3.091 – Introduction to Solid State Chemistry Lecture Notes No. 2 ...

LN–2B.A.120osp 22H 2+pyπ - bondC C -C C+ =Ethylene has a double bond, the C atoms are bondedto each other with a σ-bond (sp2-sp2 overlap) and witha π-bond (lateral p-orbital overlap)H--H-Acetylene has a triple bond: C atoms are bonded by overlapof sp-hybrid orbitals (σ-bond) and by lateral overlap of 2 setsof p-orbitals (2 π-bonds) configured at right angles to each other.-lateral overlapof p-orbitalsleads to π−bondformationsH-H-the overlappingsp2 orbitalsbetween the Catoms form a σ-bond-Figure 9Double and Triple Bonds3. METALLIC BONDINGThe stability of both covalent and metallic bonds may be regarded as arising from thepotential energy lowering experienced by valence electrons under the influence of morethan one nucleus. In metals, where the valence electrons are not as tightly bound totheir ion cores, we cannot expect the formation of strong electron-pair bonds. The bondenergies of known diatomic molecules of metallic elements are, in fact, smaller thanthose consisting of nonmetallic elements: 104 kJ/mole for Hg 2 . Only diatomic moleculesof the semi–metals have relatively high binding energies (385 kJ/mole for As 2 ,293 kJ/mole for Sb 2 and 163 kJ/mole for Bi 2 ). These values reflect multiple bonding.Much greater stability is possible in larger aggregates of atoms such as bulk metals.The known properties of metals, such as low electrical resistance and malleability,support the conceptual view that the valence electrons in metals never remain near any25