Aristotle (350 BC)

• Believed that matter was made up by 4 properties; Hot, Cold, Dry, Wet

• Questioned Democritus’s Ideas

• Believed that matter was infinitely divisible

• Ideas are summarized as a grave stone

• Caused the death of atomic theory for 2,000 years

Democritus (400 BC)

• First to propose that matter was not infinitely divisible

• Matter was composed of individual particles known as Atomos

• Atomos - Uncuttable, indivisible which refers to the smallest piece of matter

• Believed that atoms cannot be created, destroyed or further divided

• Views are modeled as Legos because they make the foundation of atomic theory

• Each of these atoms was like a Lego block: eternal, immutable, and indivisible

• Atoms come in a variety of different shapes and sizes (makes the diversity if the natural world possible)

• Reconciled the previous 2 ideas with his theory of atoms

• he held that everything in the world was made of tiny invisible blocks which join and separate to make different things

Alchemists

• Dominated the field of “Chemistry” for 2000 years

• Believed they could turn cheap metals into gold

• Used Observations and Experimentation

• Discovered several elements

Robert Boyle (1627 - 1691)

• Irish Chemist, Physicist, inventor, and scientist

• Dealt with the behavior of gases

• Believed in the idea of chemical elements

• Greek system of four elements finally died

• First chemist to preform truly quantitative experiments

Antoine Lavoisier (1743 - 1794)

• Law of conservation of mass

• Found that nitrogen gas and oxygen can be chemically separated from certain compounds

• Confirmed law of definite proportions

• Father of modern chemistry

• Lost his head in the French Revolution

Joseph Proust (1754-1826)

• Showed that the proportion by mass of the same elements in a compound is always the same (Now called the Law of Definite Proportions)

• Part of our definition of a chemical - any substance with a definite composition

John Dalton (1766 - 1844)

• Best known for is Atomic Theory

• Published the first table of atomic masses

• Dalton’s model of a atom was the “billiard ball”

• Made the Law of Multiple Proportions - When two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers

John Dalton’s Atomic Theory

1. All matter is composed of very small particles called atoms

2. Atoms of a given element are identical (Not true - isotopes)

3. Atoms cannot be created, destroyed or subdivided (not true - sub particles ie: e- , p+ , n*) (* = neutral)

4. Chemical compounds are formed when atoms of different elements combine with each other

5. Chemical reactions involve reorganization of the atoms - changes the way they are bound together (atoms themselves are not changed in a chemical reaction)

JJ Thomson (1856 - 1940)

• Used a Cathode Ray to prove the existence of negatively charged particles (AKA electrons)

• Thomson’s model of an atom is called “plum pudding”

• Won the 1906 Nobel Prize

• Came up with the charge to mass ratio of the particles (electrons) - (-1.76×10^8 C/g)

Robert Millikan (1868 - 1953)

• Used oil drops to determine the magnitude of the charge on a single electron

• Calculated the mass of an electron to be 9.11x10^-31 Kg

• Found the charge of the electron to be -1.6×10^-19

• Won Nobel Prize in 1923

Ernest Rutherford (1871 - 1937)

• Used the Gold Foil Experiment in 1911

• Highly concentrated center of positive charge (Nucleus)

• The atom is mostly empty space

• Rutherford’s Model of an atom was called “Cherry with a pit”

Eugene Goldstein (1850 - 1930)

• Discovered the proton

James Chadwick (1891 - 1974)

• Discovered the neutron

Bohr (1913)

• Said that the electrons are on different levels

• Like the planets

Schrodinger (1926 - Present)

• Todays model of the atom called the Quantum Mechanical Model

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