Discovering the Particles of the Atom

1The glow in a fluorescent tube

Switch on a fluorescent tube in a classroom and, for a moment, it flickers before it lights. Inside the glass is a gas at low pressure. A high voltage between the two ends drags a stream of tiny charged particles through that gas, and when they strike the coating on the glass, it glows. In the late nineteenth century, physicists studied exactly this kind of glow in sealed glass tubes. What they found broke Dalton's idea that the atom cannot be divided. In about 35 years, from 1897 to 1932, the electron, the nucleus, the proton and the neutron were all discovered. This lesson follows the evidence, because at A-level you must be able to say not only what is inside an atom, but how we know.

2Evidence 1: cathode rays are particles with negative charge

In a discharge tube the air is pumped out until the pressure is a tiny fraction of atmospheric pressure, and several thousand volts are applied between a metal cathode (−) and an anode (+). The glass opposite the cathode then glows green. Experiments with such tubes showed four things:

1. A metal object placed in the path casts a sharp shadow on the glowing glass, so the rays travel in straight lines from the cathode.
2. A light paddle wheel placed in the path turns, so the rays carry momentum: they are particles with mass.
3. The beam bends towards a positive plate and away from a negative one, and it is bent by a magnet. The particles are negatively charged.
4. Whatever metal the cathode is made of and whatever gas is in the tube, the particles are the same. They must be part of every atom.

In 1897, J. J. Thomson in Cambridge passed the beam between two charged plates and also through a magnetic field, and balanced one deflection against the other. From the sizes of the fields he found the charge-to-mass ratio of the particles, $\frac{e}{m}\backslash frac\{e\}\{m\}$. It was about 1800 times larger than the charge-to-mass ratio of a hydrogen ion, the lightest ion known. Either the particles carried a huge charge, or they were far lighter than any atom. Thomson concluded that they were tiny particles from inside the atom; they were later named electrons. Figure 1.1 shows the principle of his tube.

3Evidence 2: the charge of one electron

Between 1909 and 1913 Robert Millikan measured the charge on tiny charged oil drops, balancing the downward pull of gravity on each drop with an upward electric force. Every charge he measured was a whole-number multiple of one smallest value, $e=1.602\times {10}^{-19}\mathrm{C}e\; =\; 1.602\; \backslash times\; 10^\{-19\}\backslash ,\backslash mathrm\{C\}$. This is the charge of a single electron. Combined with Thomson's ratio, it gives the electron's mass (Worked example 1).

If atoms contain negative electrons but are neutral overall, they must also contain positive charge. Thomson suggested the "plum pudding" model: electrons scattered through a sphere of evenly spread positive charge, like raisins in a pudding.

4Evidence 3: the nucleus (Rutherford, 1911)

In 1909 Hans Geiger and Ernest Marsden, working with Ernest Rutherford in Manchester, fired alpha particles (fast, positive particles from radium, each with charge +2 and about four times the mass of a hydrogen atom) at gold foil only a few hundred atoms thick. A zinc sulfide screen gave a tiny flash of light wherever a particle landed, and they counted the flashes by eye through a microscope.

• Most alpha particles went straight through, as if the foil were not there.
• A few were deflected through small angles.
• A very few, roughly one in several thousand, bounced back towards the source.

The plum pudding model cannot explain the bounce-back: its positive charge is spread too thinly to turn back a heavy, fast particle. In 1911 Rutherford proposed instead that an atom's positive charge and almost all of its mass are packed into a tiny central nucleus, with the electrons moving in the mostly empty space around it. A particle that passes close to a nucleus is repelled and deflected; the rare one that heads straight at a nucleus is turned back (Figure 1.2).

5Evidence 4: the proton and the neutron

The proton. In 1886 Eugen Goldstein saw rays travelling the other way in a discharge tube, through holes in the cathode. These "canal rays" or positive rays are positive ions made when electrons knock electrons off gas atoms. Their charge-to-mass ratio depends on the gas, and is largest with hydrogen. The positive particle from hydrogen, the hydrogen nucleus, was named the proton; in 1919 Rutherford showed that protons are knocked out of nitrogen nuclei by alpha particles, so protons are part of other nuclei too.

The neutron. Nuclei are heavier than their protons alone can explain: a helium nucleus has charge +2 but mass about 4. In 1932 James Chadwick bombarded beryllium with alpha particles. The beryllium gave out a very penetrating radiation that was not bent by electric or magnetic fields, yet it knocked protons out of paraffin wax with great energy. Chadwick showed that it was a stream of neutral particles with about the mass of a proton: neutrons.

Table 1.1: The three subatomic particles

6Telling the particles apart in an electric field

A charged particle moving between charged plates is pulled towards the plate of opposite charge. For particles entering at the same speed, the sideways deflection is proportional to the ratio $\frac{\text{charge}}{\text{mass}}\backslash frac\{\backslash text\{charge\}\}\{\backslash text\{mass\}\}$. So in Figure 1.3 the electrons bend strongly towards the positive plate, the protons bend the other way and much less (same size of charge, about 1836 times the mass), and the neutrons are not deflected at all.