【Daily Question】Theoretical Foundation 2020.06.20
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Multiple-choice question: The valence electron configuration of cobalt is 3d74s2. The number of unpaired electrons in the outer orbitals of a cobalt atom is ( ). A. 1 B. 2 C. 3 D. 4 Answer: C Hint: Additional rewards will be given to those who can explain the solution process; the answer can be found in the response
This post was last edited by h20090630 on 2020-6-20 at 15:12. Analysis: 7 electrons fill the 5 degenerate d orbitals; according to Hund’s rule, there are 3 unpaired electrons.
C. Seven electrons fill the 5 degenerate d orbitals, and according to Hund’s rule, there are 3 unpaired electrons. Hund’s rule states that electrons occupying equivalent orbitals (i.e., those in the same electron shell or subshell) will occupy different orbitals as much as possible, with their spins aligned in the same direction. Later, quantum mechanics proved that such an arrangement of electrons can result in the lowest energy, so Hund’s rule can also be included within the principle of minimum energy. Hund’s rule is also known as the rule of equivalent orbitals. Electrons arranged in the same electron subshell always occupy different orbitals first, with their spin directions being the same. For example, the 3 p electrons in a nitrogen atom are distributed across 3 p orbitals and have the same spin direction. A half-filled stable structure is achieved when there are 3 electrons in the p orbitals, 5 electrons in the d orbitals, and 7 electrons in the f orbitals. In this problem, the 3d orbitals first accommodate 5 electrons in a single row, then 2 electrons fill each of the remaining 2 orbitals, leaving 3 unpaired electrons.
Explanation for answer C: 7 electrons fill the 5 degenerate d orbitals; according to Hund’s rule, there are 3 unpaired electrons.