Which of the following elements have half-filled f-orbitals in their ground state?
A. Sm, B. Eu, C. Tb, D. Gd, E. Pm
(Given: atomic number Sm = 62; Eu = 63; Tb = 65; Gd = 64, Pm = 61)
Choose the correct answer from the options given below:

Half-filled f-orbitals means the 4f subshell contains exactly 7 electrons (f⁷).

Let us check each element's electronic configuration:

A. Sm (Z = 62): [Xe] 4f⁶ 6s² — 6 f-electrons, NOT half-filled

B. Eu (Z = 63): [Xe] 4f⁷ 6s² — 7 f-electrons, half-filled ✔

C. Tb (Z = 65): [Xe] 4f⁹ 6s² — 9 f-electrons, NOT half-filled

D. Gd (Z = 64): [Xe] 4f⁷ 5d¹ 6s² — 7 f-electrons, half-filled ✔

E. Pm (Z = 61): [Xe] 4f⁵ 6s² — 5 f-electrons, NOT half-filled

Elements with half-filled f-orbitals: B (Eu) and D (Gd).