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The relative ISOTOPIC mass is the mass of an element in relation to Carbon-12. The reason
that this is the ISOTOPIC mass, is because some elements have different isotopic forms,
for example Chlorine can be found as Chlorine-35 and Chlorine-37. The isotopic mass is not
the mass number which appears on the periodic table. The number see on the periodic table
is the Relative Atomic Mass (Ar) Relative Atomic Mass (Ar) of an element is
defined as the average mass of the naturally occurring isotopes of the element relative
to the mass of an atom of C-12. On the periodic table it is the same number
as the mass number. In basic terms, it is the average mass of
the isotopic forms of the element When we think about this we must remember
that the isotopic forms of elements have different values for their naturally occurring abundance
on Earth. For example, let’s look at the isotopic
forms of Chlorine. Some Chlorine atoms have a mass of 35, and
some have a mass of 37. In most cases to take a simple average you would just add the values
numbers together and the number of values, so in this case you would add 35 + 37 divided
by 2 to get an average of 36, but this is not how you calculate the relative atomic
mass. We need to remember that we are talking about
a number of atoms, there are a certain number of Chlorine atoms with a mass of 35 and a
certain number of chlorine atoms with a mass of 37, however, within a sample the levels
of each isotope are not equal. So we take this into account when calculating
the relative atomic mass, so that we are calculating a weighted average.
Naturally occurring Chlorine is made up of 75% Chlorine-35, and 25% Chlorine-37. This
can be shown to you on a graph to show abundance data.
Where the relative isotopic mass is show along the x axis, and the relative abundance is
shown on the y axis. So say we had a simple sample of 100 atoms
of Chlorine: we would have 75 atoms of Chlorine-35. and 25 atoms of Chlorine-37.
We first find out the total mass of the sample: we do this by multiplying the mass of each
atom by the number of atoms present for each isotope and then we add them together.
For Chlorine-35 there are 75 atoms and for Chlorine-37 there are 25 atoms. So the mass
of 100 atoms would be (75x35) + (25x37) = 3550. The mass of one atom would be the mass of
the total number of atoms divided by the number of atoms. So the mass of one atom of chlorine
would be 3550 divided by 100 = 35.5 The relative atomic mass has a value closer
to 35 because there are more atoms of Chlorine-35, than Chlorine-37. It is a WEIGHTED AVERAGE.
So the Relative Atomic Mass (Ar) of an element is defined as the average mass of the naturally
occurring isotopes of the element relative to the mass of an atom of C-12. On a periodic
table it is the same number as the mass number. The number is a WEIGHTED average, and when
calculating its value, the proportion of an isotope in a sample must be taken into account.