Simple Nature - Light and Matter

The neutronIt would have been nice and simple if all the nuclei could havebeen built only from protons, but that couldn’t be the case. If youspend a little time looking at a periodic table, you will soon noticethat although some of the atomic masses are very nearly integermultiples of hydrogen’s mass, many others are not. Even where themasses are close whole numbers, the masses of an element otherthan hydrogen is always greater than its atomic number, not equalto it. Helium, for instance, has two protons, but its mass is fourtimes greater than that of hydrogen.Chadwick cleared up the confusion by proving the existence ofa new subatomic particle. Unlike the electron and proton, whichare electrically charged, this particle is electrically neutral, and henamed it the neutron. Chadwick’s experiment has been describedin detail pn p. 138, but briefly the method was to expose a sampleof the light element beryllium to a stream of alpha particles from alump of radium. Beryllium has only four protons, so an alpha thathappens to be aimed directly at a beryllium nucleus can actually hitit rather than being stopped short of a collision by electrical repulsion.Neutrons were observed as a new form of radiation emergingfrom the collisions, and Chadwick correctly inferred that they werepreviously unsuspected components of the nucleus that had beenknocked out. As described earlier, Chadwick also determined themass of the neutron; it is very nearly the same as that of the proton.To summarize, atoms are made of three types of particles:charge mass in units of location in atomthe proton’s massproton +e 1 in nucleusneutron 0 1.001 in nucleuselectron −e 1/1836 orbiting nucleusThe existence of neutrons explained the mysterious masses ofthe elements. Helium, for instance, has a mass very close to fourtimes greater than that of hydrogen. This is because it containstwo neutrons in addition to its two protons. The mass of an atom isessentially determined by the total number of neutrons and protons.The total number of neutrons plus protons is therefore referred toas the atom’s mass number.m / Examples of the constructionof atoms: hydrogen (top) andhelium (bottom). On this scale,the electrons’ orbits would be thesize of a college campus.IsotopesWe now have a clear interpretation of the fact that helium isclose to four times more massive than hydrogen, and similarly forall the atomic masses that are close to an integer multiple of themass of hydrogen. But what about copper, for instance, which hadan atomic mass 63.5 times that of hydrogen? It didn’t seem reasonableto think that it possessed an extra half of a neutron! TheSection 8.2 The Nucleus 487