Why does the atomic radii get smaller from the left to right of a period but get bigger from top to bottom of a group? Wikipedia says in an article about electron shielding "Next we take Beryllium, Be as an example. It has 2 electrons in the 2s shell and thus, these electrons will repel each other as well. Since the repulsion is a vector, the vector may point in the direction of the nucleus at some times and thus the shielding provided by the 1s electrons cannot counteract this vector that well. Thus we know that the Be atomic radius is smaller than the Li atomic radius. From left to right across a period of elements, the atomic radius decreases because of this counteracting of the electron shielding effect." I think I get that electron shielding is the decrease in an electron's attraction to the nucleus due to electrons of lower subshells and shells or rather, electrons that are closer. However, I'm not sureabout Wikipedia's explanation.