Ch9_Review - CHEMICAL BONDING Cocaine 1 Chemical Bonding...

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Unformatted text preview: CHEMICAL BONDING Cocaine 1 Chemical Bonding 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? 2 Forms of Chemical Bonds • There are 2 extreme forms of connecting or bonding atoms: 3 • Ionic—complete transfer of 1 or more electrons from one atom to another • Covalent—some valence electrons shared between atoms • Most bonds are somewhere in between. Covalent Bonding The bond arises from the mutual attraction of 2 nuclei for the same electrons. Electron sharing results. (Screen 9.5) results. 4 Electron Distribution in a Molecule • Define number of valence electrons number • Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. • HCl has _________ valence electrons 5 Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Central atom is atom of lowest electronegativity. electronegativity. Therefore, N is central 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs 6 HA + HB HA HB H Bond is a balance of attractive and repulsive forces. Cl •• •• • • shared or bond pair lone pair (LP) This is called a LEWIS ELECTRON LEWIS structure. DOT Page 1 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 7 Sulfite ion, SO32Sulfite ion, SO32Step 1. Central atom = S Step 2. Count valence electrons S= 6 3 x O = 3 x 6 = 18 Negative charge = 2 TOTAL = 26 e- or 13 pairs Step 3. Form bonds 10 pairs of electrons are 10 pairs of electrons are now left. now left. 8 Sulfite ion, SO32Sulfite ion, SO32Remaining pairs become lone pairs, first on outside atoms and then on central atom. • • 9 H NH H octet as needed. as 3 BOND PAIRS and 1 LONE PAIR. H NH H •• O S •• •• • • O S O • • O •• •• O •• •• • • Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair. O Each atom is surrounded by an octet of electrons. Carbon Dioxide, CO2 Carbon Dioxide, CO2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs 3. Form bonds. 10 Carbon Dioxide, CO2 Carbon Dioxide, CO2 4. Place lone pairs on outer atoms. • • 11 # of bonds between a pair of atoms of Bond Order 12 O •• •• C O •• •• • • Double bond Single bond O •• C O This leaves 6 pairs. •• • • 5. So that C has an octet, we shall form DOUBLE BONDS between C and O. 4. Place lone pairs on outer atoms. • • O •• C O •• • • O •• •• C O •• •• • • • • O •• C O •• • • Acrylonitrile Triple bond The second bonding pair forms a pi pi ( ) bond. bond. Page 2 Bond Order Fractional bond orders occur in molecules with resonance structures. Consider NO2•• •• 13 Formal Atom Charges • 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. 14 Carbon Dioxide, CO 2 2 +6 - (1/2)(4) - 4 = 0 •• •• 15 N N •• •• • ••• •• O O O O •• •• • ••• •• The N—O bond order = 1.5 Total # of e- pairs used for a type of bond Bond order = Total # of bonds of that type Bond order = 3 e- pairs in N—O bonds 2 N— O bonds • Formal charge = Group number – 1/2 (no. of bonding electrons) - (no. of LP electrons) • • O C O • • +4 - (1/2)(8) - 0 = 0 MOLECULAR GEOMETRY 16 17 Electron Pair Geometries Figure 9.12 VSEPR VSEPR • Valence Shell Electron Pair Repulsion theory. • Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions. repulsions. repulsions. Geometries for Four Electron Pairs Figure 9.13 18 Page 3 19 20 Bond Polarity Bond Polarity HCl is POLAR because it POLAR because has a positive end and a negative end. +δ -δ Electronegativity , is a measure of the ability of an atom in a is molecule to attract electrons to itself. Electronegativity Figure 9.9 21 H Cl• • •• •• Cl has a greater share in bonding electrons than does H. Concept proposed by Linus Pauling 1901-1994 Cl has slight negative charge (- ) and H has slight positive charge (+ ) Molecular Polarity Molecules will be polar if a) bonds are polar AND b) the molecule is NOT “symmetric” 22 All above are NOT polar Page 4 ...
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This note was uploaded on 01/11/2011 for the course ENGINEERIN MAE 107 taught by Professor Pozikrizdis during the Fall '08 term at San Diego.

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