1. Calculating Average Mass
Atomic mass is the average of all the naturally isotopes of that element.
Copper = amu
Isotope
Symbol
Composition of the nucleus
Mass in
(amu)
Relative Abundance
(% in nature)
Copper-63 63 29
29 protons
34 neutrons
62.93
69.2 %
Copper-65 65
36 neutrons
64.93
30.8 % Cu Cu x
Mass (amu)
Relative abundance
(% as a decimal)
Copper-63
62.93 amu x
0.692
= amu +
Copper-65
64.93 amu x
0.308
= amu
amu
Periodic Table of Elements

2. Your Turn (Calculate Average Mass)
Isotope
Symbol
Atomic #
neutrons
electrons
Mass Number
(p+ + n0)
Isotope Mass in (amu)
Relative abundance
Carbon-12 6C 6
= 6
6 -
6 + 6 = 12
12.000
98.89%
Carbon-13
13 – 6 = 7
6 + 7 = 13
13.003
1.11% 12 13 x
Mass (amu)
Relative abundance
(% as a decimal)
Carbon-12
amu x
0.9889
= amu +
Carbon-13
amu x
0.0111
= amu
amu
12.01 amu
Atomic # C
Carbon 6
12.011
Atomic mass
(average)

3. Your Turn (Calculate Average Mass)
Isotope
Symbol
Atomic #
neutrons
electrons
Mass Number
(p+ + n0)
Isotope Mass in (amu)
Relative abundance
Nitrogen-14 N
14.003
99.63%
Nitrogen-15
15.000
0.37%
Chlorine-35 Cl
34.969
75.77%
Chlorine-37
36.966
24.23% 14 7 7- 14
14-7=7 7 15 7 7- 15
15-7=8 7 35 17
17- 35
35-17=18 17 37 17
17- 37
37-17=20 17 x
Mass (amu)
Relative abundance
(% as a decimal)
Nitrogen-14
amu x
0.9963
= amu +
Nitrogen-15
amu x
0.0037
= amu
amu N
Nitrogen 7
14.007
amu

4. Your Turn (Calculate Average Mass)
Isotope
Symbol
Atomic #
neutrons
electrons
Mass Number
(p+ + n0)
Isotope Mass in (amu)
Relative abundance
Nitrogen-14 N
14.003
99.63%
Nitrogen-15
15.000
0.37%
Chlorine-35 Cl
34.969
75.77%
Chlorine-37
36.966
24.23% 14 7 7- 14
14-7=7 7 15 7 7- 15
15-7=8 7 35 17
17- 35
35-17=18 17 37 17
17- 37
37-17=20 17 x
Mass (amu)
Relative abundance
(% as a decimal)
Chlorine-35
amu x
0.7577
= amu +
Chlorine-37
amu x
0.2423
= amu
amu Cl
Chlorine 17
35.453
amu

5. Pb Calculate Average Mass Mass # = p+ + n0 1.37 100 + 204 206 207 208
Lead 82
207.20
Calculate Average Mass
Mass # = p+ + n0
82p
122n
82p
124n
82p
125n
82p
126n
% abundance:
1.37%
26.26%
20.82%
51.55%
Mass #
204
206
207
208
1.37
100
No amu mass shown.
Use mass # instead. x
Mass (amu)
Relative abundance
(% as a decimal)
Lead-204
204 x
0.0137 =
Lead-206
206 x
0.2626 =
Lead-207
207 x
0.2082 = +
Lead-208
208 x
0.5155 =
207.21

6. Your Turn (Calculate Average Mass)
Isotope
Symbol
Atomic #
neutrons
electrons
Mass Number
(p+ + n0)
Isotope Mass in (amu)
Relative abundance
Helium-3
2He 2 1
2 - 3
3.0160
0.0001%
Helium-4 4
4.0026 % 3 4 x
Mass (amu)
Relative abundance
(% as a decimal)
Helium-3
amu x
= amu +
Helium-4
amu x
= amu
amu
amu
Atomic # He
Helium 2
4.0026
Atomic mass
(average)