A and B have identical size and masses. A becomes A++ and B becomes B--.. Will A++ and B-- will have the same mass...???

 aWe know that atomic mass of an atom is the total sum of neutrons, protons and electrons. However, electrons contribute less than 0.06% to an atom's total mass.

 

 means atom A has lost 2 electrons from it so the mass of it is reduced by mass of two electrons.

 means atom B has gain 2 electrons, so its mass will be increased by mass of two electrons.

 

In this case you can give two answer as mass of the electrons is almost negligible to the mass of an atom, so we can say that atom  and  have the same mass. But if we dint neglect the mass of the electron then we can say that atom  is bigger in mass then atom .

Mass of a electron is 

Mass of a proton is  

Mass of a neutron is