ionisation energy

ionisation energy

1. Trends across a period

   • Ionisation energies increase when going across each period. This is because:
     • -The number of protons in the nucleus increases so there is a higher attraction on the electrons
     • -Electrons are added to the same shell, so the outer shell is drawn inwards slightly
     • -There is the same number of inner shells, so electron shielding will hardly change

2. Trends down a group

   • moving down a group, first ionisation energies decrease. this is clearest for the first three noble gasses- He, Ne and Ar
     • -The number of electron shells increases so the distance from the outer electrons increases, so, there is a weaker force of attraction from the nucleus to the outer electrons
     • -There are more inner shells, so the shielding effect on the outer electrons from the nuclear charge increases so there is a weaker attraction
     • -The number of protons in the nucleus also increases moving down a group, but the resulting increased attraction is outweighed by the increases in distance and shielding

3. The factors that effect ionisation energy are: -nuclear charge, Distance (of the electron being removed) from the nucleus and electron sheilding

4. Decreases between groups 2 to 13 and 15 to 16

   • There is a small decrease in the first ionisation energy between the group 2 and 13 elements (Be --> B, Mg --> Al)
     • This is because the group 13 elements have their outermost electron in a p orbital, whereas group 2 elements have theirs in an s orbital. P orbitals have slightly more energy than s orbitals and so are marginally further away from the nucleus. this means that the electrons in these orbitals are easier to remove so the elements have slightly lower ionisation energies
     • A similar decrease occurs between groups 15 to 16 (N --> O, P --> S) As you move from group 13 towards group 18, outer electrons are found in p orbitals. In groups 13, 14 and 15, each of the p orbitals contain only a single electron. in group 16 however, the outermost electron is now spin-paired in the px orbital. electrons that are spin-paired experience some repulsion- this makes the first outer electron slightly easier to remove so a slightly lower first ionisation energy is observed.
     • There is a sharp decrease in first ionisation energies between the end of one period and the start of another: -From He at the end of period 2 to Li at the start of period 2 -From Ne at the end of period 2 to Na at the start of period 3 -From Ar at the end of period 3 to K at the start of period 4