Isotopes and Average Atomic
Mass
4.1
Isotopes
The number of protons define the type of
atom:
– 6 protons is carbon
– 1 proton is hydrogen
– 8 protons is oxygen
– 92 protons is uranium
However the number of neutrons can vary
within a given type of atom
Hydrogen Isotopes
There are three main isotopes of hydrogen
– Hydrogen – 1p, 0n, 1e
– Deuterium – 1p, 1n, 1e
– Tritium – 1p, 2n, 1e
Relative Mass of an Atom
The mass of an atom is expressed in
atomic mass units (amu)
Atomic mass is determined based upon
carbon-12 as a standard with a mass
exactly equal to 12amu
Using this, the relative mass of every atom
on the periodic table can be determined
(i.e. oxygen 16 would have a mass of
16amu)
Isotopic Abundance
Because elements as a combination of
different isotopes, it is useful to consider
the isotopic abundance
For example, magnesium is composed of:
– Magnesium-24 – 79%
– Magnesium-25 – 10%
– Magnesium-26 – 11%
Regardless of where you find magnesium,
it will be composed of these percentages
Average Atomic Mass and the
Periodic Table
The average atomic mass that appears on
the periodic table is the average of the
mass of all the element’s isotopes
Therefore, even carbon, which is used as
the standard, does not have an average
atomic mass of 12 but rather 12.01 due to
the presence of carbon-14
Carbon-14 is about 1.1% of all carbon
while carbon-12 is the remaining 98.1%
Average Atomic Mass Calculation
masscarbon12  12
masscarbon14  14
abundancecarbon12  98.1%
abundancecarbon14  1.1%
M C  mC12 aC12  mC14 aC14
M C  12(.981)  14(.011)
M C  12.02
Average Atomic Mass
Naturally occuring silver exists as two isotopes.
From the mass of each isotope and the
isotopic abundance listed below, calculate the
average atomic mass of silver
Isotope
Atomic mass (u)
Abundance (%)
Silver-107
106.9
51.8
Silver-109
108.9
48.2
Average
Atomic
Mass
mass
 106.9
silver107
masssilver109  108.9
abundancesilver107  51.8%
abundancesilver109  48.2%
M Ag  m Ag107a Ag107  m Ag109a Ag109
M Ag  106.9(.518)  108.9(.482)
M Ag  107.9
Why do we need to know
Average atomic mass?
To calculate formula mass and molecular mass:
3 Steps to calculate the formula mass/molecular
mass
Above all Double Check to make sure you have
the correct formula
1. Count the atoms of each element
2. Multiply the number of atoms by the elements atomic
mass (round mass to hundredths)
3. Add all the masses together
Formula Mass
This is the mass of one molecule and the
unit is atomic mass units (amu)
This is a very, very small value so it is not
very useful in the laboratory.
We use moles of molecules, which is a
group of molecules.