Development of the Atom

The Hellenic Market

Fire Water Earth Air

~~

AnaxagorasAnaxagoras (Greek, born 500 B.C.)–Suggested every substance had its own kind of “seedsseeds” that clustered together to make the substance, much as our atoms cluster to make molecules.

Some Early Ideas on Matter

O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,

EmpedoclesEmpedocles (Greek, born in Sicily, 490 B.C.)–Suggested there were only four basic seeds – earth, air, fire, and water– earth, air, fire, and water. The elementary substances (atoms to us) combined in various ways to make everything.

Democritus (Thracian, born 470 B.C.)–Actually proposed the word atomproposed the word atom (indivisible) because he believed that all matter consisted of such tiny units with voids between, an idea quite similar to our own beliefs. It was rejected by Aristotle and thus lost for 2000 years.

AristotleAristotle (Greek, born 384 B.C.)–Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements– heat, cold, dryness, moisture – as basic elements which combined as shown in the diagram (previous page).

Hot + dry made fire; hot + wet made air, and so on.

The GreeksHistory of the Atom

• Not the history of atom, but the idea of the atom

• In 400 B.C the Greeks tried to understand matter (chemicals) and broke them down into earth, air, fire, and water.

• Democritus and Leucippus Greek philosophers

Greek Model

• Greek philosopher• Idea of ‘democracy’• Idea of ‘atomos’

– Atomos = ‘indivisible’– ‘Atom’ is derived

• No experiments to support idea

• Continuous vs. discontinuous theory of matter

Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

Democritus

“To understand the very large,

we must understand the very small.”

Mental Experiment – Atoms Exist

• Looked at beach

• Made of sand

• Cut sand - smaller sand

Smallest possible piece? Atomos - not to be cut

Contributionsof alchemists:

Information about elementsInformation about elements - the elements mercury, sulfur, and antimony were discovered- properties of some elements

Develop lab apparatus / procedures / experimental techniquesDevelop lab apparatus / procedures / experimental techniques - alchemists learned how to prepare acids. - developed several alloys - new glassware

Dalton Model of the Atom

Late 1700’s - John Dalton- England

Teacher- summarized results of his experiments and those of others

Combined ideas of elements with that of atoms in Dalton’s Atomic Theory

Dalton’s Atomic Theory

1. All matter is made of tiny indivisible particles called atoms.

2. Atoms of the same element are identical, those of different atoms are different.

3. Atoms of different elements combine in whole number ratios to form compounds

4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

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Dalton’s Symbols

John Dalton 1808

Legos are Similar to Atoms

Lego's can be taken apart and built into many different things.

H

H

O

O

O

O

H

H

H

H

H

HH2

H2

O2

H2O

H2O

+

Atoms can be rearranged into different substances.

Thomson Model of the Atom

J. J. Thomson - English physicist. 1897

Made a piece of equipment called a cathode ray tube.

It is a vacuum tube - all the air has been pumped out.

J.J. Thomson

• He proved that atoms of any element can be made to emit tiny negative particles.

• From this he concluded that ALL atoms must contain these negative particles.

• He knew that atoms did not have a net negative charge and so there must be balancing the negative charge.

J.J. Thomson

Thomson Model

In the nineteenth century, Thomson described the atom as a ball of positive charge containing a number of electrons.

Plum-pudding model

Electron

Positivecharge

Source ofElectricalPotential

Metal Plate

Gas-filledglass tube Metal plate

Stream of negativeparticles (electrons)

A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Television Picture Tube

Fluorescent screen

Shadow mask

Glass window

Blue beam

Green beam

Red beam

Electron gun

Electronbeam

Deflectingelectromagnets

Fluorescentscreen withphosphor dots

Red beam

Green beam

Blue beam

Shadow mask

Ernest Rutherford (1871-1937)

• Learned physics in J.J. Thomson’ lab.

• Noticed that ‘alpha’ particles were sometime deflected by something in the air.

• Gold-foil experiment

Rutherford

PAPER

Rutherford

PAPER

Animation by Raymond Chang – All rights reserved.

Rutherford ‘Scattering’

• In 1909 Rutherford undertook a series of experiments• He fired (alpha) particles at a very thin sample of gold foil• According to the Thomson model the particles would only

be slightly deflected• Rutherford discovered that they were deflected through large

angles and could even be reflected straight back to the source

particlesource

Lead collimator Gold foil

What he expected…

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What he got…richochetingalpha particles

• Hit moth driving car – no change in car direction

• Hit deer – car changes direction

Alpha particle

Large angle of deflection, must have hit massive object!

moth

deerGold Atom

Bohr Atom

The Planetary Model of the Atom

Quantum Mechanical Model

Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

Niels Bohr &Albert Einstein

Modern View

• The atom is mostly empty space• Two regions

– Nucleus • protons and neutrons

– Electron cloud• region where you might find an electron

Democritusc460-371 BC

J.J. Thomson1856-1940

Link One - The original paper in which J.J. Thomson announces his discovery of the electron to the world.

Link Two - Thomson on the number of corpuscles (electrons) in an atom.Link Three - Thomson on the structure of the atom.Link Four excerpts from Thomson's Nobel prize address

Lord Ernest Rutherford1871-1937

Link One - Geiger's paper on the gold foilLink Two - Rutherford describing the gold foil experimentLink Three - Rutherford's paper on the structure of the atom

Robert Millikan1868-1953

Niels Bohr1913-1963

Link One - Bohr's address on the spectrum of hydrogenLink Two - An article on atomic structure written by Niels Bohr

James Chadwick1891-1974

Link One - A letter on the possible existence of the neutronLink Two - Chadwick's paper on the discovery of the neutron

Werner Heisenberg

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson32.htm