How are negative ions formed?

Negative ions, also known as anions, are formed when non-metal atoms gain electrons. Non-metals have high electronegativity, which means they have a strong tendency to attract and hold onto additional electrons. When a non-metal atom gains one or more electrons, it becomes negatively charged, as there are now more electrons than protons in the atom.

This process is fundamental in the formation of ionic compounds, where non-metals typically react with metals. For example, when chlorine (a non-metal) gains an electron, it becomes a chloride ion, which is negatively charged.

The other concepts presented in the alternatives do not lead to the formation of negative ions. Metals generally lose electrons to become positive ions (cations), and losing protons is not a process that occurs in forming ions.