In 1911,
Ernest Rutherford and some of his students directed
-particles
at a thin gold foil. They placed a zinc sulphide screen near enough to
the foil to detect any alpha particles that got through.
 |
 of -scattering |
The zinc sulphide would give off a flash of light whenever struck by an
-particle.
They expected that the
-particles
would go right through the foil with hardly any deflection because in
the Thomson model, the positive and negative electric charges inside an
atom were assumed to be uniformly distributed through a solid atom.
Consequently the positively charged
-particles
would only encounter weak electric forces and so pass through the thin
foil with only slight
deflections (less than a degree).
| Thomson model Rutherford model |
 |
| -particles
undeflected -particles
scattered |
Rutherford and his associates found that although most of the
-particles
were not deviated by much, a few were deflected through very large
angles and some were even deflected in the backward direction. As
Rutherford remarked, "It was as incredible as if you fired a 15-inch
shell at a piece of tissue paper and it came back and hit you."
Rutherford’s explanation of this remarkable finding was that an atom
must not be solid, but in fact largely empty space surrounding a tiny
nucleus, in which its positive charge and nearly all its mass are
concentrated, with electrons some distance away. With this sort of atom,
it is easy to see why most alpha particles go right through a thin foil.
However, when an
-particle
comes near a nucleus, the intense electric field there causes it to be
scattered through a large angle and possibly even backwards.
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