Chapter 8 Molecular structure and covalent bonding theoriesDescribe the basic ideas of the valence shell electron pair repulsion (VSEPR) theoryDecribe the relationships between molecular shapes and polaritiesPredict whether a molecule is polar or nonpolarDescribe the basic ideas of valence bond (VB) theoryBasic understanding about hybrid orbitals

Valence shell electron pair repulsion (VSEPR) theoryEach set of valence shell electrons on a central atom is significant. The sets of valence shell electrons on the central atom repel one another. They are arranged about the central atom so that repulsions among them are as small as possible.

We define a set" of electrons as any number of electrons that occupy a localized region around an atom. Thus, an electron set may consist of a single bond, a double bond, a triple bond, a lone pair, or even a lone electron.

Central atom is any atom that is bonded to more than one other atomRegions of high electron density around the central atomEach bonded atom is counted as one region of high electron density (whether the bonding is single, double, or triple)Each unshared pair of valence electrons on the central atom is counted as one region of high electron density

According to VSEPR theory, the molecule or ion is most stable when the regions of high electron density on the central atom are as far apart as possible. The arrangement of these regions of high electron density around the central atom is referred to as electronic geometry of the central atomThe arrangement of atoms around the central atom is called the molecular geometry.

ExampleCO2, NH3, CH4, SO42- Central atom?Number of regions of high e density

FormulaCO2NH3CH4SO42-Lewis dot formula:Central atom:CNCSNumber of atoms bonded to central atom:2344Number of unsharedpairs on central atom:0100Total number of regions of high electron densityon central atom:2444

ExerciseIdentify the central atom (or atoms) in each of the following compounds or ions:HCO3-; SiO2; SO2; Al(OH)4-; BeBr2; (CH3)4Pb

Number of Regions of High Electron Density About a Central Atom

No. Regions Electronic Geometryof High ElectronDescription Line DrawingDensityAngles2linear180o3trigonal planar120o4tetrahedral109.5o5trigonal bipyramidal90, 120, 180o6octahedral; 90o; 180o

Effect of Lone Pairs

The relative magnitudes of the repulsive forces between pairs of electrons on an atom arelp/lp>>lp/bp>bp/bpWhere lp refer to lone (unshared) pairs and bp refer to bonding pairs of valence shell electrons.

Because a lone pair is held by only one nucleus, it is less confined and exerts stronger repulsions than a bonding pair.

Effect of Double BondsHCHO

Effect of Double Bondsthe double bond, with its greater electron density, repels the two single bonds more strongly than they repel each other

MOLECULAR SHAPE AND MOLECULAR POLARITYOne of the most important and far-reaching effects of molecular shape is molecular polarity, which can influence melting and boiling points, solubility, chemical reactivity, and even biological function.

In molecules with more than two atoms, both shape and bond polarity determine molecular polarity.

Polar molecules: the influence of molecular geometryFor a molecule to be polar, two conditions must both be met:1. There must be at least one polar bond or lone (unshared) pair on the central atom, and2. (a) The polar bonds, if there are more than one must not be arranged so that their polarities (bond dipoles) cancel, or (b) If there are two or more lone (unshared) pairs on the central atom, they must not be arranged so that their polarities cancel

ExampleHFCO2H2OEach bond dipole is viewed as an electronic vector with magnitude and direction

ExerciseFrom electronegativity (EN) values predict whether each of the following molecules is polar and show the direction of bond dipoles and the overall molecular dipole when applicable:

ExerciseWhich of the following molecules are polar? Why?CH4; CH3Br; CH2Br2; CHI3;CI4

Molecular Shapes with More Than One Central Atom The shapes of these molecules are combinations of the molecular shapes for each central atomCH3-CH3 and CH3-CH2-OH

Valence bond (VB) theory Describe how bonding occursVSEPR theory describes the orientations of the regions of high electron densityVB theory describes the atomic orbitals that overlap to produce the bonding with the geometry mentioned in VSEPR theory

When other atoms are nearby (in a molecule or ion), an atom can combine its valance shell orbitals to form a new set of orbitals The process is called hybridization, and the new orbitals that are formed as called hybrid orbitalsThese hybrid orbitals can overlap with orbitals on other atoms to share electrons and form bonds.

The number of hybrid orbitals obtained equals the number of atomic orbitals mixed. The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed

Relation Between Electronic Geometries and Hybridization

Regions ofAtomic Orbitals MixedHigh Electron Electronic from Valence ShellDensityGeometryof Central AtomHybridization2Linearone s, one psp3Trigonal planarone s, two p'ssp24Tetrahedralone s, three p'ssp35Trigonal bipyramidalone s, three p's, one dsp3d6Octahedralone s, three p's, two d'ssp3d2

sp hybridization

sp2 hybridization

sp3 hybridization

Compounds containing double bondsA sigma bond is a bond resulting from head-on overlap of atomic orbitals. The region of electron sharing is along and cylindrically around an imaginary line connecting the bonded atoms (bond axis)A pi bond is a bond resulting from side-on overlap of atomic orbitals. The regions of electrons sharing are on opposite sides of an imaginary line connecting the bonded atoms and parallel to this line.

Sigma bondPi bond

STRUCTURE OF C2H4:sp2 HYBRIDZATION One 2p orbital is left unhybridized

A process for obtaining sp2-hybridized carbon atoms The three sp2 orbitals that result from hybridization are directed toward the corners of a regular triangle

A model for the bonding molecular orbitals of ethane formed from two sp2-hybridized carbon atoms and four hydrogen atoms

Picture of double bonds

Compounds containing triple bondsA triple bond consists of one sigma bond and two pi bondsExample of some molecules containing triply bonded atoms are N2, HCN, C2H2

STRUCTURE OF ETHYNE sp HYBRIDZATION

The carbon-carbon triple bond consists of two bonds and one bond

Picture of triple bonds

ExerciseWrite the Lewis formula for each of the following. Indicate which bonds are polar. Indicate which molecules are polar. (a) CS2; (b) AlF3; (c) H2S; (d) SnF2.Write Lewis formulas and three dimensional structures for the following (a) BrF3; (b) BrF; (c) BrF5.

ExerciseWhat is the hybridization of the central atom in each of the following? (a) NCl3; (b) molecular AlCl3; (c) CF4; (d) SF6; (e) IO4-

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