1. The problem statement, all variables and given/known data I have a few questions here that say to identify which atom from each pair is larger. One pair is Be and B, and the other is P and Cl. 2. Relevant equations N/A 3. The attempt at a solution I know that Be is larger than B, and that P is larger than Cl. I'm just having trouble understanding why. Is there some rule of thumb that says that sizes of atoms increase when you go across a period? What is the reasoning behind this?