The actual mass of an atom is approximately 10-23 g (now that is small!) and working with it
when doing calculations can be a bit difficult and to be honest a waste of
time! Instead we use what is known as its relative
atomic mass.
The 12C
isotope is used as a standard to which all other atoms are compared. One atom
of 12C has a mass of 12 so 1/12 of an atom of 12C
has a mass of 1….so now it is a lot more easier to
work out how many times heavier an atom is compared to 1/12 of the mass of an atom of 12C.
If you are still confused, it can be put
into even simpler terms…
The average mass of an
atom of the element
1/12 of the mass of one atom of 12C
We also use the
same idea for the masses of individual isotopes – Relative isotopic mass:
The average mass of an
atom of the isotopes
1/12 of the mass of one atom of 12C
As you may or may
not know, all elements contain isotopes
(isotopes are atoms of the same element with the same number of protons but
different number of neutrons), so when calculating the relative atomic mass of
an element we need to take this into account. There are two things you need to
know when doing these calculations:
1. the relative isotopic mass of each isotope
2. the percentage of each isotope
in the element
Below are a
couple worked examples to calculate the relative atomic mass (There are a few
ways to set your working out but I find this way really simple):
1.
Chlorine consists of two isotopes – 75.5% of 35Cl and
24.5% of 37Cl.
35Cl x 75.5% à (75.5 ÷ 100) x 35 = 26.425
37Cl x
24.5% à (24.5 ÷ 100) x
37 = 9.065
26.425
+ 9.065 =
35.49 (35.5 to 3sf)
2. Magnesium consists of
three isotopes – 78.6% of 24Mg, 10.1% of 25Mg and 11.3%
of 26Mg.
24Mg x
78.6% à (78.6 ÷ 100) x 24 =
18.864
25Mg x
10.1% à (10.1 ÷ 100) x 25 =
2.525
26Mg x
11.3% à (11.3 ÷ 100) x 26 =
2.938
18.864
+ 2.525 +
2.938 = 24.3 to 3sf
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