It became heavier... because of oxidation,
CU became CUO.
The atomic mass of copper is 64,
and that of oxygen is 16
CU is 64
CUO 64+16= 80 ,
So it became heavier.
The particular situation is the same as I said above. Without air, it will not be heavy.
Of course, such a situation is an ideal state and can only be assumed. exist.