NEET Practice Questions (MCQs) with Answers & Solutions

The variability of oxidation states... arises out of incomplete filling of d orbitals in such a way that their oxidation states differ from each other by unity, e.g., $V^{II}$, $V^{III}$, $V^{IV}$, $V^{V}$. This is in contrast with the variability of oxidation states of non transition elements where oxidation states normally differ by a unit of two. (NCERT, Chapter: The d - and f - Block Elements, Page 97)

Although in the p–block the lower oxidation states are favoured by the heavier members (due to inert pair effect), the opposite is true in the groups of d-block. For example, in group 6, Mo(VI) and W(VI) are found to be more stable than Cr(VI). (NCERT, Chapter: The d - and f - Block Elements, Page 97)

In group 6, Mo(VI) and W(VI) are found to be more stable than Cr(VI). Thus Cr(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas $MoO_3$ and $WO_3$ are not. (NCERT, Chapter: The d - and f - Block Elements, Page 97)

Low oxidation states are found when a complex compound has ligands capable of $\pi$-acceptor character in addition to the $\sigma$-bonding. For example, in $Ni(CO)_4$ and $Fe(CO)_5$, the oxidation state of nickel and iron is zero. (NCERT, Chapter: The d - and f - Block Elements, Page 97 and 99)

When a particular oxidation state becomes less stable relative to other oxidation states, one lower, one higher, it is said to undergo disproportionation. (NCERT, Chapter: The d - and f - Block Elements, Page 105)

For example, manganese (VI) becomes unstable relative to manganese(VII) and manganese (IV) in acidic solution. $3Mn^{VI}O_4^{2–} + 4H^+ \rightarrow 2Mn^{VII}O_4^{–} + Mn^{IV}O_2 + 2H_2O$. (NCERT, Chapter: The d - and f - Block Elements, Page 105)

As the oxidation number of a metal increases, ionic character decreases. In the case of Mn, $Mn_2O_7$ is a covalent green oil. Even $CrO_3$ and $V_2O_5$ have low melting points. In these higher oxides, the acidic character is predominant. (NCERT, Chapter: The d - and f - Block Elements, Page 105)

$V_2O_5$ is, however, amphoteric though mainly acidic and it gives $VO_4^{3–}$ as well as $VO_2^+$ salts. In vanadium there is gradual change from the basic $V_2O_3$ to less basic $V_2O_4$ and to amphoteric $V_2O_5$. (NCERT, Chapter: The d - and f - Block Elements, Page 105)

Scandium (Z = 21) does not exhibit variable oxidation states. (NCERT, Chapter: The d - and f - Block Elements, Page 99)

The ability of fluorine to stabilise the highest oxidation state is due to either higher lattice energy as in the case of $CoF_3$, or higher bond enthalpy terms for the higher covalent compounds, e.g., $VF_5$ and $CrF_6$. (NCERT, Chapter: The d - and f - Block Elements, Page 99)