Part I

Leucippus and Democritus (460 to 370 BC) – believed that all matter was made up of tiny particles called atoms.

Aristotle (384 to 322 BC) – believed matter was made from the four elements of earth, air, fire, and water.

John Dalton - England - Late 1700’s

Teacher – he summarized results of his experiments and those of other scientists.

Dalton stated that elements consisted of tiny particles called atoms

He also called the elements pure substances because all atoms of an element were identical and that in particular they had the same mass.

He also said the reason why elements differed from one another was that atoms of each element had different masses.

He also said that compounds consisted of atoms of different elements combined together.

Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

1. Elements are made of tiny particles called atoms.

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

3. Atoms can neither be divided, created or destroyed.

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

5. Chemical reactions involve the rearrangement of atoms.

California WEB

Scientist began to wonder what an atom was like.

Was it solid throughout with no internal structure or was it made up of smaller, subatomic particles?

It was not until the late 1800’s that evidence became available that atoms were composed of smaller parts.

Thomson Model of the Atom

J. J. Thomson - English physicist. 1897Made a piece of equipment called a cathode

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

pumped out.

A Cathode Ray Tube

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

Thomson’s Experiment

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vacuum tube

metal disks

voltage source

Thomson’s Experiment

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ON

Passing an electric current makes a beam appear

to move from the negative to the positive end

Thomson’s Experiment

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ON

Thomson’s Experiment

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ON

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By adding an electric field…

he found that the moving pieces were negative.

Cathode Ray Tube Video

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 something balancing the negative charge.

J.J. Thomson

Thomson Model of the Atom

J.J. Thomson discovered the electron and knew that electrons could be emitted from matter (1897).

J.J. Thomson adapted an earlier atomic model proposed by William Thomson (Lord Kelvin) which showed that atoms consist of small, negative electrons embedded in a massive, positive sphere.

The electrons were like currants in a plum pudding.This is called the ‘plum pudding’ model of the atom.

electrons-

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