Isotopes
Isotopes are a version of the same element but they have a different mass number. The mass
number tells us the number of protons added to the number of neutrons. As the atomic number
is the same, it must be the number of neutrons that is different. For example there are two
common isotopes for Chlorine:
35
17𝐶𝑙
37
17𝐶𝑙
We take an average of the number numbers to come up with the mass number which is in the
periodic table of 35.5.
Task 1
Use this information to write a simple definition for an isotope.
......................................................................................................................
......................................................................................................................
Task 2
Calculate the number of neutrons for each of the isotopes below. Remember we do this by
subtracting the atomic number from the mass number:
Isotope
mass number
atomic number
chlorine 35
35
17
chlorine 37
37
17
bromine 79
79
35
bromine 81
81
35
number of neutrons
Task 3
Look at the table below and use your periodic table to complete the missing numbers. Find
another example of an element that has an isotope and fill in the last two rows.
atomic
number
mass
number
1
1
1
2
1
2
H
H
12
C
12
number of
protons
number of
neutrons
C
7
Na
12
23
Na
O
O
© www.teachitscience.co.uk 2016
11
24
16
18
electronic
structure
6
13
24
number of
electrons
2,6
18
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Isotopes
Answers
Task 2
Isotope
mass number
atomic number
number of neutrons
chlorine 35
35
17
18
chlorine 37
37
17
20
bromine 79
79
35
44
bromine 81
81
35
46
Task 3
atomic
number
mass
number
number of
protons
number of
neutrons
number of
electrons
electronic
structure
1H
1
1
1
0
1
1
2
1
2
1
1
1
1
12
6
12
6
6
6
2,4
13
C
6
13
6
7
6
2,4
23
Na
11
23
11
12
11
2,8,1
24
11
24
11
13
11
2,8,1
16
8
16
8
8
8
2,6
18
8
18
8
10
8
2,6
H
C
Na
O
O
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25799
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