if two substances have a the same mass but substance A has a higher density than substance B, which one will have the higher volume?

So Pure materials ,they can not have same densities. (Assuming they are not isotopes of each other.) But mixtures, compounds or different phases can have same densities. ... But mixtures, compounds or different phases can have same densities.

So I think A will have the higher volume

They are all full and don't need any more electrons in there outer shell.