How Did John Dalton Come Up With His Atomic Theory?
By Kathy Carlsen – based on information on the internet
John Dalton is known as the father of modern atomic
theory because in the early 1800’s he introduced his ideas
about the structure of matter which have provided the
foundation for today’s explanation of atomic structure.
2000 years separated Dalton’s theory and the earlier work
of a Greek philosopher, Democritus. Democritus had
reasoned if you were to cut a substance in half, cut a half in
half again and continue the process, you would get to the
smallest possible piece of that substance. He called the
smallest possible pieces atomos which meant cannot be
divided in his language.
Fast forward 2000 years to John Dalton. As a meteorologist, as he observed
evaporation of water and wondered how water and air could occupy the same
space at the same time, when solid bodies can't. He reasoned if the water
and air were composed of separate particles, they could be in the same
space by mixing in between each other. Evaporation, he thought, might be
viewed as a mixing of water particles between air particles.
Dalton performed a series of experiments on mixtures of gases. Based on his
findings, he developed the hypothesis that the sizes of the particles making
up different gases must be different. This lead him to an idea that
contradicted the current thinking of alchemists who believed it was possible
to change one substance to another and spent much of their time trying to
‘make’ gold from less valuable metals. Dalton’s theory suggested that every
single atom of an element such as oxygen is identical to every other oxygen
atom; furthermore, atoms of different elements, such as oxygen and
mercury, are different from each other. Dalton was the first to describe
elements according to their atomic weight.
When Dalton was developing his theory, he knew about a
scientific law recently established by Antoine Lavoisier –
Conservation of Mass. His experiments proved that there was
no gain or loss of mass during a chemical reaction. Dalton
reasoned this meant that the pieces of matter he called atoms
could not be destroyed or created, simply rearranged to make
new substances.
Another scientist, Joseph Proust, had developed the Law of
Definite Proportions that says pure compounds are made of
elements combined in definite proportions. For instance, the
smallest piece of water is always composed of 2 hydrogen
pieces combined with 1 oxygen piece.
In 1803 John Dalton’s combined new scientific ideas of his time along with
his experimental evidence to propose his atomic theory which contained
these points:
1. Matter is made of tiny particles called atoms which cannot be divided.
2. Each element has it own kind of atom. All atoms of the same element
identical to each other are different from atoms of other elements.
3. In chemical reactions, atoms are never created nor destroyed, but
only rearranged.
4. When elements combine to form a new compound, the smallest portion
of the compound is a group containing a definite number of atoms of
each element