PCAT Chemical Processes – Gold Electrons
 by
 Jan 03, 2018
 PCAT Question of the Day
How many electrons does gold (Au) possess that have a quantum number l = 2?
A) 10
B) 23
C) 30
D) 79
Click for Explanation
You must know that the quantum number l can take on four different values: 0, 1, 2, and 3. Furthermore, you must know that each number corresponds to a particular orbital: 0=s, 1=p, 2=d, 3=f. So the question is asking us how many d electrons gold possesses.
Since gold is in the sixth period, it has the following d orbitals: 3d, 4d, and 5d. The 3d and 4d orbitals are filled with 10 electrons each, and the 5d orbital has 10 electrons, giving a total of 30. Therefore the correct answer is C, 30.
(If youâ€™re wondering why gold has 10 electrons in its 5d orbital instead of 9, the answer is related to the high stability of a fullyfilled d subshell. From the periodic table alone, you would think that gold had 9 5d electrons, but it becomes significantly more stable if one 6s electron moves up to fill the 5d subshell. This explains why the 5d subshell holds 10 electrons!)
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