Lanthanoids || Actinoids || Contraction || Definition || Lanthanoids Series

Lanthanoids contraction :-

After lanthanium electrons are filled in f-orbital without increasing the number of orbit. Which increases the nuclear charge density,due to diffused shape of f-orbital it has poor screening effect so nuclear force of  attraction on outer most orbit is very high by which atomic size is decreased. This is called lanthanoids contraction.

Effect of lanthanoid  Contraction : –

Similar atomic Size :-

Due to lanthanoids contraction the element of second and third transition series having more or less similar atomic size like zirconium and Hf, Nb and Ta, Mo and W.

Atomic size :-

Atomic size decreases from left to right in a lanthanoids series but the difference in atomic size b/w the element is very small so all the elements look a like and having more or less similar chemical properties. Due to such similarities it is very difficult to isolate them. These are isolated by fractional distillation method.

Basic Strength :-

Basic strength decreases across the series due to decreases across the series due to decrease in atomic size. By which the bond-length between  metal and hydroxide is small so it has high bond dissociation energy.

 M – OH  Bond dissociation energy is very high.

Actinoids :-

1. Element which came after actinium is called actinoids that is element of SF – series.
2. Such elements are distributed in 7^th period and has atomic number 90 to 103.
3. Electronic configuration :-

90^Th  →  [ Rn ] 6d² 7s²             +3, +4

91^Pa  →  [ Rn ] 5f² 6d¹ 7s²        +3 , +4 , +5

92^U  →  [ Rn ] 5f³ 6d¹ 7s²        +3, +4 ,+5, +6

93^Np  → [ Rn ] 5f⁴ 6d¹ 7s²        +3, +4 ,+5, +6, +7

94^Pu → [ Rn ] 5f⁶ 7s²                +3, +4 ,+5, +6, +7

The above abnormal electronic configuration of actinoid is due to very small energy difference between 5f and 6d orbital so electron of 5f orbital easily enters in  6d orbital after middle of the actinoid series the energy difference between 5f and 6d become high so electrons are enter in d-orbital only is case of half filled and full filled condition.

Oxidation state :-

The element of actinoid series shows variable oxidation state. It is due to participation of f-orbital in bond formation so the maximium oxidation no shows by such element is +7 and the most common oxidation number shown by such element is +3.The value of oxidation no increases upto middle of the series them it decreases.

Actinides Contraction : –

Such contraction than lanthanoids contraction because f-orbital has more bigger shape than 4f-orbital. It’s screening effect is poor than 4f-orbital so strongly attract the outer nucleus most orbit towards it self.

Chemical Reactivity :-

Upto middle of the lanthanoids series elements having similar properties like lanthanoids. After middle of the series the reactivity of lanthanoids element is more than lanthanoid element.The chemistry of actinoids elements is not show smooth as that  lanthanoids. justify this statement with example.

Ans-It happens in actinoids series because f-orbital electron  participated in bond formation so such element shows variable oxidation number but In lanthanoid f-orbital participated in bond formation by which these not