Two factors that influence
the changes of atomic radii in the Periodic Table are:
- Effective nuclear charge experienced by the valence electrons: Electrons around the nucleus experience different nucleus attraction. Those electrons closer to the nucleus experience a greater attraction than those that are farther away. The actual nuclear charge experienced by an electron is called the effective nuclear charge, Zeff. Effective nuclear charge increase, nucleus attraction stronger, atomic radii decrease. Across the period, the effective nuclear charge increases as proton number increase. As a result, the attraction between the nucleus and valence electrons become stronger, causing the atomic radius to decrease.
- The principal quantum number, n, of the valence electrons: As going down a group, the number of shells increases, more inner electrons are present to shield the valence electrons from the nucleus. The valence electrons are farther from the nucleus. Thus, the attraction between the nucleus and valence electrons decreases, therefore, the atomic radius increase. Down a group, the atomic radius increases because of the increasing principal quantum number (n) of the valence electron.