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2 Theories of Matter

Continuous: Aristotle
Discontinuous: Democritus; world was made up of empty space and tiny particles called atoms, which are indivisible


"Father of modern chemistry" responsible for the law of conservation of mass


Law of Definite Proportions: for a substance the elements are always in the same ratio by mass

Dalton's Atomic Theory (1803)

1. Matter is composed of atoms that cant be broken down
2. All atoms of the same element are alike
3. Atoms of different elements are different
4. Atom can combine in different ratios to form compounds

Atom was an empty circle


noted under comstant comditions volumes of gas reactants and products were in small whole # ratios

Avagrado's Hypothesis

Equal volumes of gases under the same temperature and pressure have equal number of molecules

William Crooke (1870's)

developed the Crooke's Tube or the Cathode Ray Tube

J.J. Thomson (1897)

discovered electrons; placed the cathode ray tube in an electric field which caused the rays to bend toward the positive plate, and away from the negative plate; concluded rays were negatively charged

J.J. Thomson (1987)

e/m charge to mass ratio of an electron


"oil drop experiment" determined the charge of electrons
1.60 x 10^-19 columb
mass of electron 9.11 x 10^-28 g

Electron Mass


Rutherford "Gold Foil Experiment" (1911)

Alpha particle- helium nucleus
Came up with idea of an atom with a positiveky charged nucleus


discovered protons

Chadwick (1932)

discovered Neutrons

Roentgen (1895)

discovered x-rays


a combination of rays and particles by spontaneous breakdown of atoms' nuclei

Becquerel (1896)

discovered radioactivity by using Uranium rocks. The rocks in a dark drawer caused image on film

Ways to detect radiation (3)

Gieger Counter

Rutherford (before 1909 experiment)- discovered 3 tupes pf radiation

Beta (high energy electrons)
Gamma Ray energy

Bohr (1913)

modified Rutherford's model
proposed electrons were located in energy levels with a positive nucleus


atoms of the same element, with same # of protons but different # of neutrons

Mass Number

total number of protons and neutrons

Atomic Number

# of protons (measured by Mosely)

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