Core charge

Core charge is the effective charge experienced by an outer shell electron, taking into account the shielding effect of inner shell electrons. Core charge can be calculated by the addition of the number of protons in the nucleus minus the number of inner shell electrons and is always a positive value. For the main group elements the core charge is simply the group number.

The core charge can be used to explain a number of periodic trends such as atomic radius, 1st ionisation energy, electronegativity, oxidising and reducing strength.

Since the core charge increases as you move across a period, the outer-shell electrons are pulled more and more strongly towards the nucleus and the atomic radius decreases.

The core charge can sometimes be considered with the nuclear charge. Furthermore, the core charge is merely a man-made method of explaining the trends in the periodic table.