| Atomic
Weight and the Mole Concept |
| Atoms and molecules
are material particles and they possess mass.
The mass of one atom of an element is known
as the atomic weight of the element. Atomic
weight, generally denoted as W,
should not be confused with another concept
called atomic number of an element. |
| Atomic number
A is the simple sum of the
number of protons (Z) and
neutrons (N) the element
has. Thus A = Z + N and is
an integral number as Z and
N both are integer numbers. |
| The atomic weight
is an actual measure of the weight of the
atoms in grams or kilograms. |
| The table
below gives mass of some of the atoms: |
|
Element |
Mass in Kg |
| Hydrogen |
1.67
x 10-27kg |
| Oxygen |
26.56
x 10-27kg |
| Chlorine |
58.9
x 10-27kg |
| Uranium |
395.0
x 10-27kg |
|
| We can see that
when we deal with the masses, since they are
very small, it is inconvenient to use such
numbers in calculations. To make calculations
less cumbersome, weight of the atom of an
element is compared to the weight of standard
atom and their ratio is taken. |
| It is best to
use relative units while measuring atomic
weights. |
| For a long time,
the mass of hydrogen atom was taken as the
unit mass for measuring atomic weights. In
terms of the mass of hydrogen atom, the carbon
atom weighed 12 units, an oxygen atom weighed
16 units and an atom of chlorine weighed 35.5
units. Therefore, a molecule of CO has mass
12 + 16 unit = 28 units relative to the mass
of an H atom. |
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