Question 10
Between which kinds of atom does an ionic bond usually form?
The right answer is the one marked above.
A metal of low electronegativity and a non-metal of high electronegativity. The metal gives up electrons readily and the non-metal pulls them in hard, so a transfer takes place and ions result.
Two atoms that both pull hard on electrons, or both hold them loosely, have no reason to transfer — they share instead, giving a covalent bond. Two atoms of the same element pull equally, so a transfer is impossible.