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Chapter 7. Atomic Physics
Notes:
- Most of the material in this chapter is taken from Thornton and Rex, Chapter 8.
7.1 The Pauli Exclusion Principle
We saw in the previous chapter that the hydrogen atom could be precisely understood by
considering the Schrödinger equation, including the electrostatic potential energy that
accounts for the interaction between the nucleus and the electron. We then discovered
that the family of stationary states, which are the solutions of the Schrödinger equation
can be completely characterized by three quantum numbers (^) n , , and m. However, we
also found that the state of the electron is further characterized by its intrinsic spin, which
acts in a way akin to the angular momentum in that it provides an extra magnetic moment
and is quantized similarly. But comparisons between the
S and
L operators should
probably not be pushed too far, as for the electron the spin is half-integer with s = 1 2
independent of any other parameter, while 0 ≤ ≤ n − 1 and always an integer. As a
result, the states of the hydrogen can only be fully specified by combining the previous
three quantum numbers for the solutions of the Schrödinger equation and the magnetic
spin quantum number (^) m s
, forming the (^) n ,, m , m s
( ) foursome.^ As we will^ now^ discover
there is another very important aspect of the spin that is absolutely essential for
understanding the structure of many-electron atoms.
Although it would be in principle possible to solve the Schrödinger equation for more
complicated atoms, the presence of several interaction terms between the different
electrons makes the problem analytically intractable and basically impossible to solve.
The exact determination of the stationary states, their energy, angular momentum, etc.
must then be accomplished using computers. It is, however, possible to qualitatively
understand the structure of many-electron atoms using the results obtain for the study of
the hydrogen atom and another fundamental principle that we owe to the Austrian
physicist Wolfgang Pauli (1900-1958).
The so-called Pauli exclusion principle , which stems from Pauli’s efforts to explain the
structure of the periodic table, can be stated as follows
Two identical fermions cannot occupy the same state.
Remember that fermions have half-integer spins; the electron with s = 1 2 is therefore
one. As Pauli initial enunciation of his principle in 1925 happened as he was studying the
atomic structure, it can then stated more specifically for atomic electrons with
No two electrons in an atom can share the same set of quantum numbers n ,, m , m s
( ).
The structure of atoms and that of the periodic table can be explained with this principle
and the further assumption that atomic electrons tend to occupy the lowest available
energy states.
To see how this works, let us consider the next simplest atom after hydrogen, i.e., helium.
The helium atom (He) is composed of a nucleus made of two protons and two neutrons
for a total charge of + 2 e (a neutron has the same mass as a proton but no charge) and
two electrons. As was the case for the hydrogen atom we can expect that the lowest
energy state for an electron will consists of a 1s wave function with the electron spin
quantum number either 1 2 or − 1 2 , i.e., n ,, m , m s
( ) =^ (^1 ,^0 ,^0 ,^ ±^1 2 ).^ The second
electron is also likely to occupy a similar state, but because of the Pauli exclusion
principle, which forbids two electrons from occupying the exact same state, the quantum
numbers can only be n ,, m , m s
( ) =^ (^1 ,^0 ,^0 ,^ ^1 2 ) ;^ note^ the opposite sign^ of^ m s
for the two
electrons. We are thus left with the picture of the helium atom having its two electrons in
1s states, where one electron has its spin “up” and the other its spin “down.” The
complete ground state of the helium atom is then denoted by 1 s
2
, where the ending
superscript specifies the number of electrons in the given state (in this case, 1 s).
What would be the electronic structure for the next simplest atom, i.e., Lithium (Li),
which contains three electrons (and a nucleus made of three protons and four neutron)? It
should now be clear that the Pauli exclusion principle will forbid the lowest atomic state
to be something like 1 s
3
, since this would imply that two electrons would have to share
the same m s
number (i.e., the ending superscript cannot be greater than two for a n s
orbital). To minimize energy two electrons will still occupy the inner shell 1 s
2
, while the
third one will reside on the next unoccupied orbital with favourable energy. In this case
the next available lowest-energy electron state is 2s. The lithium atom ground state is
therefore 1 s
2
2 s
1
. One would be justified to ask why couldn’t the ground configuration be
1 s
2
2 p
1
instead? Indeed, this appears to be supported by the fact that our solution for the
hydrogen atom asserted that s and p orbitals for a given n^ number have the same energy
E
n
. The answer lies with the consideration of the precise shapes of the different orbitals.
An s orbital is said to be more penetrating than a p orbital. That is, the radial wave
function for = 0 is more concentrated closer to the nucleus than that for = 1. This
implies that an electron on a 2p orbital in the lithium atom is more likely to have the
nuclear charge of + 3 e screened by the two electrons on the 1s orbitals and “feel” an
effective charge of + e. On the other hand, an electron on the more penetrating 2s
orbital is not as screened and will see more of the nuclear charge (i.e., the effective
charge is greater than + e ), which results in a lower potential energy due to its stronger
interaction with the nucleus. The 1 s
2
2 s
1
state is thus of lower energy than the 1 s
2
2 p
1
state and the correct choice for the ground state of lithium.
Just as the orbitals are designated by letters depending on the values of the quantum
number, e.g., s, p, d, f, etc., shells are associated to the different values of the principal
quantum number n. More precisely, levels of n = 1 , 2 , 3 , 4 , … are given the capital
letters K, L, M, N, … The aforementioned n ^ orbitals are then called subshells. It
Exercises
- Use Table 8.1 to determine the ground state configuration of potassium (K, consisting
of 19 protons and 20 neutrons).
Solution.
Potassium has 19 electrons, which we must place in subshells of increasing energy. The
K and L shells are successively filled without interruption and will use up
( 2 ) + ( 2 + 6 ) = 10 electrons (for the 1 s
2
2
6
( ) subshells).^ Likewise,^ the next 8
electrons will occupy the 3 s
2
and 3p
6
subshells, but Table 8.1 tells us that final lone
electron does not go to a 3d but a 4s orbital. The ground state configuration for potassium
is therefore 1 s
2
2 s
2
2 p
6
3 s
2
3 p
6
4 s
1
.
7.2 The Periodic Table
With the understanding of the electronic structure of atoms we acquired it is now
relatively easy to understand the structure of the periodic table, which was first
introduced by the Russian chemist Dmitri Ivanovich Mendeleev (1834-1907) in 186 9
(well before the advent of quantum mechanics). Figure 1 shows the table, with the
electronic configuration specified for every element.
The periodic table is organized in groups , along the different columns, for elements
sharing the same chemical properties. A careful look of any given column will show that
the corresponding elements all have the same or similar subshell status. That is, they all
have the same number of electrons occupying a given ^ subshell, irrespective of the n
number. The rows are known as periods and correspond to the filling of subshells. As one
goes from the first element on the left to the last on the right of a period, one or more than
one subshell can be successively filled. Periods are characterized by the number of shells
(or energy levels) of electrons surrounding the nucleus. Here is a brief description of
some important groups.
Group 1 - Alkalis
This group, to which hydrogen belongs, are characterized by having one s electron in the
outer subshell. Since s electrons tend to extend relatively far from the nucleus and,
furthermore, can easily be stripped off the atom (thus forming a positive ion of charge
- 1 e ) these elements are highly reactive. Because of this tendency to give or share an
electron it is said that alkalis have a valence of + 1. This also results in them being good
electrical conductors. All elements in this group, except hydrogen, are refereed to as
alkali metals.
Group 2 - Alkaline Earths
Alkaline earths elements have their outer s subshell filled. Although we might assume
that they would then be more stable than alkalis, the fact that s orbitals are extended and
their electrons easily removed render them quite chemically reactive. They have a
valence of + 2 and are good electrical conductors.
Groups 3 to 12 – Transition Metals (or Transition Elements)
The three rows where the 3d, 4d, and 5d subshells are being filled (i.e., elements 21-30,
39 - 48, and 72-80) form the transition metals group. Several of these atoms (e.g., iron
(Fe), cobalt (Co), and nickel (Ni)) have strong magnetic moments due to the presence of
unpaired electrons in the d subshell. These electrons will see their spins aligned therefore
producing the ferromagnetic properties of the elements.
The rare earths elements consisting of the lanthanides (elements 58-71) and actinides
(elements 90-103) can also be included in this group. This is because these elements have
unpaired electrons in the f subshells (for n^ =^4 and n^ =^5 , respectively) leading also to
large magnetic moments.
Group 17 - Halogens
All elements of this group have five electrons in their outer p subshell, and therefore have
a valence of − 1. This characteristic renders them very chemically reactive; Fluorine (F)
is the most reactive element in existence. Halogens will especially bond efficiently with
alkalis, which have a valence of + 1 , to form compound such as NaCl.
Atoms in groups 13 to 16 are composed of metals (e.g., aluminum (Al), tin (Sn), and
bismuth (Bi)), non-metals (carbon (C), nitrogen (N), and oxygen (O)), and metalloids
(often semiconductors) exhibiting some properties of metals and non-metals (e.g., boron
(B), silicon (Si), Arsenic (As), and tellurium (Te)).
7.3 The Combination of Angular Momenta
We saw that transitions metals have high magnetic moments because of the effect of
unpaired electron’s spin. To understand how this comes about we must first understand
how angular momenta, orbital and intrinsic spin, combine or add up to form the total
angular momentum
J. We will consider the simple case for the combination of the spin
S and orbital angular momentum
L of a single electron. We first note that the two
momenta add vectorially
J =
L +
S. (^7.^1 )
We remember that both the orbital and spin angular momenta are quantized such that
L = ( + 1 ), L
z
= m
S = s ( s + 1 ), S
z
= m s
and m j
≤ j. From equations ( 7. 2 ) and ( 7. 3 ) we can write
J
z
= m
( ), ( 7. 4 )
or m j
= m
. It is important to realize that different values for m
, m s
, or m j
imply a
different orientation for the corresponding vectors. We therefore expect that there will be
several possibilities for both the orientation of the total angular momentum
J as well as
its magnitude J.
To get a better sense of this let us consider the case where = 1 , m
= − 1 , 0 , and 1 , and
s = 1 2 , m s
= − 1 2 and 1 2. Considering equation ( 7. 4 ) tells us that the following values
for m j
are realized
m j
m
= 1
m s
=±
1
2
m
= 0
m s
=±
1
2
m
=− 1
m s
=±
1
2
These values for the magnetic total quantum number can be grouped as follows to find
the realized values for j according to equation ( 7. 3 )
j =
, m j
j =
, m j
This result can be generalized to any pair of angular momenta of any kind (i.e., any
mixture of orbital, spin, or “intermediate” total angular momenta) with
J =
J
1
J
2
j 1
− j 2
≤ j ≤ j 1
m j
≤ j ,
where successive values for J differ by 1. It can easily be verified that equations ( 7. 6 )
are verified when
J
1
L
1
and
J
2
S
1
Let us now come back to the case of a transition metal atom and see if we can better
understand its high magnetic moment relative to elements of other groups. For example,
we consider the case of titanium (Ti), which has the [ Ar] 3 d
2
4 s
2
electronic configuration
( [ Ar] means that the inner core of titanium corresponds to the filled electronic
configuration of argon, which is (^1) s
2
2 s
2
2 p
6
3 s
2
3 p
6
). The d subshell of titanium is thus
incomplete with only two electrons. For reasons that we will not discuss here, the ground
state for such an atom with two unpaired electron on an outer subshell is realized when i)
the spin is maximized, ii) the orbital angular momentum is also maximized, and iii) the
total angular momentum is minimized, while always keeping in mind that the Pauli
exclusion principle must be verified. The total spin is then
S =
S
1
S
2
, which implies that
s = 0 or 1. Because of the first rule above we choose s = 1. Likewise, the total orbital
angular momentum is
L =
L
1
L
2
with 1
2
= 2 and therefore = 0 , 1 , 2 , 3 , and 4.
We may be tempted to choose = 4 in order to maximize the orbital momentum, but this
would go against the Pauli exclusion principle since it implies that m 1
= m 2
(remember that 1
2
and s 1
= s 2
). We must therefore settle for = 3 , which in turn
implies that the total angular momentum
J =
L +
S can span the values j = 2 , 3 , and 4.
Evidently,
J is minimized for j = 2 , which is a state possessing a significant magnetic
moment for m j
Finally, the state (or configuration) of an atom is expressed with the following notation
2 s + 1
L j
where 2 s + 1 is called the multiplicity of the state, and L is a capital letter used for the
orbital corresponding to . For example, the ground state of titanium we determined
earlier is defined with
3
F 2
Exercise
- Determined the notation for the ground state of carbon given that its configuration is
1 s
2
2 s
2
2 p
2
.
Solution.
We have two unpaired p electrons for which we maximize the spin with s = 1. The
possible values for the total orbital angular momentum are = 0 , 1 , and 2 , with the
maximum allowed by the Pauli exclusion principle of = 1. It follows that
j = 0 , 1 , and 2 , for a minimum of j = 0 and a ground state denoted by
3
P 0
