Question

Explain the following observations:
(i) Many of the transition elements are known to form interstitial compounds.
(ii) There is a general increase in density from titanium (Z=22) to copper (Z=29).
(iii) The members of the actinide series exhibit a larger number of oxidation states than
the corresponding members of the lanthanide series

Answer 1

We know that these are d and f block elements of the periodic table. Some of the properties are that the d block elements will have unpaired electrons in their d-orbital shell, the atomic size of the elements will decrease as we go down the periodic table and the f block elements have a small energy gap in them.

Step by step answer: (i) We know the d blocks elements are referred to as transition elements and f block elements are known as inner transition elements. The interstitial compounds are the compounds when a small atom is occupied in their interstitial spaces or holes. These small atoms may be Hydrogen, Oxygen, Carbon, etc.
The transition elements have defects in their crystal lattice because most of the elements exist in CCP (Cubic close packing) or HCP (Hexagonal close packing) due to that the small atoms like H, C, N, or O are trapped inside these interstitial spaces of the lattice forming interstitial compounds.

(ii) These are the elements in the first series of transition elements. We know that while moving from left to right of the periodic table the atomic size decreases due to an increase in effective nuclear charge as electron shielding remains the same. A higher effective nuclear charge causes greater attractions to the electrons pulling the electron closer to the nucleus which results in a smaller atomic radius.
We know that,

$Density = \dfrac{{mass}}{{volume}}$

As we move across the table mass increases due to an increase in the number of electrons and volume decreases due to a decrease in atomic size, resulting in the density to increase as we move from left to right of the periodic table.

(iii)The action exhibits a larger number of oxidation states than the corresponding members of the lanthanoid series because the elements in the actinide series the outer shell is 5f or 6d while for lanthanide the outer shell is 4f or 5d. The 5f, 6d, and 7s energy levels have comparable energy levels. Therefore, they have lesser energy difference in their outer shell, which makes them more vulnerable to escape and exhibit a larger oxidation state

Note: Though there is a general increase in density as we move the periodic table, it is not the case with atomic radii. The atomic radii of d block elements decrease initially, then remains constant, and finally increases again.