State and explain the trend in atomic radius down a group of the periodic table

Down a group of the periodic table, the atomic radius increases.
As we go down a group, the outermost electron of an atom is placed in a shell that is further from the nucleus. This means it experiences the electrostatic attraction of the positive nucleus less.
Furthermore, down a group, there are more inner electrons between the outermost electron and the nucleus. These inner electrons repel the outer electron, thereby shielding it from the nucleus' attractive force. Therefore the outer electron is held further from the nucleus.