Core charge is the attractive force of the nucleus of an atom on the valence electrons. It is calculated by taking the number of protons of an atom and subtracting the number of inner-shell electrons, and you will always get a positive value. Sodium is used as an example on the left.
Core charge increases across a period and stays the same down a group as shown in the figure below. Core charge is useful in explaining many of the trends of the Periodic Table. |