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Isotopes
Objectives: To learn about the terms isotope, atomic number,
and mass number.
To understand the use of the symbol ZAX to describe a
given atom.
C H E M I S T RY
All atoms of same element have
the same number of protons
(the element’s atomic number)
and the same number of
electrons.
In a free atom, the positive and
negative charges always balance to yield a net zero charge.
Atomic number: the number of
protons.
Mass number: the sum of
protons and neutrons.
W
e have seen that an atom has a nucleus with a positive charge due
to its protons and has electrons in the space surrounding the nucleus at relatively large distances from it.
As an example, consider a sodium atom, which has 11 protons in its
nucleus. Because an atom has no overall charge, the number of electrons
must equal the number of protons. Therefore, a sodium atom has 11 electrons in the space around its nucleus. It is always true that a sodium atom
has 11 protons and 11 electrons. However, each sodium atom also has neutrons in its nucleus, and different types of sodium atoms exist that have
different numbers of neutrons.
When Dalton stated his atomic theory in the early 1800s, he assumed
all of the atoms of a given element were identical. This idea persisted for
over 100 years, until James Chadwick discovered that the nuclei of most
atoms contain neutrons as well as protons. (This is a good example of how
a theory changes as new observations are made.) After the discovery of the
neutron, Dalton’s statement that all atoms of a given element are identical
had to be changed to “All atoms of the same element contain the same
number of protons and electrons, but atoms of a given element may have
different numbers of neutrons.”
To illustrate this idea, consider the sodium atoms depicted in Figure
3.10. These atoms are isotopes, or atoms with the same number of protons
but different numbers of neutrons. The number of protons in a nucleus is
called the atom’s atomic number. The sum of the number of neutrons
and the number of protons in a given nucleus is called the atom’s mass
number. To specify which of the isotopes of an element we are talking
Nucleus
Nucleus
11 protons
12 neutrons
11 protons
13 neutrons
11 electrons
23
11 Na
11 electrons
24
11 Na
Figure 3.10
Two isotopes of sodium. Both have 11 protons and 11 electrons, but they differ in the
number of neutrons in their nuclei.
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Chapter 3
Chemical Foundations: Elements, Atoms, and Ions
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about, we use the symbol
A
ZX
where
X the symbol of the element
A the mass number (sum of protons and neutrons)
C H E M I S T RY
Isotopes are “top heavy”; that
is, the mass number is on the
top and the atomic number is
on the bottom. For most isotopes, the value of A is greater
than the value of Z.
Z the atomic number (number of protons)
For example, the symbol for one particular type of sodium atom is written
Mass number
(sum of protons and neutrons)
23
11 Na
Element symbol
Atomic number
(number of protons)
The particular atom represented here is called sodium-23, because it has a
mass number of 23. Let’s specify the number of each type of subatomic particle. From the atomic number 11 we know that the nucleus contains 11
protons. And because the number of electrons is equal to the number of
protons, we know that this atom contains 11 electrons. How many neutrons
are present? We can calculate the number of neutrons from the definition of
the mass number
Mass number number of protons number of neutrons
or, in symbols,
A Z number of neutrons
We can isolate (solve for) the number of neutrons by subtracting Z from both
sides of the equation
A Z Z Z number of neutrons
A Z number of neutrons
This is a general result. You can always determine the number of neutrons
present in a given atom by subtracting the atomic number from the mass
number. In this case (23
11Na), we know that A 23 and Z 11. Thus
A Z 23 11 12 number of neutrons
In summary, sodium-23 has 11 electrons, 11 protons, and 12 neutrons.
Example 3.2
Interpreting Symbols for Isotopes
In nature, elements are usually found as a mixture of isotopes. The three
isotopes of elemental carbon are 126C (carbon-12), 136C (carbon-13), and 146C
(carbon-14). Determine the number of each of the three types of subatomic
particles in each of these carbon atoms.
(conti nued)
3.7
Isotopes
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Solution
(continued)
The number of protons and electrons is the same in each of the isotopes
and is given by the atomic number of carbon, 6. The number of neutrons
can be determined by subtracting the atomic number (Z) from the mass
number:
A Z number of neutrons
The numbers of neutrons in the three isotopes of carbon are
12
6C:
number of neutrons A Z 12 6 6
13
6C:
number of neutrons 13 6 7
14
6C:
number of neutrons 14 6 8
In summary,
Number of
Protons
Number of
Electrons
Number of
Neutrons
12
6C
6
6
6
13
6C
6
6
7
14
6C
6
6
8
Symbol
Self-Check Exercise 3.2
Give the number of protons, neutrons, and electrons in the atom symbolized by 90
38Sr. Strontium-90 occurs in fallout from nuclear testing. It can
accumulate in bone marrow and may cause leukemia and bone cancer.
Self-Check Exercise 3.3
Give the number of protons, neutrons, and electrons in the atom symbolized by 201
80Hg.
Example 3.3
Writing Symbols for Isotopes
Write the symbol for the magnesium atom (atomic number 12) with a mass
number of 24. How many electrons and how many neutrons does this
atom have?
Solution
The atomic number 12 means the atom has 12 protons. The element magnesium is symbolized by Mg. The atom is represented as
24
12Mg
Magnesium burns in air to give
a bright white flame.
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Chapter 3
and is called magnesium-24. Because the atom has 12 protons, it must also
have 12 electrons. The mass number gives the total number of protons and
neutrons, which means that this atom has 12 neutrons (24 12 12).
Chemical Foundations: Elements, Atoms, and Ions