the energy of an atom is determined by the amount of energy in its electron shell. the more energy it has the bigger those shells are. (each shell holds a certain amount of electons, but when the get more energtic they can jump to a larger shell). since the attraction of an atom is determined by

1)distance between them

2)weight of the atom

increasing the distance between the atoms, but increasing the size of the outer atomic shells reduces the attraction of the atoms. this allows more movement of the atoms.

the same is true of molucules that share electons

the more man learns, the more he realises, he really does not know anything.