Chemistry Worksheet Template Page 2
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4Isotopes
Isotopes are two or more atoms of the same element with the same number of protons but a
different number of neutrons. The existence of isotopes proves that another part of Dalton's atomic
theory is incorrect. Dalton wrote that atoms of the same element have the same physical and chemical
properties. Although isotopes have the same chemical properties, they do not have the same physical
properties.
Isotopes are written in two different ways. They can be written using their symbol with the mass number
(to the upper left) and atomic number (to the lower left) or the isotope name is written with a dash and the
mass number.
For example: Two naturally occurring isotopes of chlorine are chlorine-35 & chlorine-37. Thirty-five and
thirty-seven are the mass numbers for the two isotopes. Both isotopes have the same atomic number,
number of protons and electrons.
isotope name
atomic number
# protons
# neutrons
mass number
# electrons
chlorine-35
17
17
18
35
17
chlorine-37
17
17
20
37
17
Atomic Weight
The atomic weight of an element is the weighted average of the masses of the isotopes of that
element. The weighted average is determined using the abundance and mass of each isotope. Most
elements have more than one naturally occurring isotope.
For example, there are two naturally occurring isotopes of copper, copper-63 and copper-65. The natural
abundances of the isotopes are 69.2% and 30.8% respectively.
To determine the atomic weight:
Step 1: Multiply the mass number and the relative abundance (as a decimal). The mass of the electron is
insignificant in this calculation and is not used.
isotope name
mass number (amu)
x
abundance (as a decimal)
=
result
copper-63
63 amu
x
0.692
=
43.596
copper-65
65 amu
x
0.308
=
20.020
Step 2: Add up your results.
Atomic Weight =
63.616
On your periodic table you can see that the atomic weight is 63.546. The reason for the difference
between the actual atomic weight (as seen on the periodic table) and the calculated atomic weight is due
to the fact that the masses of the proton and neutron is not exactly 1 amu. The mass of a proton is approx
1.008 amu. The mass of a neutron is approximately 1.009 amu
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