The atomic core refers to the central part of the atom excluding the valence electrons. The atomic core has a positive electric charge called the core charge and is the effective nuclear charge experienced by an outer shell electron. In other words, core charge is an expression of the attractive force experienced by the valence electrons to the core of an atom which takes into account the shielding effect of core electrons. Core charge can be calculated by taking the number of protons in the nucleus minus the number of core electrons, also called inner shell electrons, and is always a positive value in neutral atoms.