Which quantum number differs for the two electrons present in K-shell of an atom? — Atomic Structure Chemistry Question

💡 Solution & Explanation

The K-shell corresponds to $n=1$, which has only the 1s orbital ($l=0, m=0$). The two electrons in this orbital must have opposite spins ($+1/2$ and $-1/2$) according to Pauli's exclusion principle, so they differ in their spin quantum number. Therefore, correct answer is D.