Describe what happens to the size of atoms of the elements as you go across a row and down a column in the periodic table. explain why each of these trends exists

Atom size increases down a group because there is an increase in the number of shells so the attraction between electrons and protons decreases. Atom size decreases across a period because the number of protons increases meaning that there is more attraction between the protons and electrons therefore drawing the outer shells closer to the nucleus.