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BONDING AND PHYSICAL PROPERTIES OF SOLIDS Michel Barsoum By and large the properties of all materials are traceable to the nature of the bonds holding them together. It is thus very important to understand how and why a solid is "glued" together. The very existence of the three states of matter (solids, liquids and gases) indeed life on earth as we know it, is a manifestations of a very delicate balance between the strengths of the bonds holding the atoms together that make up the elements and compounds of our solar system, and the thermal energy available to these atoms. Wherever this delicate balance is destroyed, i.e. in most of the known universe, life does not exist. There are several types of bonds that occur between atoms. These include, among others, ionic, covalent, metallic, van der Waals and hydrogen bonds. The first three are primary bonds because they are relatively strong, whereas the last two are called secondary bonds. In this handout I will discuss in detail the energetics of the ionic bond. The other types of bonds are discussed in detail in your book. 1. IONIC VERSUS COVALENT BONDS Ionic compounds generally form between very active metallic elements and active non- metals. For reasons that will become clear shortly, the requirements for an AB ionic bond to form are that A be able to lose electrons easily and B be able to accept electrons without too much energy input. This restricts ionic bonding to mostly metals from Groups 1A, IIA and part of IIIA as well as some of the transition metals, and the most active non-metals of Groups VIIA and VIA (see periodic Table). For covalent bonding to occur, on the other hand, ionic bonding must be unfavorable. This is tantamount to saying that the energies of the bonding electrons of A and B must be comparable, because if the electron energy on one of the atoms is much lower than the other, electron transfer from one to the other would occur and ionic bonds would tend to form instead. These qualitative requirements for the formation of each type of bonding, while shedding some light on the problem do not have much predictive capability as to the nature of the bond that
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will form. In an attempt to semiquantify the answer Pauling established a scale of relative electronegativity or “electrongreed” of atoms and defined electronegativity to be: the power of an atom to attract electrons to itself . With this scale it becomes a relatively simple matter to predict the nature of a bond. If the two elements forming a bond have similar electronegativities, they will tend to share the electrons between them and form covalent bonds. If, on the other hand, the electronegativity difference, X, between the two elements is large (indicating that one element is much greedier than the other) the electron will be attracted to the more electronegative element forming ions which in turn attract each other and tend to form ionic bonds. Needless to say, the
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