Question

a) The extent of actinoid contraction is greater than lanthanoid concentration. Explain.
b) Write two uses of DDT
c) Explain why ${\text{G}}{{\text{d}}^{{\text{3 + }}}}$ is colorless
d) Identify A in the following reaction ${\text{A}}\xrightarrow[{}]{{{\text{PC}}{{\text{l}}_5}}}$ Isopropylidene dichloride

Answer 1

a) You must consider the difference in the shielding effect of the $4f$ and orbitals. $5f$ orbital is farther and larger in size than $4f$ orbital.
b) DDT is the short form for Dichloro- Diphenyl Tri-chloro- ethane. It is a poisonous substance.
c) Gadolinium is an $f -$block element. To answer this question, you must recall its electronic configuration
d) Phosphorus pentachloride $\left( {PC{l_5}} \right)$ is a chlorinating agent for hydrocarbons.

Complete answer:
a) Lanthanoids and actinoids are the elements in the periodic table that correspond to the filling of the $f -$orbital. Lanthanides are the elements that correspond to the filling of $4f -$ orbitals and actinoids correspond to the filling of $5f -$ orbitals. $f -$orbitals have very poor shielding effect and cannot shield the attraction of the nucleus which results in the stronger attraction of the valence shell and decrease in size. Since, $5f$ orbital has poorer shielding than $4f$ orbital. Thus, actinoid contraction is greater than lanthanoid contraction.
b) DDT due to its poisonous properties is used as insecticide and pesticide. It also used various illnesses like typhus, malaria and body lice.
c) ${\text{G}}{{\text{d}}^{{\text{3 + }}}}$ contains 7 unpaired electrons in its $4f$ orbital. Since the $4f$ orbital is half- filled, it provides extra stability. Thus, $f – f$transitions are not possible as it destroys the symmetry of the half filled orbital and makes it unstable.
d) Phosphorus pentachloride carries out substitution reaction in geminal- alcohols and carbonyl compounds to give geminal dihalides. If the product obtained is Isopropylidene dichloride, then A is propanone.

Note:
The use of DDT is now banned due to its high levels of toxicity and the huge number of deaths caused due to its use. It is an accumulator and causes harmful effects due to biomagnification.