Periodicity

State and explain the general trend in first ionisation energies across a period.

First ionisation energies generally increase across a period. This is because

• there are the same number of electron shells so the outer e-s experience a similar degree of shielding.

• The nuclear charge increases and the atomic radius decreases

• so the electrostatic attraction between the nucleus and the outer electrons becomes stronger

requiring more energy to remove an electron

State and explain the trend in first ionisation energies down a group.

First ionisation energies decrease down a group. This is because

• there are more electron shells so the outer e-s experience more shielding and the atomic radius increases.

• Despite the increase in nuclear charge

• the electrostatic attraction between the nucleus and the outer electrons is weaker overall

requiring less energy to remove an electron.

What to mention in first ionisation energy questions?

• State the trend

• Shielding (outer electrons experiencing more/less)

• Atomic radius (increases/decreases)

• Nuclear charge (increases/decreases)

• Electrostatic attraction between the nucleus and the outer electrons is (stronger/weaker)

Why is the first ionisation energy of the Gr3 element lower than Gr2 element of the same period?

• Electron removed from the Gr3 element is from the p-subshell, whereas Gr2, it’s from the s-subshell

• As the p-subshell is higher in energy than s, the p electron requires less energy to remove, thus the ionisation energy is lower

Why is the first ionisation energy of the Gr6 element lower than Gr5 element of the same period?

• Electron removed from the Gr6 element comes from a doubly occupied p orbital, whereas Gr5 its from a singly occupied p orbital

• The electron in the doubly occupied p orbital experiences more electron repulsion, so less energy is required to remove it, resulting in a lower ionisation energy

Write the 3rd ionisation energy of aluminium.

Equations that show the increase in reactivity down group 2:

Appearance of halogens