NOTES AND PROBLEMS 39little to us if we did express it exactly in arithmetical figures.
What we are interested in is the ratio of the weights. When a
great many million million atoms of oxygen combine with the same
number of million million atoms of zinc we make the actual meas-
urement that 1 gram of oxygen combines with 4.09 grams of zinc.
Standard of Atomic Weights, O = 16. It has been interna-
tionally agreed that it is a matter of convenience to adopt the
exact number 16.000 as the atomic weight of oxygen. Exactly
16 grams is the gram atomic weight of oxygen. We may mention
as a matter of interest that 16 grams of oxygen contains 6.06 X
1023 atoms, although this enormous figure is of no practical im-
portance beyond the mere fact that it is enormous and has been
determined accurately.
The atomic weight of zinc is therefore 4.09 X 16 = 65.4 if
our assumption of the 1:1 ratio is correct. Therefore 65.4 grams
of zinc is the gram atomic weight of this element, and it likewise
contains 6.06 X 10^23 actual atoms.
Of course the fact that many cases are known where two ele-
ments can combine in different proportions to form different
compounds (multiple proportions) shows us at once that the atomic
combining ratio cannot always be 1:1. In order to establish
consistent atomic weights for all the elements on the O = 16
basis, we must either make very clever guesses as to the atomic
ratios or we must have some reliable means of finding out this
ratio. We shall state here that there are reliable methods of
doing this, one of the most useful of which will be explained in a
later section of this chapter (p. 48). In the front inside cover of
this book is printed a list of all the elements with their symbols and
their atomic weights. The atomic weights are obtained in many
cases from the combining ratio by weight with oxygen itself,
in other cases from the combining ratio with another element whose
combining ratio with oxygen is known. In every instance the com-
bining ratio by weight with oxygen must be multiplied by 16 and
divided by the atomic ratio of the element to oxygen.
PROBLEMS- The combining ratio by weight of zinc and sulphur is
2.039 : 1. Assuming the knowledge that the atomic weight
of zinc is 65.4 and that zinc and sulphur combine in the
1:1 atomic ratio, find the atomic weight of sulphur.