Question

Which of the following would form an anion? Ar K Al O

Answer 1

Answer is: O (oxygen).

1) Oxygen is an element (nonmetal) with atomic number 8 (8 protons and 8 electrons). Oxygen can form anion (O²⁻) when it gain two electrons to have stable electron configuration like closest noble gas neon (Ne) with atomic number 10.

Electron configuration of oxygen atom: ₈O 1s² 2s² 2p⁴.

Electron configuration of oxygen anion: ₈O²⁻ 1s² 2s² 2p⁶.

2) Potassium (K) and aluminium (Al) are metals, which means they lost electrons, so they form cations, not anions. Potassium form cation K⁺ and aluminium Al³⁺.

3) Argon (Ar) is noble gas, which means it has stable electron configuration, so does not form anions and cations.

Answer 2

Anion means negatively charged ion.

Among Ar, K, Al and O, only O forms anion.

Hope this helps!