Explanation:

The lanthanide contraction refers to the decrease in the size of the lanthanide atoms or ions as the atomic number increases from La to Lu. This decrease in size is due to the poor shielding of one of the 4f electrons by another in the subshell.

Shielding of electrons:

Shielding of electrons refers to the ability of electrons in the inner shells to shield the outer electrons from the attractive force of the nucleus. The more effective the shielding, the less the attraction between the outer electrons and the nucleus, and the larger the atomic size.

Factors affecting shielding:

There are two main factors that affect the shielding of electrons:

1. Distance from the nucleus: The closer an electron is to the nucleus, the stronger its attractive force, and the less effective its shielding.

2. Number of electrons: The more electrons there are between the nucleus and the electron in question, the more effective the shielding.

The role of 4f and 5d electrons in the lanthanide contraction:

The lanthanide contraction occurs because the 4f and 5d electrons are not very effective at shielding each other from the attractive force of the nucleus. The 4f electrons are close to the nucleus and are therefore strongly attracted, but they are not very effective at shielding each other because they are all in the same subshell and have the same energy. The 5d electrons are further from the nucleus and are less strongly attracted, but they are also not very effective at shielding the 4f electrons because they are in a different subshell and have a different energy.

Explanation of options:

a) Poorer shielding of 5d electrons by 4f electrons: This option is incorrect because the shielding of 5d electrons by 4f electrons is not a significant factor in the lanthanide contraction.

b) Effective shielding of one of 4f electrons by another in the subshell: This option is incorrect because the poor shielding of one of the 4f electrons by another in the subshell is the main cause of the lanthanide contraction.

c) Poor shielding of one of 4f electron by another in the subshell: This option is correct because the poor shielding of one of the 4f electrons by another in the subshell is the main cause of the lanthanide contraction.

d) Greater shielding of 5d electrons by 4f electrons: This option is incorrect because the shielding of 5d electrons by 4f electrons is not a significant factor in the lanthanide contraction.

Conclusion:

The main cause of the lanthanide contraction is the poor shielding of one of the 4f electrons by another in the subshell. This poor shielding results in a stronger attraction between the nucleus and the outer electrons, leading to a decrease in the size of the lanthanide atoms or ions as the atomic number increases from La to Lu.

It is due to a large area covered by f orbital which causes poor shielding