If an atom gains an electron, what charge will it become as an ion?
An electron has a negative charge,so adding one to an atom creates a negatively charged ion or negative ion.
Some negative ions with one atom that you encounter every day are:
Fluoride,formed from a fluorine atom and found in toothpaste
Chloride, formed from chlorine and found in ordinary table salt
Iodide, formed from iodine and also present in small amounts in table salt (iodized salt)