Question

Explain actinoid contraction.

Answer 1

In the given question we will discuss the contraction in the actionid, as these Actinides are the element in the periodic table consisting of a sequence of fifteen serial chemical elements. These elements are also called actinide elements.

Complete answer:
As we know that Actinides are elements with the atomic numbers from $90$to $103$ subsequent elements are called Actinium. Actinides includes the elements that are naturally occurring: thorium, protactinium and uranium and eleven transuranics that can be artificially formed by nuclear reactions.

The atomic size or we can say ionic radii of tri positive that is $+ 3$actinides ions decrease mainly through the thorium to Lawrencium due to increase in the nuclear charge and also in the electrons entering to the inner $\left( {n - 2} \right)f$ orbital. Thus, this steady decrease in the size along with an increasing atomic number is termed actinide contraction.

The measured decrease in the ionic radii with the increase in atomic number of actinides. In the actinide sequence, the nuclear charge and number of $5f$ electrons increase by one unit.
The size contraction in actinoids is greater as compared to that in lanthanide. These $5f$orbitals have a poorer shielding effect. The similar concentration in actinides is called lanthanide contraction.

Note: In actinoid, the nuclear charge increases within one unit at individual successive element and this new electron enters the $5f$subshell and Due to the strange shapes of f-orbitals, there stays imperfect shielding of electrons as of the nuclear attraction. As a consequence of this the size of actinoids atoms decreases.