The answer is (1) HI. H and I are both nonmetals, and remember that a molecular compound is one in which covalent bonds hold atoms together, and covalent bonds typically form between nonmetals. The other three choices represent ionic compounds (metal and nonmetal).
Isotopes are atoms of the same element that have the same number of protons and electrons but different number of neutrons. Masses of atoms are the sum of protons and neutrons. Therefore when the number of neutrons change but protons are the same, then the masses of the atoms change. isotope 1 has 8 protons , 8 electrons and 8 neutrons. isotope 2 has 8 protons, 8 electrons and 10 neutrons
mass of isotope 1 is 16 and mass of isotope 2 is 18