Question

Explain, the trends in atomic and ionic sizes of lanthanoids and also of first row elements of transition series.

Accepted Answer

The gradual decrease in atomic and ionic sizes of lanthanoids with increase in atomic number is called lanthanoid contraction. Though the contraction in size from one element to another is very small, the net contraction over the fourteen elements from $La$ to $Lu$ is appreciable. While, the atomic radii show some irregularities, but the ionic radii decreases steadily from $La$ to $Lu$ . The table gives the atomic and ionic radii of $M^{3 +}$ ions.

The atomic size of elements of $3 d$ series gradually decreases upto $Cr$ and then remains almost constant for few elements and then increases slightly towards the end of the series. The initial decrease in the atomic radii from $Sc$ to $Cr$ is due to an increase in the effective nuclear charge, then a sudden increase in the atomic radius of $Mn$ is due to its configuration of $3 d^{5} 4 s^{2}$ , which causes electron-electron repulsion. After this the size remains almost constant and again increases towards the end because of the electron-electron repulsion in the completely filled orbitals of $Cu (3 d^{10} 4 s^{1})$ and $Zn (3 d^{10} 4 s^{2})$ .

In the first transition series if we go left to right, the ions of the same charge show a progressive decrease in radius with increasing atomic numbers, because the new electron enters the $3 d$ orbital each time the nuclear charge increases by a factor of $1$ , and we know that the shielding effect of the d-electrons is not that effective. Hence, the net electrostatic attraction between the nucleus and the outermost electron increases and the ionic radius decreases.

	Catalyst		Process
A.	$T i C l_{3}$	i.	Wacker process
B.	$P d C l_{2}$	ii.	Ziegler – Natta polymerisation
C.	$C u C l_{2}$	iii.	Contact process`
D.	$V_{2} O_{5}$	iv.	Deacon’s process

Explain, the trends in atomic and ionic sizes of lanthanoids and also of first row elements of transition series.

Important Questions on The d- and f-Block Elements

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