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400 BCE
Democritus
introduced the concept of the atom as the basic building block of all matter -
370 BCE
Aristotle rejected Democritus' idea
Aristotle rejected Democritus' idea and his opinion was accepted for more than 2000 years. The idea of the atom was revived around 1800 by the English scientist John Dalton -
John Dalton
He imagined atoms as tiny, solid balls. In this model, atoms are indivisible and indestructible. This means they can't be split into smaller parts and they don't break. -
assigned atomic weights to elements
assigned atomic weights to the atoms of the 20 elements at the time. andnwould contribute to the development of the periodic table later in the 19th century. -
Michael Faraday
Faraday discovered the laws of electrolysis, a process where an electrical current run through water or another substance would separate the molecules of the substance into their component atoms. -
Eugen Goldstein
Eugene Goldstein discovered evidence for the existence of this positively charged particle. Using a cathode ray tube with holes in the cathode, he noticed that there were rays traveling in the opposite direction from the cathode rays. -
JJ Thomson
In 1897 Thomson discovered the electron and then went on to propose a model for the structure of the atom. Plum pudding model depicts the electrons as negatively-charged particles embedded in a sea of positive charge. -
Henri Becquerel discovered radioactivity
Henri Becquerel discovered radioactivity when he observed that uranium salts emitted rays that could fog photographic plates. Marie and Pierre Curie advanced this discovery in the following years, identifying radium and polonium as radioactive elements -
Ernest Rutherford
This model was based on the results of his famous gold foil experiment, which showed that atoms have a dense, positively charged nucleus at their center. -
Robert Millikan
By measuring how the droplets responded to the electric field, Millikan was able to deduce that the charge on each droplet was a multiple of a fundamental unit. This fundamental charge was later determined to be the charge of a single electron, which was approximately 1.602 × 10⁻¹⁹ coulombs. -
Neils Bohr
Bohr built on Rutherford’s nuclear model by proposing that electrons occupy specific, discrete orbits or energy levels around the nucleus. Electrons in these orbits do not radiate energy and remain stable, thus avoiding the collapse predicted by classical physics. -
theory of wave-particle duality was preposed
Louis de Broglie proposed his theory of wave-particle duality, suggesting that particles like electrons also have wave-like properties. This discovery helped cement the quantum mechanical model of the atom, which superseded Bohr’s model and contributed to the development of modern physics. -
Werner Heisenberg
Developed the Uncertainty Principle, stating that the position and momentum of a particle cannot both be precisely determined. -
Edwin Schroedinger
Developed the "electron cloud" model, proposing that electrons do not have fixed paths but exist as a cloud of probability. -
discovery of the neutron
The discovery of the neutron by James Chadwick. -
James Chadwick
Discovered the neutron, adding it to the "electron cloud" model of the atom. -
WWll begins
World War II began, fundamentally changing the course of history.