The Atomic Model Through Time

(see p. 54-55 in text)

Aristotle Aristotle (460 B.C. – 370 B.C.)(460 B.C. – 370 B.C.)

• emphasized that nature consisted of four elements: air, earth, fire, and water.

• he did not believe in discontinuous or separate atoms, but felt that matter was continuous

Democritus (460 B.C. – 370 B.C.)Democritus (460 B.C. – 370 B.C.)

• greek philosopher• first to suggest the

existence of “atoms” (Greek word “atomos” = indivisible)

• atoms indivisible and indestructible

• no experimental support

http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg

John Dalton (1766-1844)John Dalton (1766-1844)

• english chemist and schoolteacher• used scientific method to test Democritus’s

ideas• Dalton’s atomic theory

1. elements composed of atoms2. atoms of the same element are alike3. different atoms can combine in ratios to form

compounds4. chemical reactions can occur when atoms are

separated, joined, or rearranged (but atoms are not created nor destroyed)

J.J. Thompson (1856-1940)J.J. Thompson (1856-1940)

• English physicist• discovered the

electron• thought atom was

negative charges stuck in a positive charged lump– referred to as the

“plumb pudding model”

We still use Thompson’s “cathode ray tube” in TV and

other applications

Robert A. Millikan (1868-1953)Robert A. Millikan (1868-1953)

• American physicist that continued Thompson’s work on electrons

• Found the quantity of charge carried by an electron (one unit of negative charge)

• Calculated the mass of an electron (1/1840th the mass of a hydrogen atom)

Ernest Rutherford (1871-1937)Ernest Rutherford (1871-1937)

• New Zealand physicist and former student of Thompson

• proposed that the atom is mostly empty space

• positive charges and almost of the mass are in a small, centralized region called the nucleus

“Like howitzer shells bouncing off of tissue paper!”

Evidence for Rutherford’s Evidence for Rutherford’s ConclusionsConclusions

http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Generalchemistry/Atomic/BasicStructure/ErnestRutherford/ruther.gif

Rutherford Flash Animation

Try it Yourself!Try it Yourself!In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The AnswersThe Answers

Target #1 Target #2

Niels Bohr (1855-1962)Niels Bohr (1855-1962)

• former student of Rutherford• electrons found only in

specific circular paths (orbits) around the nucleus

• based on information about how the energy of an atom changes when it absorbs and emits light

• called these fixed energies “energy levels”

Erwin Schrodinger (1926)Erwin Schrodinger (1926)

• Austrian physicist • quantum

mechanical model – probability of

electron locations around the nucleus

– not an exact orbit• led to the electron

cloud model

Werner Heisenberg (1927)Werner Heisenberg (1927)

• German physicist• Heisenberg

Uncertainty Principle – impossible to know the exact position and velocity of a particle (electron) at the same time

• “the observer affects the observed”

http://www.deutsches-museum-bonn.de/ausstellungen/heisenberg/bilder/heisenberg_2.jpg