Which of the following is not a cause of lanthanoid contraction?

Filling of atomic orbitals of shells in order

The correct answer is option 3. Filling of atomic orbitals of shells in order.

The lanthanoid contraction is a phenomenon observed in the lanthanoid series (elements with atomic numbers 57 to 71, from lanthanum to lutetium). As we move across the series from left to right, the atomic and ionic radii of these elements decrease, despite the addition of more electrons. This decrease is gradual but significant enough to influence the properties of the elements that follow the lanthanoids in the periodic table.

Causes of Lanthanoid Contraction:

1. Imperfect shielding of one electron by another in the same set of orbitals:

The 4f electrons in the lanthanoids do not shield each other effectively from the nuclear charge. Shielding refers to the ability of inner electrons to reduce the effective nuclear charge felt by the outer electrons. In the case of 4f electrons, their shielding effect is poor due to their shape and penetration. This poor shielding means that as more protons are added to the nucleus (as atomic number increases), the effective nuclear charge experienced by the electrons increases. This stronger nuclear pull reduces the size of the atom.

2. Increase in nuclear charge with increasing atomic number:

As we move across the lanthanoid series, the number of protons in the nucleus increases, leading to a higher nuclear charge. The increase in nuclear charge tends to pull the electrons closer to the nucleus, which decreases the atomic radius. This is a key factor contributing to lanthanoid contraction.

3. Filling of Atomic Orbitals of Shells in Order:

This refers to the general process of how electrons are added to orbitals as we move across the periodic table. For the lanthanoids, this involves filling the 4f orbitals. However, this is not a direct cause of the lanthanoid contraction. While the filling order affects the electronic configuration, it is the poor shielding by 4f electrons and the increasing nuclear charge that are the primary drivers of the contraction.

4. Decrease in Atomic Size with Increasing Atomic Number:

This is a consequence of lanthanoid contraction, not a cause. As the effective nuclear charge increases and the electrons are pulled closer, the atomic size decreases across the series. Therefore, this is an effect rather than a contributing factor.

Conclusion:

The correct answer to the question is option 3: Filling of atomic orbitals of shells in order. This option describes the general process of electronic configuration but does not directly cause the lanthanoid contraction. The other options describe factors that contribute to the contraction, either through the imperfect shielding by 4f electrons or the increasing nuclear charge that pulls electrons closer, resulting in a decrease in atomic size.