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400 BCE
Democritus
Democritus’ mentor Leucippus, originally came up with the atomic theory. But it was adopted by Democritus.Democritus hypothesized that atoms cannot be destroyed, differ in size, shape and temperature, are always moving, and are invisible. He believed that there are an infinite number of atoms. This hypothesis was created in 465 BC. -
400 BCE
Democritus
He created the first atomic model by depending on the analogies from human sense -
350
Aristotle
The Greek philosopher Aristotle made significant and lasting contributions to nearly every aspect of human knowledge, from logic to biology to ethics and aesthetics. Though overshadowed in classical times by the work of his teacher Plato.It was at Lyceum that Aristotle probably composed most of his 200 pieces of works, of which only 31 survive. -
350
Aristotle
Aristotle adopted the concept of four elements and introduced the idea that the elements can be different on the basis of properties such as hot versus cold and wet versus dry. -
John Dalton
Democritus believed that everything was made of tiny particles called atoms and that these atoms could not be split into smaller particles. Dalton was now going to solve this 2000 year-old mystery. He carried out countless chemical reactions, and in 1808 published what we now call Dalton’s Law in his book, A New System of Chemical Philosophy. -
John Dalton
Dalton's model of the atom. John Dalton proposed that all matter is composed of very small things which he called atoms. This was not a completely new concept to the ancient Greeks. -
J.J. Thomson
When Thomson began his research career, nobody had a clear picture of how atoms might look. Thomson decided he would picture them as a kind of smoke ring and see where the mathematics would take him. Through this goal Thomson had found the discovery of electrons the first subatomic particle. -
J.J. Thomson
Thomson atomic model, earliest theoretical description of the inner structure of atoms, proposed about 1900. -
Ernest Rutherford
Starting in 1898 Rutherford studied the radiation emitted by uranium. He discovered two different types of radiation, which he named alpha and beta. By allowing radiation from uranium to pass through an increasing number of layers of metal foil, he discovered that beta particles have greater penetrating power than alpha rays. Also by the direction of their movement in a magnetic field, he deduced that alpha particles are positively charged. -
Ernest Rutherford
Rutherford's model shows that an atom is mostly empty space, with electrons orbiting a fixed, positively charged nucleus in set, predictable paths. This model of an atom was developed by Ernest Rutherford. -
Niels Bohr
Firstly, Bohr wanted to understand the behavior of electrons in the atom. His theoretical physics research focused on understanding the electron’s place in the atom. He looked to the new quantum physics of Max Planck and Albert Einstein for a solution to the behavior of electrons. Bohr couldn't find a way of bringing the quantum theory to work. Until he took Balmer’s formula and used the new quantum theory. -
Niels Bohr
In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. -
James Chadwick
In 1923 he became Rutherford’s Assistant Director of Research and continued to study the atomic nucleus.In his spare time, through the 1920s, Chadwick made varieties of attempts to find these neutral particles. Chadwick at this time did not believe he had discovered a new elementary particle. He believed the neutron to be a complex particle consisting of a proton and an electron. In 1935, James Chadwick received the Nobel Prize in Physics for his discovery of the neutron. -
James Chadwick
This model introduced the concept of sub-energy levels. Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. -
