1. As elements go across from left to right in a period, they hold their electrons more tightly, because they
have more protons in the nucleus attracting the
orbitting electrons in the electron cloud. So across a
period, does atomic radius increase or decrease?

decrease.

Explanation:

WHY? - As you go across a period, electrons are added to the same energy level. ... The concentration of more protons in the nucleus creates a "higher effective nuclear charge." In other words, there is a stronger force of attraction pulling the electrons closer to the nucleus resulting in a smaller atomic radius.