Chemistry MCQs for NEET — Practice Questions with Answers

Referring to Table 4.11 and the accompanying text: '...the maximum oxidation state increases from +4 in Th to +5, +6 and +7 respectively in Pa, U and Np...' The highest oxidation state listed is +7 for Neptunium (Np).

The text states: 'The actinoids are radioactive elements and the earlier members have relatively long half-lives, the latter ones have half-life values ranging from a day to 3 minutes for lawrencium (Z =103). The latter members could be prepared only in nanogram quantities. These facts render their study more difficult.'

The text mentions: 'The magnetic properties of the actinoids are more complex than those of the lanthanoids.' 'The chemistry of the actinoids is more complex in view of their ability to exist in different oxidation states.' 'The structural variability is obtained due to irregularities in metallic radii which are far greater than in lanthanoids.' Electronegativity trend is not described as complex or varied.

The text states: 'All the actinoids are believed to have the electronic configuration of $7s^2$ and variable occupancy of the 5f and 6d subshells.' This corresponds to $5f^{1-14}6d^{0-1}7s^2$ when expanded for the 5f and 6d subshells, with n=7, so (n-2)f = 5f, (n-1)d = 6d, ns = 7s. The option '$(n-2)f^{1-14}(n-1)d^{0-1}ns^2$' is the general definition for f-block, but 5f, 6d, 7s refers to the specific series.

The text states: 'The 5f electrons, will therefore, be more effectively shielded from the nuclear charge than the 4f electrons of the corresponding lanthanoids. Because the outer electrons are less firmly held, they are available for bonding in the actinoids.' Wait, the text actually says: 'when 5f orbitals are beginning to be occupied, they will penetrate less into the inner core of electrons. The 5f electrons, will therefore, be more effectively shielded from the nuclear charge than the 4f electrons of the corresponding lanthanoids. Because the outer electrons are less firmly held, they are available for bonding in the actinoids.' This means more effective shielding leads to less firmly held electrons and lower ionization enthalpy. So options are reversed. Let's re-evaluate. 'This is quite reasonable since it is to be expected that when 5f orbitals are beginning to be occupied, they will penetrate less into the inner core of electrons. The 5f electrons, will therefore, be more effectively shielded from the nuclear charge than the 4f electrons of the corresponding lanthanoids. Because the outer electrons are less firmly held, they are available for bonding in the actinoids.' Effective shielding means less attraction to the nucleus, hence easier to remove. So 'Weaker shielding of 5f electrons by the nuclear charge' is incorrect if it leads to lower ionization enthalpy.

The text states regarding actinoids: 'However, even the early actinoids resemble the lanthanoids in showing close similarities with each other and in gradual variation in properties which do not entail change in oxidation state.' This indicates a commonality in this specific aspect for the early actinoids resembling lanthanoids.

The text states: 'The action of boiling water on them, for example, gives a mixture of oxide and hydride...'

The text states: 'The magnetic properties of the actinoids are more complex than those of the lanthanoids.' (So I is incorrect). 'Although the variation in the magnetic susceptibility of the actinoids with the number of unpaired 5f electrons is roughly parallel to the corresponding results for the lanthanoids, the latter have higher values.' (So II and III are correct).

The text states: 'The actinoids show in general +3 oxidation state.' and also 'The actinoids resemble the lanthanoids in having more compounds in +3 state than in the +4 state.'

The text states: 'Because the distribution of oxidation states among the actinoids is so uneven and so different for the former and later elements, it is unsatisfactory to review their chemistry in terms of oxidation states.'