How does an atom become a positive ion?

An atom becomes a positive ion by losing one or more of its electrons. Positively charged ions are called cations.

Positively charged ions are called cations. Generally, metallic elements turn into positive ions due to the quantity of valence electrons they contain. Metallic ions can achieve a full valence shell if they lose their outer electron (s). When they lose these electrons, the inner shell is now their valence shell, and it's complete. When this happens, they will have more protons than electrons, giving the atom a positive charge.

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.