ca-inspire-chemistry-sampler

C04_018A

Electrons

Alpha particle paths

Evenly distributed

positive charge

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C04_019A

Gold foil

Beam of

alpha

particles

Most alpha particles

pass through the

foil with little or

no deflection

Lead block containing

an alpha-particle-

emitting source

Zinc-sulfide-

coated screen

Alpha particle

deflected at a

small angle

Alpha particle

deflected at a

large angle

Rutherford was aware of Thomson’s plum pudding model of the atom. He expected the

paths of the massive and fast-moving alpha particles to be only slightly altered by a

collision with an electron. And because the positive charge within the gold atoms was

thought to be uniformly distributed, he thought it would not alter the paths of the alpha

particles, either.

Figure 10

shows the results Rutherford expected from the experiment.

Figure 10

Based on Thomson’s model,

Rutherford expected the alpha particles to

pass through the gold atoms.

The actual results observed by Rutherford and his colleagues are shown in

Figure 11

Some alpha particles were deflected at various angles. Several particles were deflected

straight back toward the source. Rutherford likened the results to firing a large artillery

shell at a sheet of paper and the shell coming back at the cannon.

Figure 11

During Rutherford’s experiment, a beam of

alpha particles bombarded a thin gold foil. Most of the

alpha particles went through the gold foil. However, a few

of them bounced back, some at large angles.

Rutherford’s model of the atom

Rutherford concluded that the plum pudding

model was incorrect because it could not explain the results of the gold foil experiment.

Considering the properties of the alpha particles and the electrons, and the frequency

of the deflections, he calculated that an atom consisted mostly of empty space through

which the electrons move. He also concluded that almost all of the atom’s positive

charge and almost all of its mass were contained in a tiny, dense region in the center of

the atom, which he called the

nucleus.

Lesson 2 • Defining the Atom