Which of the following set of elements do not possess $f$-electrons?

Which of the following set of elements do not possess $f$-electrons?
  1. La, U, Lr
  2. La, Th, Lr
  3. La, Ac, Th
  4. Ce, Ac, Th

Solution

Electronic configurations of (i) Lanthanum (La) with $(Z=57)$ is $ [\mathrm{Xe}]_{54} \cdot 5 d^1 \cdot 6 s^2 $ (ii) Uranium (U) with $(Z=92)$ is $ [\mathrm{Rn}]_{86} \cdot 5 f^3 \cdot 6 d^1 \cdot 7 s^2 $ (iii) Lawrencium ( $\operatorname{Lr})$ with $(Z=103)$ is $ [\mathrm{Rn}]_{86} \cdot 5 f^{14} \cdot 6 d^1 \cdot 7 s^2 $ (iv)Thorium (Th) with $(Z=90)$ is $ [\mathrm{Rn}]_{86} \cdot 5 f^1 \cdot 6 d^1 \cdot 7 s^2 \equiv[\mathrm{Rn}]_{86} 6 d^2 \cdot 7 s^2 $ (due to nearly same energy of $5 f$ and $6 d$-orbitals). (v) Actinium (Ac) with $(Z=89)$ is $ [\mathrm{Rn}]_{86} \cdot 6 d^1 \cdot 7 s^2 $ (VI) Cerium (Ce) with $(Z=58)$ is $ [\mathrm{Xe}]_{54} \cdot 4 f^1 \cdot 5 d^1 \cdot 6 s^2 $ On the basis of above given electronic configuration, elements La, Ac, and Th do not possess $f$-electrons. Hence, option (c) is the correct answer

Asked in: AP EAMCET 2019 (21 Apr Shift 1)

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