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417 BCE
plato
Plato theorizes that these solid forms of matter are composed of indivisible elements shaped like triangles (contrary to the popular conception of atoms as spheres). He believes triangles must be the correct form because they can join together to make very different kinds of shapes. 417 bce -
332 BCE
aeristotle
He did not believe in atomic theory, unlike Democritus, and thought that all elements on Earth were not made of atoms except for Earth, Fire, Water and Air, themselves. Aristotle believed that all substances were made of small amounts of these four elements of matter 332 bce -
robert boyle
He believed that everything was composed of very tiny particles, an idea known as atomism. In this early atomic theory of Boyle, he referred to these tiny particles as corpuscles. He conducted many chemical experiments to try and understand more about the nature and behavior of these corpuscles. 1627 -
john dalton
A theory of chemical combination, first stated by John Dalton in 1803. It involves the following postulates: (1) Elements consist of indivisible small particles (atoms). (2) All atoms of the same element are identical; different elements have different types of atom. (3) Atoms can neither be created nor destroyed. 1803 -
newlands law of octaves
If the chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements. 1865 -
mendeleevs pd. table
Mendeleev had proposed a periodic table based on the mass number and properties of elements. Elements with similar properties were placed in the same group and arranged to increase atomic masses. He predicted some elements that were yet to be discovered and their properties too. 1869 -
photoelectric
By placing a negative charge on the electroscope, and shining the short wave UV light on top, it will discharge. 1887 -
discovery of radioactivity
The radioactivity of uranium was discovered in 1896 by Henri Becquerel who, starting from a wrong idea, progressively realized what he was observing, regularly informing the French Academy of Sciences of the progress he was doing. 1896 -
plum pudding model
the electrons were embedded in a uniform sphere of positive charge, like blueberries stuck into a muffin. 1904 -
robert millikan
Robert Millikan was a physicist who discovered the elementary charge of an electron using the oil-drop experiment. 1910 -
bohrs planetary model
the electrons encircle the nucleus of the atom in specific allowable paths called orbits. 1913 -
discovery of the proton
The discovery of the proton is credited to Ernest Rutherford, who proved that the nucleus of the hydrogen atom (i.e. a proton) is present in the nuclei of all other atoms in the year 1917. Based on the conclusions drawn from the gold-foil experiment, Rutherford is also credited with the discovery of the atomic nucleus. 1917 -
heisenberg uncertainty principle
we cannot know both the position and speed of a particle, such as a photon or electron, with perfect accuracy 1919 -
rutherfords gold foil experiment
Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus. Based on these results, Rutherford proposed the nuclear model of the atom. 1927 -
discovery of a neutron
In 1932, the physicist James Chadwick conducted an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. 1932