Thomson Model (English)

2-4: Thomson's Atomic Model

[J. J. Thomson's raisin bread model (plum pudding model)]

J. J. Thomson

considered

that the structure

of an atom is

something like a

raisin bread,

so that his atomic model

is sometimes called

the raisin bread model.

He assumed

that the basic body

of an atom is

a spherical object

containing N

electrons confined

in homogeneous jellylike

but relatively massive positive

charge distribution

whose total charge cancels

that of the

N electrons.

The schematic drawing

of this model is shown

in the following figure.

Thomson's model is sometimes

dubbed a

plum pudding model.

[Thomson's Model and Alpha Particle Scattering]

As stated

on the preceding page,

Geiger and Marsden

carried out an experiment

in which

the alpha rays

were collided

against a thin metal foil.

Their results showed

that almost all incident alpha

particles penetrate

the foil and go straight

in forward direction

but a few are scattered in very large angles.

Is it possible

to explain this result

by using

Thomson's model?

This question will be

studied below,

but

before explaining it,

let us present

the answer in advance:

The results of the alpha particle scattering cannot be explained by Thomson's atomic model.

Thomson's raisin bread model

(plum pudding model)

therefore cannot

be valid

as an atomic model.

The reason will be

explained below.

Since we need somewhat

detailed mathematical expressions,

it will be given

on the other page,

2-4-A: Alpha Particle Scattering by Thomson's model

As seen

on the other page (2-4-A),

we can consider that

the scattering angle

of the alpha particle

by Thomson's model

is at most 0.01 degrees.

The thickness of

the metal foil

in the scattering experiment

of the alpha rays

is about 10^-6 m.

When assuming

that the atoms are

tightly packed

in the metal,

there are about 10000 atoms

lining up

in the direction

of thickness,

because the size

of an atom is

approximately 10^-10m.

(See the following figure.)

When the alpha particle

collides with these atoms

in the metal foil

10000 times successively,

the scattering angle

of each individual collision

in such a multiple scattering

is less than 0.01 degrees

as discussed above.

The resultant scattering angle

is obtained

by an accumulation of

these individual scatterings

of 10000 times.

One may expect that,

even if the scattering angle

of each individual scattering

is very small like 0.01 degrees,

we can have

as large resultant angle

as

This is however

unrealistic,

because the direction

of each individual scattering

must be random,

and an accumulation

of random values would give

nearly zero only.

So that we never obtain

such a large resultant scattering

angle after the multiple

scattering.

Accordingly,

such a large scattering angle

as those obtained

in Geiger and Marsden's experiment

cannot be reproduced

by such a multiple

scattering as stated above.

Thus we can conclude that

Thomson's atomic model is not held.

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