Which of the following represents a purely covalent bond?

Ne - Ne

Na - Na

N - N

Ni - O

The answer is N - N.

Purely covalent bond is formed by sharing electrons of atoms with same electronegativity to meet the octet rule (2 or 8 electrons in the valence shell).

Ne is a noble gas and has already 8 electrons in the valence shell, so it will not form a Ne - Ne bond.

Na is an alkaly metal with 1 electron in the valence shell and it does not form a true ionic bond but what is known a metallic bond.

N is a gas that forms a diatomic molecule N - N by sharing the valence electrons in a purely covalent bond.

Ni - O is a metal - non metal compound so it has a ionic character.