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Atomic Structure of Electrons - h1015v1_42
Periodic Table Summary

Chemistry Volume 1 of 2
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Fundamentals of Chemistry DOE-HDBK-1015/1-93 THE PERIODIC TABLE Rev. 0        CH-01 Page 21 A more specific statement can now be made about which electrons are involved in chemical reactions.  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 its allotted number of electrons.  Some atoms have more than one partially-filled shell.  All of the partially-filled shells have some effect on chemical behavior, but the outermost one has the greatest effect.  The outermost shell is called the valence shell, and the electrons in that shell are called valence electrons.  The term valence (of an atom) is defined as the number of electrons an element gains or loses, or the number of pairs of electrons it shares when it interacts with other elements. The periodic chart is arranged so that the valence of an atom can be easily determined.  For the elements in the A groups of the periodic chart, the number of valence electrons is the same as the group number; that is, carbon (C) is in Group IVA and has four valence electrons.  The noble gases (Group 0) have eight in their valence shell with the exception of helium, which has two. 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 are the least reactive of all the elements. The first element in the periodic table, hydrogen, does not have properties that satisfactorily place it in any group.  Hydrogen has two unique features: (a) the highest energy shell of a hydrogen atom can hold only two electrons, in contrast to all others (except helium) that can hold 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 lose an  electron.    It  has  some  properties  similar  to  Group  IA  elements,  and  some  similar  to Group VIIA elements. The number of electrons in the outer, or valence, shell determines the relative activity of the element.  The elements are arranged in the periodic table so that elements of the same group have the same number of electrons in the outer shell (except for the Transition Groups).  The arrangement of electrons in the outer shell explains why some elements are chemically very active, some are not very active, and others are inert.  In general, the fewer electrons an element must lose, gain, or share to reach a stable shell structure, the more chemically active the element is.  The likelihood of elements forming compounds is strongly influenced by this valence shell and on the stability of the resulting molecule.  The more stable the molecules are, the more likely these molecules are to form.







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