Biomedical Engineering Reference
In-Depth Information
2
Atomic Structure and Atomic Radiation
2.1
The Atomic Nature of Matter (ca. 1900)
The work of John Dalton in the early nineteenth century laid the foundation for
modern analytic chemistry. Dalton formulated and interpreted the laws of definite,
multiple, and equivalent proportions, based on the existence of identical atoms as
the smallest indivisible unit of a chemical element. The law of definite proportions
states that in every sample of a chemical compound, the proportion by mass or
weight of the constituent elements is always the same. When two elements com-
bine to form more than one compound, the law of multiple proportions says that
the proportions by mass of the different elements are always in simple ratios to
one another. When two elements react completely with a third, then the ratio of
the masses of the two is the same, regardless of what the third element is, a fact
expressed by the law of equivalent proportions. Dalton also assumed a rule of great-
est simplicity—that elements forming only a single compound do so by means of
a simple one-to-one combination of atoms. This rule does not always hold.
These ideas were supported by the work of Dalton's contemporary, Gay-Lussac,
on the law of combining volumes of gases. This law states that the volumes of
gases that enter into chemical combination with one another are in the ratio of
simple whole numbers when all volumes are measured under the same conditions
of pressure and temperature. Avogadro hypothesized that equal volumes of any
gases at the same pressure and temperature contain the same number of mole-
cules. Avogadro also suggested that the molecules of some gaseous elements could
be composed of two or more atoms of that element.
Today we recognize that a gram atomic weight of any element contains Avo-
gadro's number,
N
0
=
10
23
, of atoms.
1)
Furthermore, a gram molecular
weight of any gas also contains
N
0
molecules and occupies a volume of 22.41 L
(liters) at standard temperature and pressure [STP, 0
◦
C(=273 K on the absolute
temperature scale) and 760 torr (1 torr
=
1 mm Hg)]. The modem scale of atomic
and molecular weights is set by stipulating that the gram atomic weight of the car-
bon isotope,
12
C, is exactly 12.000
...
g. A periodic chart, giving atomic numbers,
6.022
×
1
See Appendices A and B for physical
constants, units, and conversion factors.
