Relative atomic mass
• Atoms are too small to be weighed
individually, even with the best
balance!
• We can, however, compare the
masses of different atoms.
Relative atomic mass
• We give the lightest
atom (a hydrogen-1
atom) a mass of 1.
• All other atoms are
compared so many
times heavier than
this.
• We call this relative
atomic mass (RAM).
Relative atomic mass
• E.g. a helium atom
has a RAM of 4.
• This means a
helium atom is four
times heavier than
a hydrogen atom.
Relative atomic mass
• A magnesium atom has a RAM of 24.
How many times heavier is a
magnesium atom than a helium
atom?
• SIX times heavier! (He has a RAM of
4, 24/4 = 6)
Relative atomic mass
• Some elements have RAM that are
not whole numbers.
• This is because the elements exist as
a number of isotopes – each of which
has atoms of different mass.
• Tin has 10 different isotopes!
The RAM of chlorine
• The RAM of Cl is 35.5, how do we
explain this?
• Cl exists as two isotopes, Cl-35 & Cl37.
• In any sample of Cl, 75% of the
atoms are Cl- 35 and 25% are Cl-37.
The RAM of chlorine
• The average mass, taking into
consideration the different number of
atoms of the two isotopes, is:
• (75% x 35) + (25% x 37)
• = 35.5
Atomic Mass
• Silver is found to have two stable isotopes, one
has an atomic mass of 106.904 amu and the
other weighs 108.905 amu. The first isotope
represents 51.82 % of the mass of the element
and the second represents 48.18 %. What is
the atomic mass of the element silver?
The equation to use is %X + % Y = average
And remember to turn your percents into
fractions before multiplying.
(0.5182) 106.904 amu + (0.4818) 108.905 amu
=?
55.398 amu + 52.470 amu =?
107.868 amu !!
Now look at the periodic table to verify the
PRACTICE PROBLEMS
1A sample of neon contains three isotopes,
neon-20 (with an isotopic mass of 19.9924
amu), neon-21 (20.9939 amu) and neon-22
(21.9914 amu). The natural abundances of
these isotopes are 90.92%, 0.257 %, and 8.82
%. Calculate the atomic weight of neon.
2. There are only two naturally occuring
isotopes of copper, 63Cu and 65Cu. Calculate
the atomic mass of Cu given the ff.
mass(amu)
abundance(%)
Cu-63
62.930
69.09
Cu-65
64.928
30.91
Quiz
1. Carbon occurs naturally as two
isotopes. Calculate the atomic mass
of C, given the mass and natural
abundance of each isotopes.
2. Si the second most abundant
element in the earths’ crust is used
extensively in the electronic
industry. Calculate the atomic mass
of Si, gives the mass and
abundance of its three natural
isotopes
Isotope
Si-28
Si-29
Si-30
mass
27.977
28.976
29.974
abundance
92.21
4.70
3.09
• Zn occurs naturally as five isotopes.
Calculate the atomic mass for Zn,
given the ff. isotopes;
Isotopes
mass(amu) (%)
Zn-64
63.9291
48.89
Zn-66
65.9260
27.81
Zn-67
66.9271
4.11
Zn-68
67.9249
18.57
Zn-70
69.9253
0.62
• Given the ff. information about the
isotopes of potassium (K). Calculate
the RAM.
• Isotope abundance(%)
amu
• K-39
93.2581 38.963707
• K-40
0.0117
39.963998
• K-41
6.7302
40.961826