History of Atomic Structure

Aristotle

Democritus

Ancient Philosophy• Who: Aristotle, Democritus

• When: More than 2000 years ago 460 BC

• Where: Greece

• What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.

• Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

• Any experimental support for ideas?

Alchemists• Who: European Scientists

• When: 800 – 900 years ago

• Where: Europe

• What: Their work developed into what is now modern chemistry.

• Why: Trying to change ordinary materials into gold.

Alchemic Symbols

John Dalton

Particle Theory• Who: John Dalton

• When: 1766-1844

• Where: England, school teacher

• What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.

• Why: Building on the ideas of Democritus in ancient Greece.

4 Part Atomic Theory• 1. Elements are composed of tiny indivisible

particles (atoms).

Does this part hold true today?

• 2. Atoms of the same element are identical

and different from other atoms.

Does this part hold true today?

3. Atoms can physically mix and chemically combine in whole number ratios.

• Does this part hold true today?

4. In chemical reactions, atoms separate, join and rearrange.

Atoms cannot be changed into other atoms.

Does this part hold true today?

• Did Dalton have experimental support for his theory?

Dalton’s Solid Sphere Model of the Atom

Discovery of Electrons• Who: J. J. Thompson

• When: 1897

• Where: England, physicist

• What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged.

• Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

J. J. Thompson

• Thomson’s Cathode Ray Tube

• Thomson’s Plum Pudding Model of the Atom

• Who: Robert Milliken

• When: 1868-1953

• Where: United States, physicist

• What: Conducted an experiment to determine the quantity of charge on an electron, 1-, in his famous oil drop experiment

• Why: He knew that atoms had electrons, wanted to determine charge

• Robert Milliken

Atomic Structure I• Who: Eugene Goldstein

• When: 1850-1930

• Where: England

• What: Conducted an experiment with a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles- protons.

• Why: He concluded that they were composed of positive particles- protons

Atomic Structure I• Who: Ernest Rutherford

• When: 1911

• Where: England

• What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.

• Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Ernest Rutherford

• Who: James Chadwick

• When: 1932

• Where: England, physicist

• What: confirmed existence of the neutron

• Neutrons have no charge and have a

mass nearly equal to that of a proton

Atomic Structure II• Who: Niels Bohr

• When: 1913

• Where: England

• What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.

• Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

Niels Bohr

Electron Cloud Model• Electrons travel around the nucleus in

random orbits.

• Scientists cannot predict where they will be at any given moment.

• Electrons travel so fast, they appear to form a “cloud” around the nucleus.

Electron Cloud Model