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【正文】 ond (1/100th of strong bonds。 10 kcal/mol) 4. Hydrogen bond is an example 50 Metallic bond Ionic bond Covalent bond Van der Waals bond Electrons shared among all atoms Electrons transferred between atoms, producing ions Electrons shared among two adjacent atoms Polarization produces slight electrostatic charge between molecules No directionality No directionality directionality Not directional but affects regions of molecules High strength (25200 kcal/mol) High strength (150370 kcal/mol) High strength (125300 kcal/mol) Weak bond (1/100th of strong bonds。 10 kcal/mol) Forms between atoms with low electronegativity Forms between atoms of different electronegativity values (one high, one low) Forms between atoms with high electronegativity Hydrogen bond is an example 51 Type Ionic Covalent Metallic Secondary Bond Energy Large! Variable largeDiamond smallBismuth Variable largeTungsten smallMercury smallest Comments Nondirectional (ceramics) Directional semiconductors, ceramics polymer chains) Nondirectional (metals) Directional interchain (polymer) intermolecular Summary: Bonding 52 ? Mixed Bonds In most materials, bonding between atoms is a mixture of two of more types. The Ionic and covalent bond types represent the extremes of transfer and sharing of electrons. Real bonds are a mixture of these extremes, depending on the electronegativities of the elements involved. ? Compounds formed from two or more metals may be bonded by a mixture of metallic and ionic bonds, particularly when there is a large difference in electronegativity between the elements. 53 ? It is the electronegativities of atoms that determine what bond type forms. ? If the atoms have high electronegativity, they share atoms in a covalent bond between individual pairs of atoms. ? If the atoms have low electronegativity, they share atoms among all of the atoms in a metallic bond. ? If some of the atoms have high and some low electronegativity, the electrons are transferred from one atom to another, resulting in an ionic bond. Electronegativity and bonding 54 ? The formula is Fraction Covalent = ?E2 ? ? E is the difference in the electronegativities of the atoms. ? For SiO2, the electronegativity of Si is and of O is , so the fraction is and the bond is % covalent. ? Similarly, for Al2O3, the bond is 37% covalent and for CuO is 53% covalent. ? The more covalent the bond, the more directionality is present and the lower the tendency for the ions to pack densely around ones of the opposite sign. 55 ? Materials having a high binding energy also have a high strength and a high melting temperature. Ionically bonded materials have a particularly large binding energy. Metals have lower binding energies. Bond Binding Energy (kcal/mol) Ionic 150370 Covalent 125300 Metallic 25200 Van der Waals 10 Table Binding energies for the four bonding mechanisms 56 ? Bond length, r ? Bond energy, Eo F F r ? Melting Temperature, Tm Tm is larger if Eo is larger. Properties From Bonding: TM 57 ? Elastic modulus, E ? E ~ curvature at ro ? L F A o = E L o E l a s t i c m odu l us r l a rge r E l a s t i c M odu l us s m a l l e r E l a s t i c M od ul us E n e rgy r o unstr e tched leng th E is larger if Eo is larger. Properties From Bonding: E 58 ? Coefficient of thermal expansion, a ? a ~ symmetry at ro a is larger if Eo is smaller. ?? = a? ( T 2 T 1 ) ? L L o c oe f f. t h e rm a l e xpa ns i on Properties From Bonding: a 59 Ceramics (Ionic amp。 covalent bonding): Metals (Metallic bonding): Polymers (Covalent amp。 Secondary): Large bond energy large Tm large E small a Variable bond energy moderate Tm moderate E moderate a Directional Properties Secondary bonding dominates small T small E large a Summary: Primary Bonds 60
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