Trend of 1st Ionisation Energy Across Period 1:
- Comparing the first ionisation energies of of H and He: Neither have inner shells, so there is no shielding. He has two protons in it's nucleus, whereas ||H only has one. Therefore the Helium electrons are more strongly attracted to the nucleus and hence more difficult to remove.
- The first ionisation energy of He is thus higher than that of H.
- Since H and He are the only atoms whose outer electrons are not shielded from the nucleus, it follows that He has the highest first ionisation energy of all the elements. All elements (except H) have outer electrons which are shielded to some extent from the nucleus and thus are easier to remove.
- So therefore, Helium has the highest first ionisation energy of all the elements.