Question

how negative ions are formed

Answer 1

Negative ions are formed when atoms gains an electron
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an atom in order to complete its octet it gains an electrons (happens only when the valence electrons are more than 4 )

when an atom gains an electron it becomes  negatively charged as the number of electrons will be more than that of protons , which makes its charge negative

Answer 2

Explanation:

Metal atoms and non-metal atoms go in opposite directions when they ionise: Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions. Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions.