Electronic Configuration

 

 

Electrons are arranged in energy levels or shells, these are displayed as 2,8… e.g. ₁₃Al (2,8,3).

 

The maximum number of electron in each shell are as follows:

 

Energy level	Maximum number of electrons
n = 1	2
n = 2	8
n = 3	18
n = 4	32

 

n is referred to as the principal quantum number or shell number.

 

Sub-shells

Shells can also be divided up into the sub-shells s, p, d and f.

 

Energy level	Sub-shell	Maximum number of electrons
n = 1	1s	2
n = 2	2s, 2p	8
n = 3	3s, 3p, 3d	18
n = 4	4s, 4p, 4d, 4f	32

 

The s sub-shell holds 2 electrons

The p sub-shell holds 6 electrons

The d sub-shell holds 10 electrons

The f sub-shell holds 14 electrons

 

So instead of writing (2…) in shells you can write it in sub-shells = 1s²…

1s²

Where 2 is the number of electrons
Where 1 is the principal quantum number
Where s is the name of the sub-shell

 

 

Note the 4^th sub-shell is lower in energy than the 3^rd sub-shell so this begins to fill before 3d.

 

Electron configuration in sub-shells: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ … 

This pattern can be observed by moving across the periodic table where each letter corresponds to a block.

 

Example of the electronic configuration of Oxygen: 1s² 2s² 2p⁶ 3s² 3p⁴

(Total number of electrons 2 + 2 + 6 + 2 + 4 = 16)

 

 

Relationship between sub-shells and elements position in the periodic table

Sodium’s electronic configuration using sub-shells is: 1s² 2s² 2p⁶ 3s¹

 

It has 3 principal quantum shells (n=3) and therefore is in period three. It is in s block and its outer electron is in the s sub-shell, it also has one electron in its outer shell and so it is in group 1.

 

 

Orbital

This is the volume of space around the atom in which there is good chance of finding an electron.

 

Sub-shells are made up of orbitals. Each orbital can only hold 2 electrons but more than one orbital can form a sub-shell. 

 

• For sub-shell s there is only one orbital also known as s
• For sub-shell p there are three orbitals known as p_x, p_y and p_z because there are three orbitals the p sub-shell can hold a maximum of 6 electrons
• For the d sub-shell there are five orbitals which can all also hold two electrons, so the d sub-shell can hold a total of 10 electrons

 

 

 

Orbital shapes

 

S orbital

 

  

P orbitals

 

 

 

Order of filling orbitals

Paired electrons in the same orbital will repel each other and therefore electrons in the same orbital will spin in opposite directions to prevent repulsion.

 

Electrons therefore enter orbitals without any electrons in them first.

 

When drawing electrons in orbitals arrows are used to represent the direction the electrons are spinning. Therefore in each orbital there will be one arrow pointing downwards and the other upwards.

 

Electronic configuration of oxygen using orbitals:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note how the electrons occupy the empty orbitals first and then when these are all taken begin to pair up

 

 

Definition: Isoelectronic

Different elements, which have the same electronic configuration e.g. Na and Mg⁺ both have 11 electrons, thus their electronic configuration is the same.