In this video, we are going to learn how to
calculate the relative atomic mass of an element
from its isotopic abundances. If we consider
carbon, its relative atomic mass is very close
to 12 atomic mass units. This is because in
nature 99 atoms out of 100 atoms of a carbon
sample are carbon 12 isotopes. Another way
of saying this is that carbon-12 has a 99%
isotopic abundance.
However, not all relative atomic masses of
elements are weighted in this way. Chlorine,
for instance, exists as two isotopes: chlorine-35
and chlorine-37. How many protons and neutrons
do you think each chlorine isotope has? Remember,
the mass number for an isotope is equal to
the number protons it has plus the number
of neutrons. Pause and select from one of
the following options. Use the chemical symbols
to help you work this out.
A) 17 protons/18 neutrons (chlorine-35); 17
protons/19 neutrons (chlorine-37);
B) 17 protons/17 neutrons (chlorine-35); 17
protons/20 neutrons (chlorine-37);
C) 17 protons/18 neutrons (chlorine-35); 17
protons/20 neutrons (chlorine-37);
Both chlorine-35 and chlorine-37 isotopes
have 17 protons, but the chlorine-35 isotope
has 18 neutrons, whereas the chlorine-37 isotope
has 20 neutrons. The answer was C. Did you
get it right?
In nature, out of 100 atoms of a chlorine
sample, 75 atoms are chlorine-35 isotopes
and 25 atoms are chlorine-37 isotopes. For
you to be able to calculate the relative atomic
mass for chlorine, you have to multiply the
mass number of the isotope with its abundance
(percentage), and add these products together.
This gives the total mass number of the isotopes.
Using this data, can you calculate the total
mass of chlorine isotopes using their abundance
and mass numbers? Pause, think, choose and
continue when ready.
(75x37)+ (25x35) = 2775 + 875 = 3650.
(75x35)+ (25x37) = 2625 + 925 = 3550.
(75x25)+(35x37) = 1875+ 1295 = 3170
The answer is B. You have to multiply a 75%
abundance of the chlorine-35 isotope by its
mass number, 35. You then have to multiply
a 25% abundance of the chlorine isotope by
its mass number, 37. When added together,
this gives a total of 3,550 when the two products
are added together. The final step in calculating
the relative atomic mass is to divide your
answer by 100. This is because the abundances
are given as percentages. This means chlorine
has a relative atomic mass of 35.50.
Here's a challenge. From the following data,
calculate the relative atomic mass of uranium.
Pause and continue when ready. Make sure you
write your working out down as you do the
calculation.
Isotope
Relative abundance/%
238U
99.27
235U
0.72
234U
0.01
Hint: you add the three products this time
of (isotope mass number x relative abundance).
The correct answer and working out is displayed
on the screen.
(99.27 x 238) + (0.72 x 235) + (0.01 x 234)
= 23,626.26 + 169.2 + 2.34 = 23,797.8
Remember, you then have to divide this value
by 100. This gives 237.99. Did you get it
right?
In summary, the relative atomic mass is the
average weighted mass for the isotopes of
a particular element. To calculate this, you
need to multiply the relative abundances by
the mass number of the isotope, add these
products together for each isotope, and then
divide the total by 100 -- and that's how
you calculate the relative atomic mass of
any element from its isotopes in the periodic
table.
