Atomic theory - early models
"Earth, air, fire, and water" vs "Atomos" Dalton's atomic theory Thompson's atomic model

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".

Click for larger imageAlthough 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.

Click for larger image 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.

Dalton's atomic model

Thompson's atomic model
Click for larger image 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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The atomic and kinetic theories of matter
Atomic theory - modern models
Of atoms and elements
From atoms to molecules
Making molecules - the rules of covalency
  Question 1
Question 2
Question 3
Question 4
  Atoms at Chem4Kids
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