Scientists

Aristotle

350

Matter was continuous and was not made up of smaller particles-hyle.

Democritus

400

The world was made of two things-empty space and tiny particles called atoms. Atoms are the smallest possible particles of matter. There are different types of atoms for each material in the world.

Isaac Newton and Robert Boyle

Oct 16, 1600

Published articles stating their belief in the atomic nature of elements. Their works had no proof. They were attempted explanations of the known, with no predictions of the unkown.

John Dalton

Sep 16, 1800

Studied experiemental observations made by others concerning chemcial reactoins. All matter is composed of very small particles called atoms, and that these atoms cannot be broken down into smaller particles. Atoms were simpler than particles of air or rock. All atoms of an element were exactly alike and that atoms of different elements were quite unlike. If atoms cannot be destroyed, then they must be simply be rearranged in a chemical change. Total numer/type of atoms remain the same.

Dalton2

Oct 17, 1800

Now he says all atoms of an element contain the same number of protons but they can contain different numbers of neutrons.

Lavoisier

Dec 17, 1810

When a chemical change occurred in a closed system, the mass after a chemical change equaled the mass before the change. A closed system is a system that cannot exchange matter with its surroundings.

J. L. Gay-Lussac

Feb 17, 1820

Studied gas reactions at constant temperature and pressure. Under constant conditions, the volume of reacting gases and gaseous products are in the ratio of small whole numbers.

Joseph Proust

Aug 17, 1820

observed that specific substances always contain elements in the same ratio by mass. No matter where the sample is obtained, how it is obtained, or how large it is, the ratio of the mass of sodium to the mass of chlorine never changes-law of definite proportions.

Amadeo Avogadro

Oct 17, 1830

equal volumes of gases, under the same conditions, have the same number of molecules.

Henri Becquerel

Oct 17, 1896

Matter containing uranium exposes sealed photographic film. Marie Curie then found that rays are given off by elements uranium and radium.

J. J. Thomson

Oct 17, 1897

Did reasearch on cathode rays. Cathode rays consist of electrons. Built a cahtode-ray tube to subject the rays to both a magnetic field and an electric field. Measured the bending of the path of cathode rays and was able to determine the ratio of an electron's charge to its mass. Also showed that rays consisted of a postive charge.

Robert Millikan

Oct 17, 1900

obtained the first accurate measurement of an electron's charge. As oil was sprayed from a brass atomizer, electrons were transferred from the atomizer to the oil droplets. Correctly concluded that the small charger on the droplet must be the charge of a single electron.

Albert Einstein

Oct 17, 1905

explained the origin of the energy released during nuclear changes. Einstein hypothesized that mass and energy are equivalent. E=mc^2.

Lord Rutherford and team

Oct 17, 1912

Subjected a very thin sheet of gold foil to a stream of positively charged subatomic particles. They found that most of the particles passed right through the sheet. The atom is mostly empty space. They also found that a few particles were deflected at large angles.

Henry Moseley

Oct 17, 1913

studied X rays produced in X ray tubes with anodes of different metals. He found that wavelength of the X rays is characteristic of the metal used as the anode.

James Chadwick

Oct 17, 1932

High energy particles with no charge and with essentially the same mass as the proton. These particles are now known as neutrons.