Page 25 - Chemistry_Form_2

Chemistry
for Secondary Schools

An atom is very small and it would be difficult to measure its actual mass.
To overcome this difficulty, chemists developed a simpler way to express the
mass of an atom. This involved expressing the mass of an atom in relation to
a chosen standard atomic mass. The carbon atom was chosen as the standard
atom (reference atom) and its mass was arbitrarily chosen as 12 units (not actual
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value). Then, using an instrument called a mass spectrometer, all the other atoms
were compared to this standard atom. This reference is called the Carbon-12
scale. For example, it was found that the:

(a) magnesium atom was twice as heavy as the reference atom; so its mass was
put at 24.

(b) hydrogen atom was 1 as heavy as the reference atom; so its mass was put
at 1. 12
1
(c) helium atom was as heavy as the reference atom; so its mass was put at
4. 3

The mass of an atom obtained by comparing it with the arbitrary mass of a
carbon-12 atom is called its relative atomic mass (R.A.M. or A ). The relative
r
atomic mass of an element is the average mass of one atom of the element

relative to 1 the mass of one atom of carbon-12. Therefore, R.A.M. may not
12 th
necessarily be a whole number.

Average mass of atom of an element
That is, A ! 1
r
-12 atom
12 th the mass of carbon -12 atom

Table 1.4 gives the atomic numbers and relative atomic masses of the first 20
elements in the periodic table. The relative atomic masses of such elements are
obtained by calculating the average mass of all the isotopes of each element.

For isotopic elements, the relative atomic mass (R.A.M.) can be calculated using
the following formula:

Relative atomic mass (R.A.M.) = ∑ isotopic mass × percentage abundance

Note: ∑ = Summation

18 Student’s Book Form Two

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