Refer to the data in the Mathematics Tables, pages 44 – 46, in answering this question - Leaving Cert Chemistry - Question 5 - 2002

The first ionisation energies generally increase across the period from lithium (Li) to neon (Ne). This trend is influenced primarily by three factors:

1. Increasing Nuclear Charge: As we move across the period from Li to Ne, the atomic number increases, leading to a greater positive charge in the nucleus. This increased nuclear charge attracts electrons more strongly, making it harder to remove an outer electron.

2. Decreasing Atomic Radius: The addition of electrons occurs in the same shell, which does not significantly increase the distance of the outermost electron from the nucleus. The effective nuclear charge felt by the outermost electrons increases, thereby reducing the atomic radius and increasing ionisation energy.

3. Electron Shielding: While electron shielding does occur, the relatively small number of inner shell electrons in the second period results in less shielding effect, further contributing to the increase in ionisation energy across this period.

In Group II, the first ionisation energies decrease as we move down the group. This trend is attributed to:

1. Increasing Atomic Radius: Each subsequent element has an additional electron shell, which increases the distance of the outer electrons from the nucleus. This results in a weaker attraction between the outermost electron and the nucleus.

2. Increased Shielding Effect: With more inner electron shells, the outermost electrons are more effectively shielded from the attractive force of the nucleus, making them easier to remove.

3. Effective Nuclear Charge: Although the nuclear charge increases down the group, the effect of increased atomic radius and greater shielding more than offsets this, leading to a decrease in ionisation energy.

The increase in ionisation energy values is attributable to the increased stability of the electronic configurations after each electron is removed. As we remove electrons, the positive charge of the nucleus remains unchanged while the total number of electrons decreases. This results in an increased effective nuclear charge experienced by the remaining electrons, which leads to a stronger attraction between the nucleus and the electrons, thereby requiring more energy to remove additional electrons.

The dramatic increase in ionisation energy from the second to the third ionisation of magnesium can be explained by the fact that the removal of the third electron involves breaking into a new electron shell. Once the second electron is removed, the atom achieves a stable electronic configuration. The third electron is removed from a filled shell, where there is significantly greater nuclear attraction experienced by that outer electron compared to the inner electrons that were previously removed. Hence, this transition results in a much higher ionisation energy requirement.

The next dramatic increase in ionisation energy would likely occur between the 10th and 11th ionisation energies, where the electron is likely removed from a new shell (main level) after reaching a stable electron configuration.