Fundamentals of ChemistryDOE-HDBK-1015/1-93THE PERIODIC TABLERev. 0CH-01Page 21A more specific statement can now be made about which electrons are involved in chemicalreactions. Chemical reactions involve primarily the electrons in the outermost shell of an atom.The term outermost shell refers to the shell farthest from the nucleus that has some or all of itsallotted number of electrons. Some atoms have more than one partially-filled shell. All of thepartially-filled shells have some effect on chemical behavior, but the outermost one has thegreatest effect. The outermost shell is called the valence shell, and the electrons in that shellare called valence electrons. The term valence (of an atom) is defined as the number ofelectrons an element gains or loses, or the number of pairs of electrons it shares when itinteracts with other elements.The periodic chart is arranged so that the valence of an atom can be easily determined. For theelements in the A groups of the periodic chart, the number of valence electrons is the same asthe group number; that is, carbon (C) is in Group IVA and has four valence electrons. Thenoble gases (Group 0) have eight in their valence shell with the exception of helium, which hastwo.The arrangement in which the outermost shell is either completely filled (as with He and Ne)or contains eight electrons (as with Ne, Ar, Kr, Xe, Rn) is called the inert gas configuration.The inert gas configuration is exceptionally stable energetically because these inert gases arethe least reactive of all the elements.The first element in the periodic table, hydrogen, does not have properties that satisfactorilyplace it in any group. Hydrogen has two unique features: (a) the highest energy shell of ahydrogen atom can hold only two electrons, in contrast to all others (except helium) that canhold eight or more; and (b) when hydrogen loses its electron, the ion formed, H , is a bare+nucleus. The hydrogen ion is very small in comparison with a positive ion of any other element,which must still have some electrons surrounding the nucleus. Hydrogen can either gain or losean electron. It has some properties similar to Group IA elements, and some similar toGroup VIIA elements.The number of electrons in the outer, or valence, shell determines the relative activity of theelement. The elements are arranged in the periodic table so that elements of the same grouphave the same number of electrons in the outer shell (except for the Transition Groups). Thearrangement of electrons in the outer shell explains why some elements are chemically veryactive, some are not very active, and others are inert. In general, the fewer electrons an elementmust lose, gain, or share to reach a stable shell structure, the more chemically active the elementis. The likelihood of elements forming compounds is strongly influenced by this valence shelland on the stability of the resulting molecule. The more stable the molecules are, the morelikely these molecules are to form.
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