Question

Is there any effect in the mass of an atom if it becomes negatively charged?
I know that atomic mass and mass number are two different things but the change in electrons (although may not affect the mass number) may affect the atomic mass, wouldn't it?

Answer 1

Technically, adding an electron to an atom makes the atom and ion and adds the mass of an electron (about 9E-28 g which is quite small). but, yes, it does change. It does not affect the atomic number or the mass number but the atomic mass does change for ion but it is hardly worth taking into account.