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Isotopes
The chemical properties of an element depend on the number of protons and electrons
inside the atom. So if a neutron or two is added or removed from the nucleus, then the
chemical properties will not change. This means that such an atom would remain in the
same place in the periodic table. For example, no matter how many neutrons we add or
subtract from a nucleus with 6 protons, that element will always be called carbon and have
the element symbol C (see the periodic table). Atoms which have the same number of
protons (i.e. same atomic number Z), but a different number of neutrons (i.e. different N and
therefore different mass number A), are calledisotopes.
Interesting Fact:
In Greek, “same place” reads as ίσoςτόπoς (isos topos). This is why atoms which have the
same number of protons, but different numbers of neutrons, are called isotopes. They are in
the same place on the periodic table!
Definition 1: Isotope
Isotopes of an element have the same number of protons (same Z), but a different
number of neutrons (different N).
The chemical properties of the different isotopes of an element are the same, but they might
vary in how stable their nucleus is. We can also write elements as E–A where the E is the
element symbol and the A is the atomic mass of that element. For example Cl–35 has an
atomic mass of 35 u (17 protons and 18 neutrons), while Cl–37 has an atomic mass of 37
u (17 protons and 20 neutrons).
In nature the different isotopes occur in different percentages. For example Cl–35 might
make up 75% of all chlorine atoms on Earth, and Cl–37 makes up the remaining 25%. The
following worked example will show you how to calculate the average atomic mass for these
two isotopes:
Example 1: The relative atomic mass of an isotopic
element
Question
The element chlorine has two isotopes, chlorine–35 and chlorine–37. The abundance of
these isotopes when they occur naturally is 75% chlorine–35 and 25% chlorine–37.
Calculate the average relative atomic mass for chlorine.
Answer
Calculate the mass contribution of chlorine–35 to the average
relative atomic mass
75% of the chlorine atoms has a mass of of 35 u.
Contribution of Cl–35 = (75100×35) = 26,25 u.
Calculate the contribution of chlorine–37 to the average relative
atomic mass
25% of the chlorine atoms has a mass of of 37 u.
Contribution of Cl–37 = (25100×37) = 9,25 u.
Add the two values to arrive at the average relative atomic mass of
chlorine
Relative atomic mass of chlorine = 26,25 u + 9,25 u = 35,5 u.
If you look on the periodic table (see front of book), the average relative atomic mass for
chlorine is 35,5 u.
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Isotopes and atomic mass
Exercise 1: Isotopes
Problem 1:
Atom A has 5 protons and 5 neutrons, and atom B has 6 protons and 5 neutrons. These
atoms are:
1. allotropes
2. isotopes
3. isomers
4. atoms of different elements
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Answer 1: d.
atoms of different elements
Problem 2:
For the sulphur isotopes, 3216S and 3416S, give the number of:
1. protons
2. nucleons
3. electrons
4. neutrons
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Answer 2:
a) The number of protons is the subscript number (Z) so: i) 16 ii) 16
b) The number of nucleons is the superscript number (A) so: i) 32 ii) 34
c) The number of electrons in a neutral atom is the same as the number of protons and so
for both i) and ii) the number of electrons is 16
d) The number of neutrons (N) is the number of nucleons - the number of protons or: A - Z =
N.
For i) A−Z=32−16=16 and for ii) A−Z=34−16=18.
Problem 3:
Which of the following are isotopes of 3517Cl?
1. 1735Cl
2. 3517Cl
3. 3717Cl
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Answer 3:
3. 3717Cl
Problem 4:
Which of the following are isotopes of U–235? (E represents an element symbol)
1. 23892E
2. 23890E
3. 23592E
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Answer 4:
To answer this question look at the definition of an isotope. An isotope must have the same
number of protons. The neutrons can be different. So we can rule out b) as that will not be
an isotope since the number of protons is different. c) is U-235 so it is not an isotope.
Therefore the isotope must be a).
Problem 5:
Complete the table below:
Isotope
Z
A
Protons
Neutrons
Electrons
Carbon–12
Carbon–14
Iron–54
Iron–56
Iron–57
Table 1
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Answer 5:
Carbon-12 means that A is 12. So we can easily fill in the A table. Z is the elements position
on the periodic table, so for carbon it is 6 and for iron it is 26. The number of protons = the
number of electrons = Z. And the number of neutrons = Z – A.
Isotope
Z
A
Protons
Neutrons
Electrons
Carbon-12
6
12
6
6
6
Carbon-14
6
14
6
8
6
Iron-54
26
54
26
28
26
Iron-56
26
56
26
30
26
Iron-57
26
57
26
31
26
Problem 6:
If a sample contains 19,9% boron–10 and 80,1% boron–11, calculate the relative atomic
mass of an atom of boron in that sample.
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Answer 6:
19,9100×{10}=1,99u is the contribution to the relative atomic mass from boron-10.
And the contribution from carbon-14 is: 80,1100×11=8,811u.
Now we add these two together and get the relative atomic mass: 1,99u+8,811u=10.8u.
Problem 7:
If a sample contains 79% Mg–24, 10% Mg–25 and 11% Mg–26, calculate the relative
atomic mass of an atom of magnesium in that sample.
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Answer 7:
79100×24=18,96u is the contribution to the relative atomic mass from Mg-24.
The contribution from Mg-25 is: 10100×25=2,5u
And the contribution from Mg-26 is: 11100×26=2,86u.
Now we add these two together and get the relative atomic
mass: 18.96u+2.5u+2.86=24.3u.
Problem 8:
For the element 23492U (uranium), use standard notation to describe:
1. the isotope with 2 fewer neutrons
2. the isotope with 4 more neutrons
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Answer 8:
1. 23292U
2. 23092U
Problem 9:
Which of the following are isotopes of 4020Ca?
1. 4019K
2. 4220Ca
3. 4018Ar
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Answer 9:
2. 4220Ca
Problem 10:
For the sulphur isotope 3316S, give the number of:
1. protons
2. nucleons
3. electrons
4. neutrons
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Answer 10:
a. 16
b. 35
c. 16
d. 19