Explain why Xenon had a lower first ionisation enthalpy than Neon. (3 marks)

Xenon is further down Group 8 of the Periodic Table than Neon, hence has its outermost electrons in higher energy shells further away from the nucleus. Therefore, there are more full intermediate energy levels of electrons in Xenon which are able to shield the outermost electrons more than Neon, such that they feel less of the nuclear charge so are attracted less to the nucleus. Less energy is absorbed to remove an electron and hence Xenon has a lower first ionisation energy.