The Atomic lindens and Radioactivity he configuration of electrons in an atom determines whether and how the atom T bonds to form compounds. It also dictates the melting and freezing temperatures and the thermal and electrical conductivity as well as the taste, texture, appearance, and color of substances. Changes in elec- Radioactivity, a tool of tron energies produce visible light. Larger changes produce X-rays. medicine. This chapter goes even deeper into the atom—to the atomic nucleus. 39.1 The Atomic Nucleus It would take 30 000 carbon nuclei to stretch across a single carbon atom. The nucleus within the atom is as inconspicuous as a cookie crumb in the middle of the Rose Bowl football stadium. Despite the small size of the nucleus, much has been learned about its structure. The nucleus is composed of particles called nucleons, which when electrically charged are protons, and when electrically neutral are neutrons.* Neutrons and protons have close to the same mass, with the neutron's being slightly greater. Nucleons have nearly 2000 times the mass of electrons, so the mass of an atom is practically equal to the mass of its nucleus alone. The positively charged protons in the nucleus hold the nega- tively charged electrons in their orbits. Each proton has exactly the same magnitude of charge as the electron, but the opposite sign. So in an electrically neutral atom, there are as many protons in the * Protons and neutrons are themselves composed of subnuglear particles called quarks. Are quarks themselves made of still smaller particles? They may be, but so far there is no evidence or theoretical reason for believing so. Theoretical physicists say today that quarks are the elementary particles of which all nucleons and other strongly interacting 2 LabOrcitorii Manual 97 particles are made. 009 nucleus as there are electrons outside. The number of protons in the nucleus therefore determines the chemical properties of that atom, because the positive nuclear charge determines the possible struc- tures of electron orbits that can occur. The number of neutrons in the nucleus has no direct effect on the electron structure, and hence does not affect the chemistry of the atom. The principal role of the neutrons is to act as a sort of nuclear Figure 39.1 A cement to hold the nucleus together. Nucleons are bound together The number of electrons that by an attractive nuclear force appropriately called the strong force. 11 nucleus is surround the atomic The nuclear force of attraction is strong only over a very short matched by the number of pro- (Figure 39.2). Whereas the electrical force between charges tons in the nucleus. distance decreases as the inverse square of the distance, the nuclear force decreases tar more rapaly. When: two otteleunb ate just few nucleon diameters apart, the nuclear force they exert on each other -00-- is nearly zero. This means that if nucleons are to be held together by the strong force, they must be held in a very small volume. Nuclei are tiny because the nuclear force is very short-range. It is an interesting feature of quantum mechanics that particles Figure 39.2 A held close together have large kinetic energy and tend to fly apart. So, The nuclear strong force is a although the nuclear force is strong, it is only barely strong enough to very short-range force. For hold a pair of nucleons together. For a pair of protons, which repel nucleons very close or in con- tact, it is very strong (large force each other electrically, the nuclear force is not quite strong enough to vectors). But a few nucleon keep them together. When neutrons are present, however, the attrac- diameters away it is nearly zero tive strong force is increased relative to the repulsive electric force (small force vectors). (since neutrons have no charge). Thus, the presence of neutrons adds to the nuclear attraction and keeps protons from flying apart. The more protons there are in a nucleus, the more neutrons are needed to hold them together. For light elements, it is sufficient to have about as many neutrons as protons. For heavy elements, extra neutrons are required. The most common form of lead, for example, has 82 protons and 126 neutrons, or about one and a half times as many neutrons as protons. For elements with more than 83 protons, even the addition of extra neutrons cannot stabilize the nucleus. Figure 39.3 • A strong attractive nuclear force 39.2 Radioactive Decay acts between nearby protons A and B, but not significantly One of the factors that sets a limit on how many stable nuclei can exist between A and C. The longer- is the instability of the neutron. A lone neutron will spontaneously range electric force repels pro- decay into a proton plus an electron (and also an antineutrino, a tiny tons A and C as well as A and B. particle we will not discuss here). Out of a bunch of lone neutrons, The mutual repulsion of all the about half of them will decay in 11 minutes. Particles that decay in this protons in a heavy nucleus tends or in similar ways are said to be radioactive. A lone neutron is to make such nuclei unstable. radioactive. The rules of radioactivity inside atomic nuclei are governed by the mass-energy equivalence. Particles decay only when their com- bined products have less mass after decay than before. The mass of a neutron is slightly greater than the total mass of a proton plus elec- tron (and the antineutrino). So when a neutron decays, there is less mass after decay than before. Decay will not spontaneously occur for reactions where more macs results. The reverse reaction, a proton decaying into a neutron, can occur only with external energy input. 610 Chapter 39 The Atomic Nucleus and Radioactivity • tit !tithe tar d ne. er Figure 39.4 A efes A neutron—proton combination is al: stable hut a neutron by itself is fe MAGNET unstable and turns into a proton aapr by emitting an electron (as well ogby as an antineutrino—not shown). !. 'Sam pas y afy, ' en. to RADIUM SAMPLE LEAD BLOCK AT BOTTOM OF HOLE en( to he lc - c fo Figure 39.5 A The separation of alpha, beta, and gamma rays in a magnetic field. The rays tronlds come from a radioactive source placed at the bottom of a hole drilled in a rt. lead block. trone ieni tts, ea All the elements heavier than bismuth (atomic number 83) exane, decay in one way or another. Thus, these elements are radioactive. mes , Their atoms emit three distinct types of rays. The rays are named Figure 39.6 A protts, alpha, beta, and gamma, respectively, after the first three letters of the Greek alphabet, a, p, and y. Alpha rays have a positive electric An alpha particle contains two leus. protons and two neutrons bound charge, beta rays are negative, and gamma rays are electrically neu- together and is identical to a tral. The three rays can be separated by putting a magnetic field helium nucleus. across their path (Figure 39.5). An alpha ray is a stream of particles that are made of two pro- tons and two neutrons and that are identical to the nuclei of helium atoms. These are called alpha particles (Figure 39.6). LIGHT RAY A beta ray is simply a stream of electrons. An electron is ejected from the nucleus when a neutron is transformed into a proton. It may seem that the electrons are "buried" inside the neutron, but this is not true. An electron does not exist in a neutron any more than a W AN spark exists inside a rock about to be scraped across a rough surface. The electron that pops out of the neutron, like the spark that pops GAMMA RAY out of the scraped rock, is produced during an interaction. !d by A gamma ray is massless energy Like visible light, gamma rays C0111- are simply photons of electromagnetic radiation, but of much higher if ass of a frequency and energy. Visible light is emitted when electrons jump elec- - Figure 39.7 A s less from one orbit to another of lower energy Gamma rays are emitted A gamma ray is simply electro- cur for when nucleons do a similar sort of thing inside the nucleus. Some- magnetic radiation, much higher ton times there are great energy differences in nuclear energy levels, so in frequency and energy per put.. the photons (gamma rays) emitted carry a large amount of energy. photon than light and X-rays. 611 39.3 Radiation Penetrating Power There is a great difference in the penetrating power of the three types R AD I OACT I V E of rays emitted by radioactive elements. Alpha rays are the easiest to SOURCE stop. They can be stopped by a reasonably heavy piece of paper or a few sheets of thin paper. Beta rays go right through paper but are stopped by several sheets of aluminum foil. Gamma rays are the stop and require lead or other heavy shielding to ot most difficult to block them. An alpha particle is easy to stop because it is relatively slow and its double-positive charge InLeiauts with thc mclocuies it encounters along its path. It slows down as it shakes many of these molecules apart and leaves positive and negative ions in its wake. Even when traveling through nothing but air, an alpha particle will come to a stop after only a few centimeters. It soon grabs up a couple of stray electrons and becomes nothing more than a harmless helium atom.