Quantum Physics

918 Chapter 28 Atomic PhysicsWOLFGANG PAULI (1900–1958)An extremely talented Austrian theoreticalphysicist who made important contributionsin many areas of modern physics,Pauli gained public recognition at the ageof 21 with a masterful review article onrelativity that is still considered one of thefinest and most comprehensive introductionsto the subject. Other major contributionswere the discovery of the exclusionprinciple, the explanation of the connectionbetween particle spin and statistics,and theories of relativistic quantum electrodynamics,the neutrino hypothesis, andthe hypothesis of nuclear spin.CERN/Courtesy of AIP Emilio Segre Visual ArchivesTABLE 28.3Number of Electrons in Filled Subshells and ShellsNumber of Number ofElectrons in Electrons inShell Subshell Filled Subshell Filled ShellK (n 1) s( 0) 2 2s( 0) 2L (n 2)p( 1) 68s( 0) 2M (n 3) p( 1) 6 18}d( 2) 10s( 0) 2}p( 1) 6N (n 4)d( 2) 1032f( 3) 14state for which n 1 and 0, and the superscript indicates that one electron ispresent in this level.Neutral helium has two electrons. In the ground state, the quantum numbers for1these two electrons are 1, 0, 0, and 1, 0, 0, 1 22. No other possible combinations ofquantum numbers exist for this level, and we say that the K shell is filled. The heliumelectronic configuration is designated as 1s 2 .Neutral lithium has three electrons. In the ground state, two of these are in the1s subshell and the third is in the 2s subshell, because the latter is lower in energythan the 2p subshell. Hence, the electronic configuration for lithium is 1s 2 2s 1 .A list of electronic ground-state configurations for a number of atoms is providedin Table 28.4. In 1871 Dmitri Mendeleev (1834–1907), a Russian chemist,arranged the elements known at that time into a table according to their atomicmasses and chemical similarities. The first table Mendeleev proposed containedmany blank spaces, and he boldly stated that the gaps were there only becausethose elements had not yet been discovered. By noting the column in which thesemissing elements should be located, he was able to make rough predictions abouttheir chemical properties. Within 20 years of this announcement, the elementswere indeed discovered.The elements in our current version of the periodic table are still arranged sothat all those in a vertical column have similar chemical properties. For example,consider the elements in the last column: He (helium), Ne (neon), Ar (argon), Kr(krypton), Xe (xenon), and Rn (radon). The outstanding characteristic of theseelements is that they don’t normally take part in chemical reactions, joining withother atoms to form molecules, and are therefore classified as inert. Because ofthis “aloofness,” they are referred to as the noble gases. We can partially understandtheir behavior by looking at the electronic configurations shown in Table 28.4,page 919. The element helium has the electronic configuration 1s 2 . In otherwords, one shell is filled. The electrons in this filled shell are considerably separatedin energy from the next available level, the 2s level.The electronic configuration for neon is 1s 2 2s 2 2p 6 . Again, the outer shell isfilled and there is a large difference in energy between the 2p level and the 3slevel. Argon has the configuration 1s 2 2s 2 2p 6 3s 2 3p 6 . Here, the 3p subshell is filledand there is a wide gap in energy between the 3p subshell and the 3d subshell.Through all the noble gases, the pattern remains the same: a noble gas is formedwhen either a shell or a subshell is filled, and there is a large gap in energy beforethe next possible level is encountered.The elements in the first column of the periodic table are called the alkalimetals and are highly active chemically. Referring to Table 28.4, we can understandwhy these elements interact so strongly with other elements. All of these alkali}