The VSEPR Theory Advanced Chemistry Ms. Grobsky

Determining Molecular Geometries In order to predict molecular shape, we use the Valence Shell Electron Pair Repulsion (VSEPR) theory This theory proposes that the geometric arrangement of groups of atoms about a central atom in a covalent compound is determined solely by the repulsions between electron pairs present in the valence shell of the central atom ▫The molecule adopts whichever 3-D geometry minimizes the repulsion between valence electrons

Determining Molecular Geometries To determine the shape of a molecule, we distinguish between: ▫Lone pairs (non-bonding pairs) ▫Bonding pairs (those found between two atoms)  Multiple bonds are considered as ONE bonding pair even though in reality, they have multiple pairs of electrons All electrons are considered when determining 3-D shape AX m E n A - central atom X – surrounding atom E – non-bonding valence electron group m and n - integers

Electron Group Repulsions and the Five Basic Molecular Shapes

Factors Affecting Electron Repulsion (And therefore, Bond Angles!) Two factors that affect the amount of electron repulsion around an atom: ▫Multiple bonds  Exert a greater repulsive force on adjacent electron pairs than do single bonds  Result of higher electron density  Distorts basic geometry! ▫Non-bonding (lone) pairs  Lone pairs repel bonding pairs more strongly than bonding pairs repel each other

The Effect of Non-Bonding Electrons on Bond Angles Remember, electron pairs of bonding atoms are shared by two atoms, whereas the nonbonding electron pairs (lone pairs) are attracted to a single nucleus ▫As a result, lone pairs can be thought of as having a somewhat larger electron cloud near the parent atom This “crowds” the bonding pairs and the geometry is distorted! ▫Bond angles change!

Factors Affecting Bond Angles Double BondsNon-Bonding (Lone) Pairs

The Single Molecular Shape of Linear Electron-Group Arrangement AX2 Examples ▫CS2, HCN, BeF2 A XX

The 2 Molecular Shapes of Trigonal Planar Electron-Group Arrangement Trigonal PlanarBent AX 3 Examples ▫SO 3, BF 3 AX 2 E Examples ▫SO 2 A X X X X XA E

The 3 Molecular Shapes of the Tetrahedral Electron-Group Arrangement Tetrahedral AX 4 Examples ▫CH 4, SiCl 4, SO 4 2-, ClO 4 - AX 3 E Examples ▫NH 3, PF 3, ClO 3, H 3 O+ Trigonal Pyramidal Bent AX 2 E 2 Examples ▫H 2 O, OF 2, SCl 2 A X X X X A X X X E A E E X X

The 4 Molecular Shapes of the Trigonal Bipyramidal Electron-Group Arrangement AX 5 Examples ▫PCl 5, PF 5, AsF 5, SOF 4 Trigonal Bipyramidal See-SawT-ShapedLinear AX 4 E Examples ▫SF 4, XeO 2 F 2, IF 4 +, IO 2 F 2 - AX 3 E 2 Examples ▫ClF 3, BrF 3 AX 2 E 3 Examples ▫XeF 2, I 3 -, IF 2 -

The 3 Molecular Shapes of the Octahedral Electron-Group Arrangement Octahedral AX 6 Examples ▫SF 6, IOF 5 AX 5 E Examples ▫BrF 5, XeOF 4, TeF 5 - Square Pyramidal Square Planar AX 4 E 2 Examples ▫XeF 4, ICl 4 -

What You Need to Know From All of This Five BASIC geometries of covalent compounds and their bond angles (ideal bond angles) ▫Linear (AX2) ▫Trigonal planar (AX3) ▫Tetrahedral (AX4) ▫Trigonal bipyramidal (AX5) ▫Octahedral (AX6) The following “special” geometries of covalent compounds with lone pairs ▫AX2E ▫AX3E ▫AX2E2

Steps in Determining a Molecular Shape Refer to front of Page 237!

Electronegativities determine polarity since it measures a nucleus’ attraction or “pull” on the bonded electron pair ▫When two nuclei are the same, sharing is equal  Non-polar ▫When 2 nuclei are different, the electrons are not shared equally  Polar ▫When electrons are shared unequally to a greater extent, IONIC Bonds can be polar while the entire molecule is not ▫Determined by geometry!  More on this later! Dipole moment ▫Separation of the charge in a molecule (slightly positive/slightly negative poles) ▫IF octet rule is obeyed AND all the surrounding bonds are the same (even if they’re very polar), then the molecule is NONPOLAR  Example: CCl4 Electronegativity

VSEPR and Polarity Knowing the geometry of a molecule allows one to predict whether it is polar or nonpolar ▫A bond between unlike atoms is usually polar with a positive end and a negative end The symmetry of the molecule determines polarity ▫A diatomic molecule containing two different atoms is polar  HF, CO ▫A diatomic molecule containing the same two atoms is nonpolar  N2, O2 ▫A polyatomic molecule may be nonpolar even if it contains polar bonds because, in such cases, the polar bonds are counteracting each other  CO2, CH4 = nonpolar

VSEPR Symmetry and Molecular Polarity