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300 BCE
Aristotle & Democritus
Aristotle and Democritus lived more than 2000 years ago in Greece. Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”. Aristotle and Democritus used observation and inference to explain the existence of everything. -
Period: 300 BCE to
The Historical Development of Atomic History
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John Dalton
In England in 1808, John Dalton built on the ideas of Democritus in ancient Greece. Dalton described atoms as tiny particles that could not be divided. He thought each element was made of its own kind of atom. -
J.J. Thompson
Also in England almost 90 years later in 1897, J. J. Thompson was using a cathode ray and discovered that electrons were smaller particles of an atom and were negatively charged. Thompson knew atoms were neutrally charged, but couldn’t find the positive particle. Thompson proposed the plum pudding model of an atom. He believed the atom was a soup of positive charge with electrons in side (like plums in a pudding). -
Ernest Rutherford
Ernest Rutherford in England, 1911, conducted an experiment, where he bombarded metal foil with alpha particles. Rutherford was able to isolate positive particles in the atom. He also decided that the atoms were mostly empty space, but had a dense core, known as the nucleus. Rutherford’s model of an atom had the electrons revolving around a central core (nucleus) which contained the positively changed protons. -
Niels Bohr
Niels Bohr in England in 1913, was trying to show why the negative electrons were not sucked into the nucleus of the atom. He proposed that electrons travelled in fixed paths around the nucleus and can exist at different levels. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus. -
James Chadwick
In 1932 James Chadwick noted that the atom also consists of a neutral particle called a neutron. He also noted that the number of electrons equals the number of protons to make the whole atom neutral. -
Modern Models
Electrons move so fast that it seems to fill the space, with a cloud like appearance around a dense nucleus. We also now know that protons and neutrons are made up of three quarks each. A quark is a fast-moving point of energy, of which there are several varieties.
