Q4. Compare the actinoids with the lanthanoids with reference to (i) electronic configuration, (ii) oxidation states and (iii) chemical reactivity, and (iv) atomic and ionic sizes. (NCERT Exercises 4.33 and 4.20.)
(i) Lanthanoids fill 4f with 6s; actinoids fill 5f with 7s, sometimes with a 6d electron, and their 5f orbitals are less buried. (ii) Lanthanoids are mainly +3, with a few +2 and +4 ions; actinoids have a wider range, with the early ones reaching +6 (U) and +7 (Np, Pu) (NCERT page 112); beyond the NCERT text, No(II) is also important (Wikipedia, “Nobelium”). (iii) Both are reactive metals; the early lanthanoids resemble calcium, the later aluminium (NCERT page 110), while the actinoids are highly reactive, especially when finely divided; hydrochloric acid attacks all of them, but most are only slightly affected by nitric acid, owing to protective oxide layers, and alkalies have no action (NCERT page 113). Beyond the NCERT text: uranium itself does dissolve in nitric acid (Wikipedia, “Uranium”). (iv) Both series contract across; NCERT describes a greater actinoid contraction, though individual numerical comparisons require matched oxidation states and radius conventions (page 112).
Q5. Uranium forms UO22+, the uranyl ion. Find the oxidation state of uranium in it, and say whether this is uranium’s highest state in NCERT Table 4.11.
x + 2(−2) = +2, so x = +6, which is uranium’s highest state in NCERT Table 4.11 (page 112): all six of its outer electrons, 5f3 6d1 7s2, are used (reasoned).
Q6. Why does NCERT say the ionisation enthalpies of the early actinoids are lower than those of the early lanthanoids?
When the 5f orbitals begin to be occupied they penetrate less into the inner core of electrons, so the 5f electrons are more effectively shielded from the nuclear charge than the 4f electrons of the corresponding lanthanoids; the outer electrons are less firmly held and are available for bonding (NCERT page 113). Give that in an exam. Strictly, the measured first ionisation energies of thorium and uranium are higher than those of cerium and neodymium (NIST Atomic Spectra Database; see the slip on the page).
Q7. Why is the study of the later actinoids so difficult?
NCERT’s reason (page 112): the later actinoids have half-lives from a day down to 3 minutes for lawrencium and could be prepared only in nanogram quantities (page 112). Strictly, a half-life belongs to a particular isotope, and the amount made depends on the production route (reasoned).