What happens to the atomic mass as you go across a period?
As you move across a period, the atomic mass increases because the atomic number also increases. When the atomic number increases, this means that there are more protons and neutrons that add to the atomic mass of an atom.
The atomic mass for any given atom mainly comes from the mass of the protons and neutrons. Although the atomic number is also equal to the number of electrons, the electrons are so light that they are insignificant to the mass of an atom.