Why does ionisation energy show exceptions in certain positions in the periodic table?

Ionisation energy shows exceptions due to electron shielding and electron pair repulsion in certain positions in the periodic table.

Ionisation energy is the energy required to remove an electron from an atom. Generally, it increases across a period from left to right and decreases down a group. This is because as you move across a period, the number of protons in the nucleus increases, pulling the electrons closer and making them harder to remove. Conversely, as you move down a group, the outer electrons are further from the nucleus and easier to remove.

However, there are exceptions to this trend due to electron shielding and electron pair repulsion. Electron shielding refers to the phenomenon where inner shell electrons shield outer shell electrons from the full attractive force of the nucleus. This makes the outer electrons easier to remove, thus reducing the ionisation energy. This is particularly noticeable in the transition metals, where the 3d electrons shield the 4d electrons, causing a smaller than expected increase in ionisation energy across the series.

Electron pair repulsion occurs when two electrons occupy the same orbital. Electrons are negatively charged and repel each other, so when two electrons share an orbital, they push each other away. This makes one of the electrons easier to remove, thus reducing the ionisation energy. This is particularly noticeable in the p-block elements, where the second ionisation energy is often lower than expected because removing the second electron relieves the electron pair repulsion.

For example, the ionisation energy of aluminium is lower than that of magnesium, even though aluminium is to the right of magnesium in the periodic table. This is because the outer electron in aluminium is in a 3p orbital, which is shielded by the 3s electrons, whereas the outer electron in magnesium is in a 3s orbital, which is closer to the nucleus and less shielded. Similarly, the ionisation energy of sulfur is lower than that of phosphorus, because removing an electron from sulfur relieves the electron pair repulsion in the 3p orbital.