Ch9_Bonding

Ch9_Bonding - 1 CHEMICAL CHEMICAL BONDING BONDING 4 Ionic...

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Unformatted text preview: 1 CHEMICAL CHEMICAL BONDING BONDING 4 Ionic Ionic Bonds Bonds of 1 or more electrons from one atom to another • Covalent—some valence electrons shared between atoms • Most bonds are somewhere in between. The bond arises from the mutual attraction of 2 nuclei for the same electrons. Electron sharing results. electrons. sharing (Screen 9.9) HA HB Bond is a balance of attractive and repulsive forces. Page 1 3 • Ionic—complete transfer Covalent Bonding HA + HB Forms of Chemical Bonds • There are 2 extreme forms of connecting or bonding atoms: 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 Essentially complete electron transfer from an element of low IE (metal) element low to an element of high to high affinity for electrons (nonmetal) 2 Na(s) + Cl2(g) ---> 2 Na+ + 2 ClTherefore, ionic compds. exist primarily between metals at left of periodic table (Grps 1A and 2A and transition metals) and nonmetals at right (O and 2 Chemical Bonding Problems and questions — 5 Chemical Bonding Objectives Objectives are to are understand: 1. valence e- distribution 1. distribution in molecules and ions. 2. molecular structures 2. structures 3. bond properties and 3. and their effect on molecular properties. 6 7 Electron Electron Distribution Distribution in Molecules in Molecules G. N. Lewis G. 1875 - 1946 1875 • Electron distribution is depicted with H • Valence electrons are distributed as shared or BOND shared BOND PAIRS and and unshared or unshared or core and core and valence electrons B 1s2 2s2 2p1 Core = [He] and valence = 2s 2 2p1 Cl H • • •• 9 •• + Cl • • •• H •• Cl • • •• lone pair (LP) Overlap of H (1s) and Cl (2p) Note that each atom has a single, unpaired electron. This is called a LEWIS This LEWIS ELECTRON DOT structure. ELECTRON structure. 10 Bond Formation A bond can result from a “head-tohead” overlap of atomic orbitals head” overlap of on neighboring atoms. •• shared or bond pair LONE PAIRS. Electrons are divided between 8 shared or BOND PAIRS and shared BOND and unshared or LONE PAIRS. unshared LONE Lewis electron dot structures Valence Electrons Valence Electrons Bond and Lone Pairs • Valence electrons are distributed as Rules of the Game No. of valence electrons of a main group atom = Group number •For Groups 1A-4A (14), no. of bond pairs = group number. •For Groups 5A (15)-7A (17), BP’s = 8 - Grp. No. 11 Rules of the Game •No. of valence electrons of an atom = Group number •For Groups 1A-4A (14), no. of bond pairs = group number • For Groups 5A (15)-7A (17), BP’s = 8 - Grp. No. •Except for H (and sometimes atoms of 3rd and higher periods), BP’s + LP’s = 4 Br [Ar] 3d10 4s2 4p5 Core = [Ar] 3d10 and valence = 4s 2 4p5 This observation is called the OCTET RULE Page 2 12 Building a Dot Structure 13 Ammonia, NH3 Building a Dot Structure 3. Form a single bond between the central atom and each surrounding atom 1. Decide on the central atom; never H. Central atom is atom of lowest Central affinity for electrons. 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 •• NH •• • • O •• S •• •• •• 10 pairs of electrons are 10 pairs of electrons are 10 now left. now left. now • • Each atom is surrounded by an octet of elect rons. electrons. O S O 17 18 Carbon Dioxide, CO2 Carbon Dioxide, CO2 1. 2. 3. Carbon Dioxide, CO2 Carbon Dioxide, CO2 Central atom = _______ Valence electrons = __ or __ pairs Form bonds. O • • O O H electrons), while H shares 1 pair. Remaining pairs become lone pairs, first on outside atoms and then on central atom. •• Step 1. Central atom = S Step 2. Count valence electrons 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 16 O 15 Sulfite ion, SO32Sulfite ion, SO32- HNH 4. Remaining electrons form LONE PAIRS to complete octet asH needed. 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 Sulfite ion, SO32Sulfite ion, SO32- • • 14 C O 4. Place lone pairs on outer atoms. • • •• This leaves 6 pairs. •• O •• C •• O • • •• •• C O • • •• 5. So that C has an octet, we shall form DOUBLE BONDS between C and O. 4. Place lone pairs on outer atoms. • • •• O • • •• O •• C •• O •• • • • • O •• C The second bonding pair forms a The Page 3 O • • •• pi (π) bond. pi Double and even triple bonds are commonly observed for C, N, P, O, and S 19 H2CO •• • • C2F4 O •• O •• C O •• S •• O bring in left pair •• • • •• O •• •• S O •• S O • • •• This leads to the following structures. • • O •• •• S •• O •• •• • • •• O • • •• O S •• • • These equivalent structures are called RESONANCE STRUCTURES. The RESONANCE The true electronic structure is a HYBRID of true HYBRID of the two. • • •• 22 Urea, (NH2)2CO O •• OR bring in right pair •• •• •• •• 23 Urea, (NH2)2CO 1. Number of valence electrons = 24 e2. Draw sigma bonds. 24 Urea, (NH2)2CO 3. Place remaining electron pairs in the molecule. O HN H C 21 OR bring in right pair • • • • 3. Form double bond so that S has an octet — but note that there are two ways of doing this. • • Sulfur Dioxide, SO2 Sulfur Dioxide, SO2 bring in left pair 1. Central atom = S 2. Valence electrons = 18 or 9 pairs SO3 • • 20 Sulfur Dioxide, SO2 Sulfur Dioxide, SO2 •• • • N H Page 4 H •• HN H O C • • •• N H H 25 Urea, (NH2)2CO Violations of the Octet Rule Usually occurs with B and elements of higher periods. 4. Complete C atom octet with double bond. •• O •• HN • • 26 Boron Trifluoride Trifluoride • Central atom = _____________ • Valence electrons = __________ or electron pairs = __________ • Assemble dot structure •• C H •• N • • H •• H BF3 3 SF4 4 • • F •• F B • • F •• 28 Sulfur Tetrafluoride , SF4 Tetrafluoride, • Central atom = • Valence electrons = ___ or ___ pairs. • Form sigma bonds and distribute electron pairs. •• • • •• F S •• •• • • F •• •• F •• • • •• F •• • • 5 pairs around the S 5 pairs around the S atom. A common atom. A common occurrence outside the occurrence outside the 2nd period. 2nd period. Page 5 • • • • The B atom has a The B atom has a share in only 6 pairs share in only 6 pairs of electrons (or 3 of electrons (or 3 pairs). B atom in pairs). B atom in many molecules is many molecules is electron deficient. electron deficient. 27 ...
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