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Dalton’s table shows symbols and
atomic weights of different elements.
Dalton was drawn to atomic theory
through meteorology, when he asked
himself why air and water particles
could mix.A CENTURY OF PROGRESS
a mixture of different gases. As he
experimented, he observed that a
given quantity of pure oxygen will
take up less water vapor than the
same amount of pure nitrogen,
and he jumped to the remarkable
conclusion that this is because
oxygen atoms are bigger and
heavier than nitrogen atoms.
Weighty matters
In a flash of insight, Dalton
realized that atoms of different
elements could be distinguished
by differences in their weights.
He saw that the atoms, or “ultimate
particles,” of two or more elements
combined to make compounds in
very simple ratios, so he could
figure out the weight of each atom
by the weight of each element
involved in a compound. Very
quickly, he figured out the atomic
weight of each element then known.
Hydrogen, Dalton realized, was
the lightest gas, so he assigned it
an atomic weight of 1. Because
of the weight of oxygen that
combined with hydrogen in water,
he assigned oxygen an atomic
weight of 7. However, there was a
flaw in Dalton’s method, because
he did not realize that atoms of the
same element can combine. He
always assumed that a compound
of atoms—a molecule—had only
one atom of each element. ButAn inquiry into the
relative weight of the ultimate
particles of bodies is a subject,
as far as I know, entirely new.
John DaltonSee also: Joseph Proust 105 ■ Dmitri Mendeleev 174–79
John Dalton Born into a Quaker family in
England’s Lake District in 1766,
John Dalton made regular
observations of the weather from
the age of 15. These provided
many key insights, such as that
atmospheric moisture turns to rain
when the air cools. In addition to
his meteorological studies, Dalton
became fascinated by a condition
he and his brother shared: color
blindness. His scientific paper on
the subject gained him admission
to the Manchester Literary and
Philosophical Society, of which
he was elected president in 1817.
He wrote hundreds of scientificpapers for the Society, including
those about his atomic theory.
The atomic theory was quickly
accepted, and Dalton became a
celebrity in his own lifetime—
more than 40,000 people
attended his funeral in
Manchester in 1844.Key works1805 Experimental Enquiry into
the Proportion of the Several
Gases or Elastic Fluids,
Constituting the Atmosphere
1808–27 New System of
Chemical PhilosophyDalton’s work had put scientists on
the right track, and within a decade
Italian physicist Amedeo Avogadro
had devised a system of molecular
proportions to calculate atomic
weights correctly. Yet the basic
idea of Dalton’s theory—that each
element has its own unique-sized
atoms—has proved to be true. ■