Ch9_Review

Ch9_Review - CHEMICAL BONDING 1 The bond arises from the...

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Unformatted text preview: CHEMICAL BONDING 1 The bond arises from the mutual attraction of 2 nuclei for the same electrons. Electron sharing results. (Screen 9.5) results. HA + HB HA HB Bond is a balance of attractive and repulsive forces. 4 1 or more electrons from one atom to another • Covalent—some valence electrons shared between atoms • Most bonds are somewhere in between. 5 • Define number of valence electrons Define number • Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. LONE • HCl has _________ valence electrons HCl has •• Cl •• sh a r e d o r b ond pair • • lone pair ( LP) This is called a LEWIS ELECTRON This LEWIS structure. Page 1 3 • Ionic—complete transfer of Electron Distribution in a Molecule H Forms of Chemical Bonds • There are 2 extreme forms of connecting or bonding atoms: Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties? Cocaine Covalent Bonding Chemical Bonding 2 DOT Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Central atom is atom of lowest Central electronegativity. Therefore, N is central Therefore, 2. Count valence electrons H = 1 and N = 5 and Total = (3 x 1) + 5 Total = 8 electrons / 4 pairs electrons 6 Building a Dot Structure 3. Form a single bond between the central atom and each surrounding atom 4. Remaining electrons form LONE PAIRS to complete octet as needed. 3 BOND PAIRS and 1 LONE PAIR. H N 7 Step 1. Central atom = S Step 2. Count valence electrons Step S= 6 3 x O = 3 x 6 = 18 18 Negative charge = 2 Negative TOTAL = 26 e- or 13 pairs Step 3. Form bonds H H H •• N H 10 pairs of electrons are 10 pairs of electrons are 10 now left. now left. now Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair. 10 C •• O •• C • • •• •• • • •• O •• O This leaves 6 pairs. O •• • • O •• O S •• • • •• O •• • • Each atom is surrounded by an octet of electrons. O 11 Bond Order # of bonds between a pair of atoms 4. Place lone pairs on outer atoms. •• C O Double bond • • •• Single bond 5. So that C has an octet, we shall form DOUBLE BONDS between C and O. 4. Place lone pairs on outer atoms. • • S • • •• O •• C •• O •• • • • • O •• C The second bonding pair forms a pi pi Page 2 9 •• • • Carbon Dioxide, CO2 Carbon Dioxide, CO2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs 3. Form bonds. O O Sulfite ion, SO32Sulfite ion, SO32Remaining pairs become lone pairs, first on outside atoms and then on central atom. O H Carbon Dioxide, CO2 Carbon Dioxide, CO2 8 Sulfite ion, SO32Sulfite ion, SO32- O • • •• ( ) bond. bond. Acrylonitrile Triple bond 12 13 Bond Order Fractional bond orders occur in molecules with resonance structures. Consider NO2•• •• N Total # of e- pairs used for a type of bond Bond order = Total # of bonds of that type 3 e - pairs in N—O bonds 2 N— O bonds MOLECULAR GEOMETRY VSEPR VSEPR • Valence Shell Electron alence hell Pair Repulsion theory. air Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions. repulsions. Carbon Dioxide, CO 2 2 • Atoms in molecules often bear a charge (+ or -). • The predominant resonance structure of a molecule is the one with charges as close to 0 as possible. N •• •• •• • ••• O O O O •• •• • ••• •• The N—O bond order = 1.5 Bond order = Formal Atom Charges 14 15 +6- ( 1 / 2 )( 4 ) - 4 = 0 • Formal charge = Group number – 1/2 (no. of bonding electrons) - (no. of LP electrons) •• • • O C •• O • • +4- ( 1 / 2 )( 8 ) - 0 = 0 16 17 Electron Pair Geometries Electron Geometries Figure 9.12 • Most important factor in determining geometry is relative repulsion between electron pairs. Page 3 Geometries for Geometries for Four Electron Pairs Figure 9.13 18 Bond Polarity Bond Polarity 19 Electronegativity , is a measure of the ability of an atom in a molecule to attract electrons to itself. HCl is POLAR because it POLAR has a positive end and a negative end. +δ -δ •• H Cl• • •• 20 Cl has a greater share in has bonding electrons than does H. Concept proposed by Linus Pauling 1901-1994 Cl has slight negative charge (- ) and H has Cl (slight positive charge (+ ) slight Molecular Polarity 22 Molecules will be polar if a) bonds are polar AND AND b) the molecule is NOT “symmetric” All above are NOT polar All NOT Page 4 Electronegativity Figure 9.9 21 ...
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