L.O. To understand isotopes and
atomic mass
Most naturally-occurring carbon exists as carbon-12, about
1% is carbon-13 and a much smaller amount is carbon-14.
6 protons
6 protons
6 protons
6 neutrons
7 neutrons
8 neutrons
6 electrons
6 electrons
6 electrons
Particle
Mass
Charge
proton
1
+1
neutron
1
0
electron
almost 0
-1
The atoms of an element contain equal numbers of
protons and electrons and so have no overall charge.
The isotopes of an element are virtually identical in their
chemical reactions.
This is because they have the same number of protons and
the same number of electrons.
The uncharged neutrons make no difference to chemical
properties but do affect physical properties such as melting
point and density.
Natural samples of elements are
often a mixture of isotopes.
Hydrogen-1 makes up the vast majority of the naturallyoccurring element but two other isotopes exist.
hydrogen
deuterium
tritium
1 proton
1 proton
1 proton
0 neutrons
1 neutron
2 neutrons
1 electron
1 electron
1 electron
About 75% of naturally-occurring chlorine is chlorine-35 and
25% is chlorine-37.
17 protons
17 protons
18 neutrons
20 neutrons
17 electrons
17 electrons
Almost all of naturally-occurring oxygen is oxygen-16 but
about 0.2% is oxygen-18.
oxygen-16
8
8
8
What are the
particle
numbers in
each isotope?
protons
neutrons
electrons
oxygen-18
8
10
8
Atomic Mass of elements
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The atoms of each element have a different mass.
The relative atomic mass is a way of saying how heavy
atoms are in comparison to others.
Carbon is given a relative atomic mass (RAM) of 12.
The RAM of other atoms compares them with carbon.
Eg. Hydrogen has a mass of only one twelfth that of carbon
and so has a RAM of 1.
Below are the RAMs of some other elements.
Element
Symbol
Times as heavy as carbon
R.A.M
Helium
He
one third
4
Beryllium
Be
three quarters
9
Molybdenum
Mo
Eight
96
Krypton
Kr
Seven
84
Oxygen
O
One and one third
16
Silver
Ag
Nine
108
Calcium
Ca
Three and one third
40
Complete the Isotopes and atoms sheet
Many elements are a mixture of isotopes. The RAM given in
the periodic table takes account of this.
To calculate the RAM of a mixture of isotopes,
multiply the percentage of each isotope by its
atomic mass and add them together.
For example, chlorine exists as two isotopes:
chlorine-35 (75%) and chlorine-37 (25%).
RAM of chlorine = (75% x 35) + (25% x 37)
= (0.75 x 35) + (0.25 x 37)
= 26.25 + 9.25
= 35.5
Bromine contains 50.5% bromine-79 and 49.5% bromine-81.
What is the RAM of naturally-occurring bromine?
RAM of bromine = (50.5% x 79) + (49.5% x 81)
= (0.505 x 79) + (0.495 x 81)
= 39.895 + 40.095
= 79.99
= 80 (the RAM is usually rounded
to the nearest whole number)
1.
2.
3.
4.
5.
Calculate the average atomic mass of iron if its abundance
in nature is 15% iron-55 and 85% iron-56.
What is the average atomic mass of silicon if 92.21 % of its
atoms have a mass of 27.977 amu, 4.07 % have a mass of
28.976 amu, and 3.09 % have a mass of 29.974 amu?
Calculate the average atomic mass for neon if its
abundance in nature is 90.5% neon-20 (19.922 amu), 0.3%
neon-21 (20.994 amu), and 9.2% neon-22 (21.991 amu).
Calculate the average atomic mass of silver if 13 out of 25
atoms are silver-107 and 12 out of 25 atoms are silver-109.
Calculate the average atomic mass of chromium.
Isotope
6.
Cromium
– 50
Chromium – 52
Chromium – 53
Chromium – 54
Mass (amu)
49.946
51.941
52.941
53.939
Relative Abundance
0.043500
0.83800
0.095000
0.023500