Ion Core
Index
Ion Core refers to the full inner electronic quantum states of an atomic system. Full quantum states have spherical symmetry, and additional electrons may occupy incompletely filled quantum states.

The electronic structure of Iron is (1s)2(2s)2(2p)6(3s)2(3p)6(3d)6(4s)2. The ion core of this atom has the Argon electronic structure: (1s)2(2s)2(2p)6(3s)2(3p)6. The (3d) quantum states can accept 10 electrons, and this state is only partly full in iron. The (3p) state is full, containing two electrons of opposite electron spin quantum number, ms.