Ionisation energy is defined as the minimum amount of energy required to remove an electron from a neutral gaseous atom or ion. The ionisation energy of an atom depends on various factors like atomic size, nuclear charge, and electron shielding.

Explanation:

Hafnium (Hf) has the highest ionisation energy among the given elements. This is because of the following reasons:

1. Nuclear Charge: The ionisation energy increases with an increase in the nuclear charge. Hafnium has the highest nuclear charge among the given elements, which explains why it has the highest ionisation energy.

2. Shielding Effect: The shielding effect is the tendency of the inner electrons to shield the outer electrons from the attraction of the nucleus. As we move from left to right in the periodic table, the number of electrons increases, but the shielding effect remains constant. Hence, the outermost electrons experience a greater attraction towards the nucleus, leading to an increase in ionisation energy.

3. Atomic Size: The ionisation energy decreases with an increase in atomic size. As we move from top to bottom in the periodic table, the atomic size increases, and hence, the ionisation energy decreases. However, this trend is not applicable in the case of the given elements because they belong to the same group and have similar atomic sizes.

Therefore, we can conclude that Hafnium, with the highest nuclear charge, has the highest ionisation energy among the given elements.

Due to weak shielding effect of f orbital Hf have higher ionisation energy.