Within a horizontal row of the periodic table how does mass change?

Answer 1

Within a horizontal row, mass of the elements increases as you go to the right. This is because as the atomic number increases, so does the number of protons and electrons.

This means that the total mass of the atom also increases because protons weigh approximately 1 Atomic Mass unit.

Answer 2

Generally a single element's mass will change (and stay the same element) only if the number of neutrons changes in the nucleus. Although an element's mass does change by the loss of electrons, it is exceptionally negligible. Should an element change in mass by losing a proton, it then becomes a different element entirely.