Ionisation energy

Define Ionisation energy

The amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions

Ionisation energy is affected by:

• Atomic radius

• Number of protons

• Shielding- repulsion by inner shells of electrons

• Spin pair repulsion

Ionisation trends

Down a group

Ionisation energies decrease down the group because

• Bigger atomic radius

• More shielding

• Weaker attraction between nucleus and electrons

Across a period

Ionisation generally increases because

• Smaller atomic radius

• More protons

• Stronger attraction between nucleus and electron

• Across a period the nuclear charge increases

Dips in the trend

There is a light decrease in IE between beryllium and boron

Beryllium had full sub-shell stability, as the highest occupied shell is complete. Boron has one electron in a higher 2p sub-shell, which is easier to remove due to shielding, hence its first IE is lower than that of beryllium.

There is a slight decrease in the first IE of nitrogen and oxygen

Nitrogen has half shell stability. Oxygen has one 2p orbital which has a pair of electrons and paired electrons repel (spin pair repulsion), so one of these electrons is easier to remove, hence it has a lower first IE

Ionisation energy

Measured under standard conditions which are 298K and 101KPa

The unites of IE are kilojoules per mole (KJ/mol)