What has a larger atomic radii? Li or Cs and Li or O Explain. (using CER)

Answer 1

First of all, I have no idea what CER stands for. It has many meanings in chemistry/science/medicine, etc.

Atomic radius increases going from right to left across a period, and going from top to bottom within a group. Comparing Li (Group I, period 2) to Cs (Group 1, period 6), Cs would have a larger atomic radius. It is in the same group, but in period 6 vs. period 2. The reason is that in the same group, as you go down the group, you are adding more and more shells, so the radius increases.

Comparing Li and O, they are both in the same period (period 2), but O is far to the right in group VI compared to Li in group I. So, they both have n=2, or 2 shells, but O has more protons in the nucleus pulling the electrons closer. Thus, Li would have a larger atomic radius.