I have no clue what this is asking me

The difference between the enthalpy of H2O(I) and H20(g) is H2O(I) will have less energy than compared to H2O(g).                                                

Explanation:

The enthalpy of H20(I) is more stable than H20(g) because H20(I) will have more liquid and H2O(g) will have more gas. The enthalpy of H20(I) is very exothermic than H2O(g). H20(I) has lesser energy than compared to gas because it has particles weaker than ice.So it is necessary to specify the physical state of water in the thermochemical equation. ΔH=ΔHgas−ΔHliquid.                                                                                                                                        

         

I'm not sure but i will try to find out

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.