The atomic radius cannot be measured directly because the electron cloud does not have a definite boundary. So, one way to measure the size of an atom is by calculating the bond radius, which is half the distance between the nuclei of two bonded atoms. Electron shielding (down a group, not across a period), effective nuclear charge, and the energy level that the outermost electrons occupy plays an major role in determining the atomic radius. On the periodic table, the atomic radius increases down a group because the energy level of the atom down the group increases from top to bottom causing the electrons to have more energy. The effective nuclear charge remain constant down a group because of electron shielding prevents the valence electrons