Simple Nature - Light and Matter

that fly off to see what was inside them. The approach has beencompared to shooting a clock with a rifle and then studying thepieces that fly off to figure out how the clock worked.Discussion QuestionsA The diagram, showing alpha particles being deflected by a goldnucleus, was drawn with the assumption that alpha particles came in onlines at many different distances from the nucleus. Why wouldn’t they allcome in along the same line, since they all came out through the sametube?B Why does it make sense that, as shown in the figure, the trajectoriesthat result in 19 ◦ and 20 ◦ scattering cross each other?C Rutherford knew the velocity of the alpha particles emitted by radium,and guessed that the positively charged part of a gold atom had a chargeof about +100e (we now know it is +79e). Considering the fact that somealpha particles were deflected by 180 ◦ , how could he then use conservationof energy to derive an upper limit on the size of a gold nucleus? (Forsimplicity, assume the size of the alpha particle is negligible compared tothat of the gold nucleus, and ignore the fact that the gold nucleus recoilsa little from the collision, picking up a little kinetic energy.)8.2.4 The structure of nucleiThe protonThe fact that the nuclear charges were all integer multiples of esuggested to many physicists that rather than being a pointlike object,the nucleus might contain smaller particles having individualcharges of +e. Evidence in favor of this idea was not long in arriving.Rutherford reasoned that if he bombarded the atoms of a verylight element with alpha particles, the small charge of the targetnuclei would give a very weak repulsion. Perhaps those few alphaparticles that happened to arrive on head-on collision courses wouldget so close that they would physically crash into some of the targetnuclei. An alpha particle is itself a nucleus, so this would be a collisionbetween two nuclei, and a violent one due to the high speedsinvolved. Rutherford hit pay dirt in an experiment with alpha particlesstriking a target containing nitrogen atoms. Charged particleswere detected flying out of the target like parts flying off of cars ina high-speed crash. Measurements of the deflection of these particlesin electric and magnetic fields showed that they had the samecharge-to-mass ratio as singly-ionized hydrogen atoms. Rutherfordconcluded that these were the conjectured singly-charged particlesthat held the charge of the nucleus, and they were later namedprotons. The hydrogen nucleus consists of a single proton, and ingeneral, an element’s atomic number gives the number of protonscontained in each of its nuclei. The mass of the proton is about 1800times greater than the mass of the electron.486 Chapter 8 Atoms and Electromagnetism