1. Consider the He atom. It has 2 electrons, each with its own spin,
and . Adding spin angular momenta means adding vectors.
With this in mind, what are the total spin quantum numbers of
this system? Remember that spin is a form of angular momentum
(A) S = 0 or 1; mS = -1, 0, +1
(B) S = -1, 0 or 1; mS = -1, 0, +1
(C) S = 0; mS = 0
(D) S = 1; mS = -1, 0, +1
(E) S = 1; mS = 0, 1 z z z

2. OR Consider the He atom. It has 2 electrons, each with its own spin,
and . Adding spin angular momenta means adding vectors.
With this in mind, what are the total spin quantum numbers of
this system? Remember that spin is a form of angular momentum
(A) S = 0 or 1; mS = -1, 0, +1
(B) S = -1, 0 or 1; mS = -1, 0, +1
Wrong, because S ≥ 0 !
(C) S = 0; mS = 0
Wrong, because the vectors can add
(D) S = 1; mS = -1, 0, +1
Wrong, because the vectors can cancel
(E) S = 1; mS = 0, 1
Wrong, because the vectors can add, and because m goes from –S to S z z z
1 ħ
0.5 ħ =
0.5 ħ
0.5 ħ OR z z z
0.5 ħ =
-0.5 ħ
vectors cancel!

3. As a general rule, we cannot distinguish one particular electron
from another (can’t fix labels on them). If electrons are
indistinguishable, which of the spin configurations is incorrect?
(A) Spin(1,2) = (1)(2)
(B) Spin(1,2) = (1)(2)
(C) Spin(1,2) = (1)(2)
(D) Spin(1,2) = (1)(2) + (1)(2)
(E) Spin(1,2) = (1)(2) - (1)(2)

4. As a general rule, we cannot distinguish one particular electron
from another (can’t fix labels on them). If electrons are
indistinguishable, which of the spin configurations is invalid?
(A) Spin(1,2) = (1)(2)
(B) Spin(1,2) = (1)(2)
(C) Spin(1,2) = (1)(2) we would have to know which one
points up and which points down
(D) Spin(1,2) = (1)(2) + (1)(2)
(E) Spin(1,2) = (1)(2) - (1)(2)