Atomic Theory 1

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Atomic Theory

Theories in science are proposed to explain the evidence available at the time.

As new evidence is discovered theories are adapted to explain new data.

This is the nature of Science.

In the future this will continue.

DemocritusPlato

460-370 BC

Atoms make up the world

Aristotle

Plato

384-322 BC

Four Elements

Teacher of Aristotle

Aristotle

Wind

Fire

Water

Earth

dryness

wetness

coldness hotness

conflictharmony

The Four Element Theory PropertiesForces

The Four Element Theory lasted for about 2 thousand years because no one tested the theory with scientific experiments.

It was not a scientific theory- which is tested by experiment.

John Dalton

1766-1844

Atomic Theory

Dalton’s Atomic Theory- 1808

Evidence1. The Law of Conservation of Mass2. The Law of Constant Composition- water is 11 % H

and 89 % OTheory1. Each atom is an indestructible and unique spherical

particle2. Atoms combine in simple whole number ratios to

form compounds

Hydrogen

Oxygen Lithium

Dalton Theory- Water

1 g H1 g H

16 g O 18g O

16 g O

1 g H 1 g H

Dalton Theory- Water

18g O

= 11% H

16 g O

1 g H 1 g H

Dalton Theory- Water

2 g H

18 g H2Ox 100% 89 % O

Still11 % H89 % O

Having a molecule (atoms combine in simple whole # ratios) explains the law of constant composition.

No matter how much water you have its still 11% H

2H2 + O2 2H2O

If the atoms are not destroyed then the mass does not change

J. J. Thomson 1871-1937J. J. Thomson 1871-1937

The Thomson Theory of the AtomEvidence1. The electrical nature of matter

beta particlesstatic electricitycurrent electricity

Theory1. The atom is made up of positive material with negative particles throughout- like blueberries in a blueberry muffin.

+ + - + - + +

- + -+ - + - + - + + - +

+ - + -+ - + - +

+ - + -

The Thomson Atom 1903

Ernest Rutherford 1871-1937Ernest Rutherford 1871-1937

The Rutherford Atom 1911SimulationEvidence: Gold Foil Experiment1. 99.99 % of alphas are not deflected2. 0.01 % of alphas are radically deflectedTheory1. Most of the atom is empty space.2. There is a small dense nucleus in the center of the

atom that makes up most of the mass. Electrons circle the nucleus randomly.

The Rutherford Atom 1911Scale: nucleus is home plate; atom is the baseball field

Be

-

-

-

-Nucleus- 4 protons and 5 neutrons

Electrons

Nucleus- is small but has most of the mass

Cornelis Escher 1898-1972Cornelis Escher 1898-1972

Discovery of the Nucleus

Size of the Atom