Answer:
(i) Interstitial compounds
Transition metals can trap some atoms like hydrogen, boron,
carbon, nitrogen etc., in vacant spaces in their crystal lattice forming
interstitial compounds. Carbon and nitrogen always occupy octahedral holes; hydrogen
is smaller and always occupies tetrahedral holes. As only transition metals
form such compounds, the d electrons are, therefore, presumably involved
in the bonding. The structure of the metal often changes during the formation
of such compounds. The composition of these compounds is generally
non-stoichiometric, e.g.,but may
approach regular stoichiometry and a regular structure, e.g., TiC and VN.
The later transition elements of the first series form non-stoichiometric
carbides with irregular structures, such as which are
more reactive than the interstitial carbides of the early transition elements.
These interstitial compounds are of much importance, i.e.,
carbon steels are interstitial iron-carbon compounds in which the interstitial carbon
prevents the iron atoms from sliding over one another, making iron harder,
stronger but more brittle. [1]
(ii) When we move along a period left to right, force of attraction
between valence shell electron and nucleus increases. Thus, atomic size
decreasesIn a
period, n remains constant
Thus,
Atomic volume is Thus
directly related to radius.
Since, density [1]
Volume also decreases along a period, Thus density increases.
(iii) The sum of the first three ionisation energies of
the lanthanoids is comparatively low, so the elements are highly electropositive.
They readily form ions
by losing both the 6s and one 5d electrons. In the absence of 5d
electron, one of the electrons present in the (n -2)f orbitals is lost. Besides
+ 3 oxidation state, some of the lanthanides show other oxidation state
Actinides generally show + 3 oxidation state. But due to
comparable energies of 5f, 6d and 7s level, variety of oxidation
states is shown by actinoids. The maximum oxidations state increase from + 4
(in Th) to + 5 in Pa, + 6 in U and + 7 in Np and Pu. [1]
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