Which atom has the greatest first ionisation energy? A H B He C Li D Ne - AQA - A-Level Chemistry - Question 14 - 2019 - Paper 3

First ionisation energy generally increases across a period in the periodic table due to increasing nuclear charge, which attracts the electrons more strongly. Additionally, it generally decreases down a group due to increased distance between the nucleus and the outermost electrons.

1. H (Hydrogen) has a lower first ionisation energy compared to Helium.
2. He (Helium) has a higher first ionisation energy due to its lack of shielding and greater effective nuclear charge.
3. Li (Lithium) has a lower first ionisation energy as it is in the same group as H but further down, which increases distance from the nucleus.
4. Ne (Neon) has a high first ionisation energy as it is a noble gas with a full outer shell.

Based on the above analysis, the atom with the greatest first ionisation energy is B: He (Helium).