Isotopes and Average
Atomic Mass
Chemistry 11- Chapter 1 Section 4
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
Trend in the number of neutrons
as atomic number increases
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 (u)
 In the past, hydrogen was used as a standard
of 1 u but because of the isotopes, the standard
was changed to carbon
 Atomic mass is determined based upon carbon12 as a standard with a mass exactly equal to
12u. One twelve of C-12 is equal to 1u
 Using this, the relative mass of every atom on
the periodic table can be determined (i.e.
oxygen 16 would have a mass of 16u)
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
Mass spectrometer
 Highly specialized piece equipment that
can identify the mass of an element or
compound in a mixture and its relative
abundance
 It directs the element in gas form across
a tube and the beam is deflected by a
magnetic field…heavy elements keep
going straight, light elements veer of
quickly. The detector plate senses the
elements as they hit it.
Isotopes and radioisotopes
 Some isotopes are stable but not all
 All elements after Bismuth are unstable
 Unstable isotopes break down- the nucleus,
and transmutate to new elements and give off
radiation in the process.
 They are described as radioactive and break
down in a predictable way called half life
 There are many uses for radioisotopes
especially in the medical and research field
Nuclear radiation
3 types: alpha, beta and gamma
 Alpha= heavy, positive helium nucleus
=
2 P and 2 N, can be stopped by paper
 Beta= negatively charged electron
=
can be stopped by aluminum or plexiglass
(thick see-through plastic)
 Gamma= high energy electromagnetic
radiation
=
no mass or charge, can be stopped by thick
lead
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
mass
 12
carbon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
masssilver107Atomic
 106.9 Mass
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
You try
 1. Chlorine has two naturally occurring
isotopes. One has a mass of 34.969 and an
abundance of 75.77% and the other a mass of
36.966 and abundance of 24.34%. Calculate
the average atomic mass.
 2. Calculate the average atomic mass of
oxygen given the three isotopes abundance are
99.759%, 0.037%, 0.204% for the masses
15.995, 16.995 and 17.999 respectively.
 1. 35.494
 2. 15.999