Section: Chemical and Physical Foundations of Biological Systems

23) How many electrons does gold (Au) possess that have a quantum number l = 2?

1. 9
2. 23
3. 29
4. 79

You must know that the quantum number l can take on four different values: 0, 1, 2, 3. Further, 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 it is in the sixth period, it has the following d orbitals: 3d, 4d, 5d. The 3d and 4d orbitals are filled with 10 electrons each, and the 5d orbital has 9 electrons, giving a total of 29. Therefore the correct answer is C, 29.