Electron Configurations and Valence Electrons

Most electron configurations follow predictable patterns, but exceptions exist for lanthanides, actinides, and some transition metals in groups VIB (6) and IB (11). These deviations are attributed to the enhanced stability provided by half-filled or completely filled d sublevels.

Elements within the same group share the same number of electrons in their last-occupied principal energy level, known as the valence level or shell. Electrons in this highest-energy level are called valence electrons. Electrons in lower principal energy levels are core or inner electrons. The similar chemical properties of elements in the same group are directly linked to their number of valence electrons. While d-sublevel electrons can sometimes be considered valence for transition metals, their behavior is less predictable than main-group valence electrons.

One method to find valence electrons is by examining the electron configuration. For example, silicon (Si) has four valence electrons in its n=3 principal energy level (3s and 3p sublevels). Electrons constituting a noble-gas configuration are typically core electrons. For main-group elements, the number of valence electrons equals the electrons in the highest-energy s and p sublevels. A more convenient method involves using the periodic table: the period number corresponds to the valence level number for s and p sublevels, and the Roman numeral group number (for main-group elements, excluding helium) directly indicates the number of valence electrons. For instance, silicon in Group IVA (14) has four valence electrons, and bromine in Group VIIA (17) has seven.