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Antoine Lavoisier
Discovered when a chemical change occured in a closed system, the mass did not differentiate. He then concluded that matter cannot be created or destroyed. This became known as the law of conservation of mass. -
Period: to
Scientists over the ages
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Joseph Proust
Noticed substances always have elements in the same ratio as mass. Became known as the law of definite proportions. -
John Dalton
Hypothesized that atoms combine to form compounds. This later lead to another hypothesis known as the law of multiple proportions, stating that the ratio of masses of one element combined with that of another element can be expressed in small whole numbers. -
Dmitri Mendeleev
Created the periodic law, that states properties of elements were their own periodic functions of their own weights. He also arranged the elements into 7 different groups. -
G. J. Stoney
Hypothesized that electricity was made of negative particles. He called them electrons. -
Henri Becquerel
Found that rays are given off by the elements uranium and radium. This helped better the understanding of radioactivity, the phenomenon of rays being produced spontaneously by unstable atomic nuclei. -
J.J. Thomson
Discovered that cathode rays consist of electrons. -
Max Planck
Creates the Quantum Theory which states that instead of energy being given constantly,it is given off in little packets called quanta. -
Robert Millikan
Obtained the first accurate measurement of an electron's charge by using a device consisting of an atomizer, charged plates, and a microscope. -
Albert Einstein
Explains the origin of energy released during nuclear changes. It later became the very commonly used formula, E=mc squared. -
A Team of Scientists: Niels Bohr, Hans Geiger, Ernest Marsden
Created many experiments to better understand the atom. Concluded that the atom was mostly empty space, and that the core of the atom, now called the "nucleus", was extremely smaller than it was thought to be. -
Henry Moseley
Found that wavelength of the X rays is characteristic of the metal used as the anode. An anode was a positive electrode. This implied that the wavelength depends on the number of protons in the nucleus of the atom and is always the same for a given element. -
Lord Rutherford
Predicted there was a third particle within the atom. This theory was later proved by another scientist. -
Walter Bothe
Created the first evidence of a third particle within the atom. This third particle would be known later as a neutron. -
James Chadwick
Repeated the work of a scientist called Walter Bothe and found high energy particles that had no positive or negative charge with the same mass as the proton. These are now known as neutrons.
