1. Main Menu Great Pyramids 2900 BCE Atomic Theory Founders of
Law of Conservation of Mass
1785
Atomic Theory of Matter
1803
Law of Definite
Proportions
1797
Radioactivity
Value of Pi
240 BCE
Algebra
820
Admission charged
for a concert
1672
Civil War
1st McDonald’s
1955
440 BCE
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2. Leucippus and Democritus
Leucippus of Miletus (circa B.C.E.) originated the concept of the atom
Democritus of Abdera (circa 460 –371 B.C.E.) was a pupil of Leucippus.
Atomic Theory
Matter is made up of indivisible particles called atoms
- atom comes from the Greek “atomos” meaning uncuttable
Atoms are completely solid with no internal structure
Atoms differ in size, shape and weight

3. Total mass of Reactants
Antoine Lavoisier
Born 1743 – Died 1794
French Chemist
Law of Conservation of Mass
First modern chemical textbook
Law of Conservation of Mass
In a chemical reaction, mass is neither created nor destroyed
Total mass of Reactants
Total mass of Products =

4. Joseph Proust Born 1754 – Died 1826 French Chemist
1797 Law of Definite Proportions (also known as Law of Constant Composition)
Law of Definite Proportions
Showed that copper carbonate must always be composed of the same fixed proportions of copper, carbon and oxygen.
Generalized that the elements in all compounds are present in fixed proportions by weight.

5. John Dalton Born 1766 - Died 1944 British Chemist and Physicist
Developed the atomic theory of matter
Dalton’s Atomic Theory
Each element is composed of extremely small particles called atoms
All atoms of a particular element are identical, atoms of different elements have different properties
Atoms are neither created nor destroyed in a chemical reaction
Compounds contain atoms of two or more elements

6. Wilhelm Roentgen Born 1845 – Died 1923 German Physicist
Awarded Nobel Prize in 1901 for the discovery of the X-rays
1895 – discovered X-rays (called them Roentgen’s rays)
While experimenting with a Crookes tube, he discovered a new type of ray emitted from the tube which would cast shadows of solid objects
These rays pass through most substances, including the soft tissue of the body, and leave bones and solid metals visible

7. Antoine-Henri Becquerel
Born 1852 – Died 1908
French Scientist
1896 – discovered radioactivity as a spontaneous emission of radiation by a material and called the emissions becquerel rays.
Observed the image of uranium crystals on photographic plates.
Showed that this radiation differed from X-rays because it could be deflected by a magnetic field and therefore must contain charged particles.

8. Marie Curie Pierre Curie Born 1867 – Died 1934
1898 – observed that thorium compounds also emit becquerel rays.
Invented the word “radioactivity” to describe this behavior.
Pierre Curie
Born 1859 – Died 1906
1898 – Together they discovered polonium and radium by fractionation of pitchblende, which is an ore (metal-bearing mineral mass) which has a uranium yield between 50-80%.

9. Ernest Rutherford Born 1871 – Died 1937
1899 – discovered alpha and beta rays from uranium
Exposed a beam of both alpha and beta rays to a magnetic field.
The beta rays bent in the field, the alpha rays did not.
(Bending in a magnetic field is the sign of charged particles)
Beta radiation is a stream of particles not a ray.
The charge-to-mass ratio of beta radiation was shown to be the same as that of a beam of electrons
Beta radiation is a stream of fast-moving electrons

10. Rutherford (continued)
1902 – was able to bend the beam of alpha radiation in both an electric and magnetic field proving it was a stream of particles and not a ray.
Alpha particles deflect in the opposite direction of beta particles and are therefore positively charged
Later found that the charge-to-mass ratio of an alpha particle is the same as that of a hydrogen ion
Alpha particles have a mass that is twice that of a hydrogen atom.
1908 – Rutherford established that alpha particles were helium atoms with two electrons missing

11. Paul Villard X-rays -> 102 – 10-3 nm
1900 – discovered gamma rays
They were later given the name gamma rays by Rutherford.
The gamma rays were emitted from radium and were not affected by electric or magnetic fields
Recognized them as being different from X-rays because they had a much greater penetrating depth
This is due to gamma rays having a much shorter wavelength than X-rays.
X-rays -> 102 – 10-3 nm
gamma rays -> 10-1 – 10-5 nm