Fundamentals of ChemistryDOE-HDBK-1015/1-93THE PERIODIC TABLERev. 0CH-01Page 19Figure 5 Electron Shells of AtomsChemical activity can also be determined from position in the periodic table. The most activemetals are the members of the Alkali Family, e.g., cesium (Cs) and francium (Fr). The mostactive nonmetals are the members of the Halogen Family, e.g., fluorine (F) and chlorine (Cl).The Noble Gases in Group 0 are inert. The activity of metals decreases when proceeding to theright in the periodic table; the activity of nonmetals decreases when proceeding to the left.Atomic Structure of ElectronsBased on experimental data, it is known that chemical reactions involve only the electrons inatoms. In fact, only some of the electrons are involved. Because chemical properties areperiodic, there must also be a periodic characteristic about electrons. This characteristic is themanner in which electrons are arranged in the atom. Electrons are in constant motion aroundthe nucleus. They have both kinetic and potential energy, and their total energy is the sum ofthe two. The total energy is quantized; that is, there are definite, discrete values of total energythat atomic electrons can possess. These energy states can be visualized as spherical shellsaround the nucleus separated by forbidden areas where electrons cannot exist in a stable state.This sort of arrangement is illustrated in Figure 5.It is customary to speak of electron shells around the nucleus, and the shells are referred to bynumber. The first, or No. 1, shell is the one nearest the nucleus; the second, or No. 2, shell isnext; then the third, or No. 3, shell; and so on in numerical order. In general, electrons closerto the nucleus have a lower energy state. Atomic electrons always seek the lowest energy stateavailable.
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