Question

as you move across the periodic table, atoms tend to get smaller. this is because... (options: the atoms have more mass, the atoms have less mass, they have more protons, the atoms have more neutrons)

Explanation:

When moving across a period (left to right) in the periodic table, the number of protons in the nucleus increases. The increased positive charge of the nucleus pulls the electron cloud (which is in the same energy level for a period) more strongly, resulting in a smaller atomic radius. Mass (related to protons + neutrons) doesn't directly cause size decrease, and neutrons don't impact the electrostatic attraction that shrinks the atom. So the correct reason is that atoms have more protons.

Answer:

They have more protons