Electronic structure

Aufbau’s principle

electrons enter the lowest energy orbital possible

Hund’s rule

electrons prefer to occupy orbitals on their own and only pair up when no empty orbitals of the same energy are available.

Electrons

they have fixed energies, and they move around in the nucleus in certain regions called shells / energy / levels.

the further a shell is away from the nucleus..

the higher its energy and the larger its quantum number

shells

shells are divided into sub-shells and different electron shells have a different number of sub-shells.

1st electron shell

lowest energy, contains one sub-shell(1s) holds 2 electrons in the shell

2nd electron shell

contains two sub-shells (2s,2p) holds 8 electrons in the shell.

3rd electron shell

contains 3 sub-shells (3s,3p,3d) holds the maximum number of 18 electrons in the shell

4th electron shell

highest energy, contains four sub-shells (4s,4p,4d,4f) holds a maximum number of 32 electrons in the shell

sub-shells

have a different number of orbitals and each orbital can hold up to 2 electrons

s sub shell

has 1 orbital so holds 2 electrons

p sub shell

has 3 orbitals and holds 6 electrons

d sub shell

5 orbitals so holds 10 electrons

f sub shell

7 orbitals so holds 14 electrons

atomic orbitals of lower energy

atomic orbitals of lower energy are filled first so the lower main energy level is filled first and the sub-shells within it.

atomic orbitals of the same energy

atomic orbitals of the same energy fill singly before pairing. this is because electrons repel each others.