The VSEPR Theory. Advanced Chemistry Ms. Grobsky. Determining Molecular Geometries. In order to predict molecular shape, we use the V alence S hell E lectron P air R epulsion (VSEPR) theory
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 AXmEn A - central atom X – surrounding atom E – non-bonding valence electron group m and n - integers
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 Bonds Non-Bonding (Lone) Pairs
The Single Molecular Shape of Linear Electron-Group Arrangement • AX2 • Examples • CS2, HCN, BeF2 X X A
The 2 Molecular Shapes of Trigonal Planar Electron-Group Arrangement Trigonal Planar Bent • AX3 • Examples • SO3, BF3 • AX2E • Examples • SO2 X E X A A X X X
The 3 Molecular Shapes of the Tetrahedral Electron-Group Arrangement Tetrahedral Bent Trigonal Pyramidal • AX4 • Examples • CH4, SiCl4, SO42-, ClO4- • AX3E • Examples • NH3, PF3, ClO3, H3O+ • AX2E2 • Examples • H2O, OF2, SCl2 X E E A A A E X X X X X X X X
The 4 Molecular Shapes of the TrigonalBipyramidal Electron-Group Arrangement TrigonalBipyramidal See-Saw T-Shaped Linear • AX5 • Examples • PCl5, PF5, AsF5, SOF4 • AX4E • Examples • SF4, XeO2F2, IF4+, IO2F2- • AX3E2 • Examples • ClF3, BrF3 • AX2E3 • Examples • XeF2, I3-, IF2-
The 3 Molecular Shapes of the Octahedral Electron-Group Arrangement Octahedral Square Planar Square Pyramidal • AX6 • Examples • SF6, IOF5 • AX5E • Examples • BrF5, XeOF4, TeF5- • AX4E2 • Examples • XeF4, ICl4-
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) • Trigonalbipyramidal (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