Which of the following best explains why the 4s orbital is filled before the 3d orbitals in ground-state electron configurations for the first‐row transition elements?
a) The principal quantum number of 4s is higher, so it must always be filled before any subshell with a lower principal quantum number.
b) The 4s subshell is filled first because its lower angular-momentum quantum number (ℓ=0) places it at a lower energy than any subshell with ℓ>0 in the same principal level.
c) Electrons in 4s experience a lower energy than those in 3d primarily because the 4s orbital penetrates closer to the nucleus, resulting in a greater effective nuclear charge.
d) Hund’s rule forbids pairing of electrons in the 3d orbitals until the 4s subshell is completely filled.