Everything Science Grade 10

CHAPTER 4. THE ATOM 4.3

Rutherford’s alpha-particle scattering

experiment

ESAAW

Radioactive elements emit different types of particles. Some of these are positively charged
alpha (α) particles. Rutherford wanted to find out where the positive charge in an atom is.
He carried out a series of experiments where he bombarded sheets of gold foil with alpha
particles (since these would be repelled by the positive nucleus). A simplified diagram of
his experiment is shown in figure 4.5.

radioactivesubstance

αparticles

C

C

B

B

A

A

Zinc Sulfide screen

b

b

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b

b

b

b

b

b

b

b

b

b

b

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b A

B

C

αparticles

nucleus of

gold atom

(a) (b)

gold sheet

Figure 4.5: Rutherford’s gold foil experiment. Figure (a) shows the path of theαparticles
after they hit the gold sheet. Figure (b) shows the arrangement of atoms in the gold sheets
and the path of theαparticles in relation to this.

See video: VPald at http://www.everythingscience.co.za

Rutherford set up his experiment so that a beam of alpha particles was directed at the
gold sheets. Behind the gold sheets was a screen made of zinc sulphide. This screen
allowed Rutherford to see where the alpha particles were landing. Rutherford knew that
theelectronsin the gold atoms would not really affect the path of the alpha particles,
because the mass of an electron is so much smaller than that of a proton. He reasoned
that the positively chargedprotonswould be the ones torepelthe positively charged alpha
particles and alter their path.

If Thomson’s model of the atom was correct then Rutherford would have observed mostly
path C in figure 4.5. (C represents alpha particles that are reflected by the positive nu-
cleus). What he found instead was that most of the alpha particles passed through the foil
undisturbed and could be detected on the screen directly behind the foil (path A). Some
of the particles ended up being slightly deflected onto other parts of the screen (path B).
The fact that most particles passed straight through suggested that the positive charge was
concentrated in one part of the atom only.

Through this experiment he concluded that the nucleus of the atom is positively charged

Chemistry: Matter and Materials 69