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300
Greek Philosophers (300 B.C.)
In the mid 300 B.C. era, "earth, air, fire, and water" were believed to be the sole elements. This theory was popularized by philosophers such as Aristotle -
400
Democritus (400 B.C.)
Hypthosized through philosophical reasoning that matter was composed of small, indivisible particles called atoms. The size, weight, and shape of these particles explained the differences in matter. -
Lavoisier
How to illustrate the scientific idea of conservation of mass
Observed that the mass of a chemical was the same before a chemical reaction in a sealed container as it was after the reaction. This led to the Law of Conservation of Mass which states that mass can neither be created nor destroyed, only changed. -
Proust
Chemistry 5.8a Law of Definite Proportions
Observed that the compostion of water is always 11% hydrogen and 89% oxygen by mass. This led to the Law of Definite Proportion which states that elements that compose a compound are always in a certain proportion by mass. -
Dalton and and the Atomic Theory
John Dalton's Atomic TheoryDalton, the father of atomic theory, proposed his atomic theory in 1803:
1. Matter is composed of indivisible particles known as atoms. 2. Atoms of the same elements are identicle, and atoms of any one element are different from those of another element. 3. Matter cannot be destroyed or created, only changed.
4. Elements that compose compounds are always in certain proportions by mass and atoms of different elements can combine to form compounds. -
Eugen Goldstein (expirementation from 1869-1896)
He used a tube filled with hydrogen gas to confirm the existance of positively charged particles called protons. He found that the mass of a proton was 1.67x10-24 grams (0.00000000000000000000000167 grams!). To make calculations easier, he defined this as one atomic mass unit (1 amu). -
J.J. Thomson (expirementaiton from 1895-1897)
J.J. Thompson's expirement
He experimented with a cathode ray tube which was a tube with gas and a completed electrical circuit (like a neon sign). He found that the rays bent toward a positively charged plate and bent away from a negatively charged plate. This led to the discovery of the electron. -
Robert Milikan (experimentation from 1909-1911)
Oil Drop Experiment
Milikan's Oil Drop Experiment concluded electrons are negatively charged and that the mass of one electron is 1/1840 the mass of a proton- it essentially has no mass. -
Rutherford (expirementation from 1909-1911)
Gold Foil Experiment
Prior to Rutherford's expirement, scientists hypothesized the atomic structure to look like plum pudding (a solid with a positive mass, dotted with negatively charged electrons). Rutherford found, however, after his gold foil experiment that atoms are mostly empty space, have a small dense center called the Nucleus, and that Nucleuses are positively charged. -
James Chadwick
Due to the fact that atoms of neon were found to have different masses there must be a third particle that effects the mass but has no charge. This third particle is called the neutron and was found by Chadwik in 1932.
