electrons and negative don’t really have a lot in common, but by process of elimination, you know they’re negative.