Ionisation energy

Definition of first ionisation energy

The energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of 1+ ions

Is ionisation an endothermic or exothermic proccess?

Endothermic

Factors which affect ionisation energy

• Nuclear charge 

• Atomic radius 

• Electron shielding 

Nuclear charge 

The greater the number of protons, the greater the nuclear charge 

Atomic radius

The greater the atomic radius, the greater the distance between the nucleus and the outermost electrons and the smaller the nuclear attraction on the outermost electrons

Electron shielding

Inner shell electrons repel the outer shell electrons resulting in shielding. The greater the number of inner shells, the greater the shielding experience by the outermost electrons and the smaller the nuclear attraction on the outer most electrons

What is the trend in successive ionisation energies?

Each successive ionisation energy is more endothermic than the one before

Definition of second ionisation

The energy required to remove one electron from each ion in one mole of gaseous 1+ ions of an 

What is the trend in successive ionisation energies?

• Increase with ionisation energy

• More endothermic

Why do successive ionisation energies increase?

• As each electron is removed, the outer shell is drawn closer to the nucleus (less repulsion between electrons in same shell)

• Atomic radius slightly decreases

• Electrons are removed form an increasingly positive species 

What does a large jump in ionisation energy indicate?

The electron is being removed from a different shell

Trend in first ionisation energies down a group

• Decrease down a group 

• Atomic radius increases as the number of electron shells increases

• Shielding increases

• Nuclear attraction to outermost electrons decreases

• Nuclear charge increases because the number of proton increases, but this is outweighed by the other two factors 

Trend in first ionisation across a period

• Increases across the period

• Nuclear charge increases as the number of protons increase

• Nuclear attraction on outermost electrons increases - there is a greater attraction between the nucleus and outer electron

• Outermost electrons experience the same shielding as the electrons are in the same shell and there is the same number of inner shells

• Hence atomic radius decreases

• More energy required to remove outer shell electron

Why is there a dip in first ionisation energy between group 2 and 3?

• Due to a change in the type of orbital

• p sub-shell has a slightly higher energy than the s sub-shell

• It requires less energy to remove the electron from the p sub-shell

Why is there a dip in first ionisation energy between group 5 and group 6?

• Due to pairing of electrons

• When two electrons are in the same orbital they repel each other

• Therefore it takes less energy to remove a paired electrons