EXAMPLE 3.1
How many neutrons are there in the
Number of Neutrons
235
U
92
nucleus?
Solution
Simply subtract the atomic number Z (number of protons) from the nucleon
number A (number of protons plus neutrons).
A – Z = number of neutrons
235 – 92 = 143
There are 143 neutrons in the nucleus.
Exercise 3.1A
How many neutrons are there in the 131
53 I nucleus?
Exercise 3.1B
A potassium isotope has 21 neutrons in its nucleus. What is the nucleon number and name of the isotope?
EXAMPLE 3.2
Number of Neutrons
How many neutrons are there in the strontium-90 nucleus?
Solution
From the periodic table on the inside front cover, we find the atomic number of strontium is 38. The nucleon
number is given as 90. The number of neutrons is therefore
90 – 38 = 52
Exercise 3.2A
How many neutrons are there in the molybdenum-90 nucleus?
Exercise 3.2B
Use the AZ X notation to represent the isotope of uranium having 148 neutrons.
CONCEPTUAL EXAMPLE 3.3
Isotopes
Refer to the following isotope symbols, in which we use the letter X as the symbol for all elements so that the
symbol will not identify the elements. (a) Which are isotopes of the same element? (b) Which have the same
nucleon number? (c) Which have the same number of neutrons?
16
8X
16
7X
14
7X
14
6X
12
6X
Solution
a.
Isotopes of the same element will have the same atomic number (subscript). Therefore, 167 X and 147 X
are isotopes of nitrogen (N), and
b.
14
6X
and
12
6X
are isotopes of carbon (C).
The nucleon number is the superscript, so 168 X and
16 X
7
have the same nucleon number. The first is an
isotope of oxygen, and the second an isotope of nitrogen. 147 X and
14
6
X also have the same nucleon
number. The first is an isotope of nitrogen, and the second an isotope of carbon.
c.
To determine the number of neutrons, we subtract the atomic number from the nucleon number. We
find that
16
8X
and
14 X
6
each have eight neutrons (16 – 8 = 8 and 14 – 6 = 8 respectively).
Exercise 3.3A
Which of the following are isotopes of the same element?
90
37 X
90
35 X
88
37 X
88
38 X
Exercise 3.3B
How many different elements are represented in Exercise 3.3A?
93
38 X
EXAMPLE 3.4
Electron Shell Capacity
What is the maximum number of electrons in the fifth shell (fifth energy level)?
Solution
The maximum number of electrons in a given shell is given by 2n2. For the fifth level, n = 5, and so we have
2 x 52 = 2 x 25 = 50
Exercise 3.4A
What is the maximum number of electrons in the fourth shell (fourth energy level).
Exercise 3.4B
What is the lowest shell (value of n) that can hold 18 electrons?
EXAMPLE 3.5
Main-Shell Electron Configurations
What are the main-shell electron configurations for (a) fluorine and (b) aluminum?
Solution
a.
Fluorine has the symbol F (Z = 9) it has nine electrons. Two of these electrons go into the first shell,
and the remaining seven go into the second level.
F 2 7
b.
Aluminum (Z = 13) has 13 electrons. Two go into the first shell, eight go into the second, and the
remaining three go into the third shell.
Al 2 8 3
Notice that fluorine is in group 7A of the periodic table and that it has seven outer-shell electrons.
Aluminum is in group 3A, and it has three electrons in its outermost shell.
Exercise 3.5
Give the main-shell electron configurations for (a) beryllium and (b) magnesium. These elements are in the
same group of the periodic table. What do you notice about the number of electrons in their outermost shells?
EXAMPLE 3.6
Subshell Notation
Without referring to Table 3.3, use subshell notation to write out the electron configuration for (a) oxygen and
(b) sulfur. What similarity of features do the electron configurations exhibit?
Solution
a.
b.
Oxygen (Z = 8) has eight elections. Place them in the lowest unfilled energy sublevels. Two go into
the 1s orbital and two into the 2s orbital. That leaves four electrons to be placed in the 2p subshell.
The electron configuration is 1s22s22p4.
Sulfur atoms have 16 electrons each. The electron configuration is 1s22s22p63s23p4. Note that the total
of the superscripts is 16 and that we have not exceeded the maximum capacity for any sublevel.
Both O and S have electron configurations with four electrons in their highest energy sublevel
(outermost subshell).
Exercise 3.6A
Without referring to Table 3.3, use subshell notation to write out the electron configurations for (a) fluorine
and (b) chlorine. What similarity of features do the electron configurations exhibit?
Exercise 3.6B
Use Figure 3.16 to write the electron configurations for (a) titanium (Ti) and (b) gallium (Ga).
EXAMPLE 3.7
Valence Shell Electron Configurations
Write out the subshell notation for the electrons in the highest main shell for (a) strontium (Sr) and (b) arsenic (As).
Solution
a.
b.
Strontium is in group 2A and thus has two valence electrons in an s subshell. Because strontium is in
the fifth period of the periodic chart, its outer main shell is n = 5. Its outer electron configuration is
therefore 5s2.
Arsenic is in group 5A and the fourth period. Its five outer (valence) electrons are in the n = 4 shell
and have the configuration 4s24p3.
Exercise 3.7A
Write out the subshell notation for the electrons in the highest main shell for (a) rubidium (Rb), (b) selenium
(Se), and (c) germanium (Ge).
Exercise 3.7B
The valence electrons in aluminum have the configuration 3s23p1. Gallium is directly below aluminum, and
indium is directly below gallium in the periodic table. Use only this information to write the configuration for
the valence electrons in (a) gallium and (b) indium.