The electronic configuration 1s² 2s² 2p⁶ 2d¹ is impossible because:

1. The subshells available in a shell are limited by its principal quantum number $n$. 2. For $n=2$ the allowed values of the azimuthal quantum number are 0 and 1, giving only s and p subshells. 3. A d subshell needs an azimuthal quantum number of 2, which first becomes available at $n=3$. 4. So 2d cannot exist, and any configuration naming it is impossible. 5. The 2p subshell holding six electrons is entirely correct, since three orbitals take two electrons each. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 4: The Atom_