history of the atom
In a nutshell:
- Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today).
- John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century.
- In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.
- In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.
- In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.
- We can think of an atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).
'We think there is color, we think there is sweet, we think there is bitter, but in reality there are atoms and a void.'
DEMOCRITUS, C. 460 – C. 370 BC
From around the 5th century B.C.E., Ancient Greeks, under the leading influence of philosophers Plato and Aristotle, believed all matter was made of a combination of four elements: water, air, fire and earth. Meanwhile, philosophers Leucippus and Democritus believed matter was made of small, invisible and unbreakable blocks of different nature and first developed the concept of the atom. However, since Aristotle and other prominent thinkers of the time strongly opposed their idea of the atom, their theory was overlooked and essentially buried for almost two millenia, until the 16th and 17th centuries. Alchemists were the earliest chemists (~1500 AD), but were not really scientists - they did not have models to interpret their experimental results. To learn more about Alchemists, click here.
In time, Lavoisier’s groundbreaking 18th century experiments accurately measured all substances involved in the burning process, proving that “when substances burn, there is no net gain or loss of weight.” Lavoisier established the science of modern chemistry, which gained greater acceptance because of the efforts of John Dalton (1766-1844), who modernized the ancient Greek ideas of element, atom, compound, and molecule; and provided a means of explaining chemical reactions in quantitative terms.
Dalton's atomic theory stated that
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties.
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
Many previously unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoretical foundation in chemistry and remains valid in modern chemical thought.
At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. He also proposed a different model for the atom, theorising that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding – but with lots more space in between).
In 1911, Ernest Rutherford interpreted the results of a gold foil experiment and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (centre of the atom).
The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.
In time, Lavoisier’s groundbreaking 18th century experiments accurately measured all substances involved in the burning process, proving that “when substances burn, there is no net gain or loss of weight.” Lavoisier established the science of modern chemistry, which gained greater acceptance because of the efforts of John Dalton (1766-1844), who modernized the ancient Greek ideas of element, atom, compound, and molecule; and provided a means of explaining chemical reactions in quantitative terms.
Dalton's atomic theory stated that
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties.
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
Many previously unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoretical foundation in chemistry and remains valid in modern chemical thought.
At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. He also proposed a different model for the atom, theorising that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding – but with lots more space in between).
In 1911, Ernest Rutherford interpreted the results of a gold foil experiment and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (centre of the atom).
The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.