As an atom becomes an ion, what happens to its mass number?
An atom's mass number tells you how many protons and neutrons said atom has in its nucleus.
#color(blue)("mass number" = A = "no. of protons" + "no. of neutrons")#
The number of protons an atom has in its nucleus is given by the atomic number,
Now, a neutral atom will always have equal numbers of protons, which are positively charged particles located in the nucleus, and electrons, which are negatively charged particles tha surround the nucleus.
This balance between the number of protons and the number of electrons gives the atom an overall zero net charge.
When an atom becomes an ion, this balance is disrupted. By losing electrons, the atom develops an overall positive net charge.
Likewise, by gaining electrons, the atom develops an overall negative net charge.
So what does this have to do with mass number? Absolutely nothing.
Gaining or losing electrons has no impact on the number of protons and neutrons an atom has, which is another way of saying that the mass number is not affected by changes in the net charge.