Question

Define Lanthanide Contraction:

Answer 1

Hint: We should recall about the concepts of the Lanthanoid in the periodic table. It consists of chemical elements in the periodic table from lanthanum to lutetium (atomic numbers 57–71).

Complete step by step answer:
In the periodic table, Lanthnoids show a certain change in size of atoms. We should know Lanthanoid contraction in chemistry as the steady decrease in the size of the atoms and ions of the rare earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71).
We will observe that for each consecutive atom the nuclear charge is more positive by one unit, accompanied by a corresponding increase in the number of electrons present in the 4f orbitals surrounding the nucleus. The 4f electrons very imperfectly shield each other from the increased positive charge of the nucleus, so that the effective nuclear charge attracting each electron steadily increases through the lanthanide elements, resulting in successive reductions of the atomic and ionic radii. Due to Lanthanide contraction, Niobium(Nb)and Tantalum(Ta)show similar atomic and ionic size.
We should know that lanthanum ion, $L{{a}^{3+}}$ has a radius of 1.061 angstroms, whereas the heavier lutetium ion, \[L{{u}^{3+}},\]has a radius of 0.850 angstrom. Because the lanthanoid contraction keeps these rare earth ions about the same size and because they all generally exhibit the +3 oxidation state, their chemical properties are very similar, with the result that at least small amounts of each one are usually present in every rare earth mineral. We should know that the lanthanoid contraction also is a very significant factor in the extremely close chemical similarity of zirconium (atomic number 40) and hafnium (atomic number 72) of the IVb group of the periodic table. Because of the lanthanoid contraction, heavier hafnium, which immediately follows the lanthanides, possesses a radius nearly identical to the lighter zirconium.

Note:
Here we should note that lanthanides and actinides form a group that appears almost disconnected from the rest of the periodic table. This is the f block of elements, known as the inner transition series. This is due to the proper numerical position between Groups 2 and 3 of the transition metals. Actinides are typical metals. All of them are soft, have a silvery colour (but tarnish in air), and have relatively high density and plasticity. Some of them can be cut with a knife.