Bond Strength • Bond energy (D) : Energy required to break a bond between two atoms • Values listed are averages • As bond energy increases, bond length tends to decrease
Bond Energy and Enthalpy • Bond energies can be used to estimate H° rxn • H = D(bonds broken) – D(bonds formed) • Example: CH 4 (g) + Cl 2 (g) CH 3 Cl(g) + HCl(g)
Molecular Shapes • V alence S hell E lectron P air R epulsion • Electrons pairs seek to be as far apart as possible • Predicts shapes by determining the arrangement which maximizes the distance between electron pairs
VSEPR • Bond Pairs: • Any bond between two atoms is a Bond Pair • For VSEPR, multiple bonds are considered to be a single Bond Pair • Lone Pairs: • Nonbonding pairs of electrons around an atom
VSEPR • Electron - pair geometry: • Linear (2 pairs) • Trigonal planar (3 pairs) • Tetrahedral (4 pairs) • Trigonal bipyramidal (5 pairs) • Octahedral (6 pairs)
VSEPR • Molecular geometry: the shape formed by the atoms in the molecule • Nonbonding domains need more room than bonding domains • Nonbonding domains can alter bond angles
Polarity of Molecules • For a molecule to be polar: • the bonds must be polar • the dipole vectors must not cancel each other out • Generally, for any molecule AB x : • It will be non-polar if all domains are bonding • It will be polar if there are nonbonding domains (exceptions = linear, square planar)
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