Atomic Radius Tutorial
Atomic Radius (radii) Trends and the Periodic Table
Fallen Down Snowman
Because as we go down a group, we have electrons in higher and higher energy levels which are farther away from the nucleus. The electron distance from the nucleus determines the size of the atom.
As we go across (left to right) a period on the periodic table, the atoms get smaller. Why?
Increased nuclear charge, while the electrons that are added to the same principal energy level.
This is curious since as we go across a period, we are adding electrons, just like we did going down a group. But, the electrons we are adding are all in the same principal energy level and therefore not any farther away from the nucleus. At the same time, the number of protons are increasing as we go across a period. There is less shielding of these outer electrons. This increases the effective nuclear charge which pulls the electrons in closer towards the nucleus.