Spin diagrams give a visual representation of electron configurations along a graph. Each arrow represents an electron in the atom. The direction of the arrow represents the spin the electron has.
The Pauli Exclusion Principle
The Pauli Exclusion Principle states that each orbital may contain no more than two electrons.
This introduced a property of electrons called spin, which has two states ‘up’ or ‘down’. The spin of an electron in the same orbital must spin in different directions.
Hund’s rule states that single electrons occupy all empty orbitals within a sub-level before they start to form pairs in orbitals.
If two electrons enter the same orbitals there is a repulsion between them due to their negative charges. The most stable configuration is with single electrons in different orbitals.
1s2 2s2 2p1
1s2 2s2 2p2
1s2 2s2 2p3
1s2 2s2 2p4
1s2 2s2 2p5
1s2 2s2 2p6
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s2
1s2 2s22p6 3s2 3p1
1s2 2s22p6 3s2 3p2
1s2 2s22p6 3s2 3p3
1s2 2s22p6 3s2 3p4
1s2 2s22p6 3s2 3p5
1s2 2s22p6 3s2 3p6
1s2 2s22p6 3s2 3p6 4s1
1s2 2s22p6 3s2 3p6 4s2
After the 1s2 2s22p6 3s2 3p1 format, there is a break in the logical pattern in that 4s sub-level fills before the 3d sub level.
This is because the 4s sub level is of lower energy than the 3d sub level. Once the 3dd sub level is filled, then the 4p sub level is filled
Electron Configuration of Transition Metals
When transition metals form ions, it is the 4s electron that are removed before the 3d electron.
1s2 2s22p6 3s2 3p6 4s2 3d3
1s2 2s22p6 3s2 3p6 4s2 3d5
If there is a positive charged ion then to remove electrons, they are taken from the highest energy level.
e.g. Ni = 1s2 2s22p6 3s2 3p6 4s2 3d5
Ni2+= 1s2 2s22p6 3s2 3p6 3d5
As there is a 2+ charge on the second nickel, two electrons are taken from the highest energy level, which is the 4s shell.
e.g. Fe = 1s2 2s22p6 3s2 3p6 4s2 3d6 (or) [Ar] 4s2 3d6
Fe2+ = 1s2 2s22p6 3s2 3p6 3d6 (or) [Ar] 3d6