accurately, and consequently the part in them due to
nitrogen is a little uncertain, as will be seen in the table. All we can
tell by this method is that the true weight is very near 14. The
equivalent can however be determined very accurately, and we have seen
that it is some multiple or submultiple of the true atomic weight.
Since molecular-weight determinations have shown that in the case of
nitrogen the atomic weight is near 14, and we have found the equivalent
to be 7.02, it is evident that the true atomic weight is twice the
equivalent, or 7.02 x 2 = 14.04.

~Summary.~ These, then, are the steps necessary to establish the atomic
weight of an element.

1. Determine the equivalent accurately by analysis.

2. Determine the molecular weight of a large number of compounds of the
element, and by analysis the part of the molecular weight due to the
element. The smallest number so obtained will be approximately the
atomic weight.

3. Multiply the equivalent by the small whole number (usually 1, 2, or
3), which will make a number very close to the approximate atomic
weight. The figure so obtained will be the true atomic weight.

~Molecular weights of the elements.~ It will be noticed that the molecular
weight of nitrogen obtained by multiplying its density by 28.9 is 28.08.
Yet the atomic weight of nitrogen as deduced from a study of its gaseous
compounds is 14.04. The simplest explanation that can be given for this
is that the gaseous nitrogen is made up of molecules, each of which
contains two atoms. In this respect it resembles oxygen; for we have
seen that an entirely different line of reasoning leads us to believe
that the molecule of oxygen contains two atoms. When we wish to indicate
molecules of these gases the symbols N_{2} and O_{2} should be used.
When we desire to merely show the weights taking part in a reaction this
is not necessary.

The vapor densities of many of the elements show that, like oxygen and
nitrogen, their molecules consist of two atoms. In other cases,
particularly among the metals, the molecule and the atom are identical.
Still other elements have four atoms in their molecules.

While oxygen contains two atoms in its molecules, a study of ozone has
led to the conclusion that it has three. The formation of ozone from
oxygen can therefore be represented by the equation

3O_{2} = 2O_{3}.

~Other methods of determining molecular weights.~ It will be noticed that
Avogadro's law gives