An unknown substance has a melting point of -77°C. Is this substance likely to be an ionic compound?

No. it,s not possible.

Explanation:

Because you know that ionic compound is having large inter-molecular forces. -77C* is for a compound having v.very small inter-molecular forces.you must know that M.B is directly varing with inter-molecular forces.

because u need two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei to form isotopes.

so since they all have the same shape? i mean maybe because m& m and reeses are both chocolates