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``Plum Puddings'' vs. Rutherford

The discovery of that the ELECTRON was such an incredibly lightweight particle with such a huge charge made it perfectly clear that an ATOM must be something like a ``plum pudding'' - a homogeneous, featureless matrix of positive charge (carrying most of the mass) with the electrons embedded in it like raisins. Otherwise the electrons were apt to be moving, and this was unthinkable! If they were in motion but stayed inside the atom, then they must be continually changing direction. That means they must be accelerated, and by that time everyone understood only too well that

$$\hbox{\large\sl accelerated charges radiate! }$$

Specifically, an accelerated charge (especially one with such a large charge-to-mass ratio) must always radiate away energy in the form of electromagnetic waves -- it is a sort of antenna - and so the normal quiescence of matter ``proves'' that the electrons must be at rest in their atoms; this can only be so if they are ``stuck'' in a ``plum pudding'' of positive charge.^23.6

In about 1910 a new type of ``radioactivity'' was discovered: certain nuclei spontaneously emit ``$\alpha$ rays'' which were shown to have a   ${q \over m}$  ratio nearly 4000 times smaller than electrons and where therefore much heavier particles. Soon afterwards, Ernest Rutherford set out to demonstrate the correctness of the ``plum pudding'' model of atoms by SCATTERING these  $\alpha$  particles off gold atoms comprising a thin gold foil.

The picture is analogous to firing cannon balls at great slabs of gelatin in which are embedded many small marbles. The cannon balls will lose a lot of energy going through the gelatin walls, but they certainly won't change their direction of motion much.

To Rutherford's astonishment, most of the  $\alpha$  particles passed right through the target foil without being deflected or losing much energy - indicating that what seemed to be ``solid'' metal was actually composed mainly of sheer vacuum. Even more alarmingly, some of the  $\alpha$  particles bounced backward off the gold atoms - indicating that the mass of the gold atom was almost all concentrated in a tiny hard kernel of positive charge some 10,000 to 100,000 times smaller than the size of the atoms themselves!

As Rutherford himself put it, ``It is like firing shells at a piece of paper handkerchief and having them bounce back at you.''