Atomic theory - early models
The ancient Greek philosophers were the first to put
theories of what made up matter, based on observation and hypothesis
only, with no experimentation. The modern atomic theory still contains
elements of the first Greek theories and is still evolving.
Dalton's atomic theory came as an inspiration and explanation for the
huge amount of chemical information that faced the early chemists of
the 18th and 19th centuries. Subsequent theories
were the sole product of experimentation, discoveries, and scientific
research based on Dalton's ideas.
One such model that developed from Dalton's ideas was Thompson's model.
J.J Thompson, with his discovery of the electron, used Dalton's model
but changed it so that it incorporated electrons.
The study of the history of atomic theory forms an excellent vehicle
for the development of an understanding of current scientific theory.
This topic investigates early theories, up to the discovery of electrons
and the atomic nucleus. The second topic in this series, Atomic
theory - modern models, explores the theories put forward from 1911
which incorporated these and other important developments in atomic
theory.
"Earth, air, fire and water"
vs "Atomos"
Greeks philosophers were the first to record their
ideas about the physical properties of the world around them. The first
theories of matter were put forward by Empedocles in 450 BC, he proposed
that all matter was composed of four elements - Earth, air, fire and
water.
Later, Leucippus and Democritus suggested matter was made up of tiny
indestructible particles continuously moving in empty space. They called
them "atomos", which is where we get the modern word "atom".
Although
today's modern "atomic theory" is based on the "atomos"
theory; the "Earth, air, fire, and water" theory was the accepted
theory for over 2000 years. This is because Aristotle proclaimed his
support for the "Earth, air, fire and water" model!
Dalton's atomic theory
John Dalton, an English schoolmaster, proposed the first modern atomic
theory. John Dalton was a teacher of mathematics and science by the
age of 12; his life was a most remarkable mix of study and inspiration.
In 1803 Dalton developed a system of chemical symbols for the known
elements and compounds of time. In addition, he proposed that a chemical
combination of different elements occurred in simple numerical ratios
by weight.
In 1808, Dalton developed his masterpiece: "The New System of
Chemical Philosophy". He suggested that all elements are composed
of tiny, indestructible particles, he called atoms. Dalton proposed
that the atoms of any particular element were all alike and had the
same weight. Although Dalton did not foresee the discovery of the even
smaller subatomic particles, his ideas, or postulates, still hold up
pretty well today and can be used as the first step in learning about
atoms.
Thompson's atomic model
In 1897, J.J. Thompson discovered the electron, a negatively charged
particle much smaller than an atom. Electrons were found to be present
in all atoms, this meant that Dalton's solid model of the atom had to
be adapted. Although a number of models were put forward at this time,
Thompson's model, was the most accepted.
Thompson held that atoms were spheres of positively charged matter
in which electrons were embedded, known as the plum-pudding model. This
simple model still retained Dalton's solid atom model and adapted it
to incorporate electrons.
Thompson's model was accepted for just over a decade. It was replaced
in 1911 when British physicist Ernest Rutherford and his team discovered
the atomic nucleus.
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