Radon and Other Noble Gases The elements in the last column of the periodic table are all very stable, mono-atomic gases. Until 1962, they were called inert gases because they did not react with other elements to form molecules. Xenon tetrafluoride, XeF4, was produced in that year. All these elements except radon are used as luminous gases in lighting (neon lights). Outline • Nobel Gas Elements • Radon and Health • Chemistry • Homework Noble Gases Each of the noble gases, in the last column of the periodic table, has its last electron shell completely filled. The elements with a filled shell configuration are the most stable and have the lowest tendency of all elements to gain electrons, lose electrons, or share electrons in chemical bonds. Chemistry 102 Prof. Shapley page 1 Ionization Energy The ionization energy the the amount of energy required to remove an electron from an atom. The highest energy electron is the first one to be removed. Ionization energy is one measure of the stability of an atom. Filled shell electronic configurations are most stable. When one type of orbital (s, p, or d) is either filled or half filled, the atom is more stable than atoms with other partially filled s, p, or d orbitals. Because they have stable, filled shell configurations, it is difficult to remove an electron from any of the noble gases. The chart below shows ionization energy through the periodic table. Each peak is one of the noble gases. Note that ionization energy decreases from top to bottom of each column because the outer shell electrons become farther removed from the nucleus. Helium Helium is the second most abundant element in the universe, after hydrogen. On Earth, its atmospheric concentration is only 0.00052 % of all gases. It is produced by radioactive decay of heavy elements and has a significant concentration (~7 %) in natural gas wells. We're all familiar with one use of helium gas. It is lighter than air and is used to fill blimps and party balloons. In liquid form, it is used to cool superconducting magnets. Chemistry 102 Prof. Shapley page 2 Neon, Argon, Krypton Neon, argon, and krypton are present in the sun and other stars. On the right you can see the atomic emission of neon spectrum of neon in sunlight. Argon is the only inert gas with a significant concentration in the atmosphere (0.9 % of all atmospheric gas molecules) but the others are present in very small quantities. These elements produce light when an electric discharge passes through the gas. Click here to see a simulation of a neon light. Why? Think of our discussion of the hydrogen atomic emission spectrum. The energy from the electric discharge causes an electron to be excited to another energy level. Neon, with its filled n=2 shell, has an electron excited to the n=3 or n=4 shell. When the electron falls back down to the ground state, it gives up its extra energy as a photon of visible light. Radon and Health Radon is a radioactive element that is one of the products of the radioactive decay of uranium. Its most stable isotope is 222Rn with a half life of 3.8 days. 222Rn 218Po + 4He Radioactive elements can decay through loss of: 1. an alpha particle (to an element with 2 fewer protons and 2 fewer neutrons), 2. a beta particle (to an element with 1 fewer neutron and 1 more proton), 3. a positive beta particle or positron (to an element with 1 fewer protons and 1 more neutrons), 4. a gamma ray, 5. of combinations of the above. Here is one uranium decay series that includes radon. Chemistry 102 Prof. Shapley page 3 238U 234Th 234Pa 234U 230Th 226Ra 222Rn 218Po 214Pb 214Bi 214Po 210Pb 210Bi 210Po 206Pb (stable) The alpha particle (helium nucleus) is released with a lot of energy and it interacts strongly with human lung tissue. Because the decay products of radon are solids that are also radioactive, they can lodge in the lung and cause serious damage over time. Radon and its decay products are a potential source of lung cancer. Radon can seep into houses through cracks in basement walls or through well water. Because radon is derived from the radioactive decay of uranium in the soil and the uranium is not evenly distributed, some parts of the country have more exposure than others. All parts of Illinois are in the EPA's zone 1 or zone 2 designation with significant to moderate radon exposure. Chemistry 102 Prof. Shapley page 4 Noble Gas Chemistry The noble gases are the least reactive of all the elements but the heavier ones do form some molecules. Helium and neon never form molecules. They have completely filled electron shells with no have-filled orbitals available for making covalent bonds and they have very high ionization energies so they don't form ions. Argon Chemistry Atomic argon has a filled shell configuration with filled 3s and 3p orbitals. For neutral atoms, the 3d orbitals are above the 4s orbitals in energy. However, for ions and molecules, the 3d is lower in energy than the 4. Chemistry 102 Prof. Shapley page 5 As another element, such as fluorine, begins to interact with argon the energy difference between the Ar 3p and Ar 3d becomes fairly small. It takes a small amount of energy to excite an electron from the filled 3p level. The resulting excited state atom has 2 half-filled orbitals available for making covalent bonds. Krypton Chemistry The energy difference between shells decreases as the energy level increases. The energy difference between the shell n=4 and n=5 (for krypton) is smaller than the difference between n=3 and n=4 (for argon). Because of this, kryton is more reactive and makes more kinds of molecules than does argon. Krypton can make 2 bonds to fluorine, as does argon. The molecule KrF2 has 2 covalent bonds between krypton and fluorine atoms. After 2 electrons in different 4p orbitals are excited to 4d orbitals, krypton can make 4 bonds to fluorine. The molecule KrF4 has 4 covalent bonds between krypton and fluorine. Chemistry 102 Prof. Shapley page 6 Krypton also forms compounds with oxygen. Radon Chemistry Radon can be oxidized to RaF2. Very little reaction chemistry is know for radon. It hasn't been studied extensively because it decays so quickly. Chemistry 102 Prof. Shapley page 7.