Chemical Bonding. Chemical Bonding

Transcription

1 ocaine EMIAL BDIG 1 hemical Bonding Problems and questions ow is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? an we predict the structure? ow is structure related to chemical and physical properties? 2 Forms of hemical Bonds There are 2 extreme forms of connecting or bonding atoms: Ionic Ionic complete transfer of 1 or more electrons from one atom to another ovalent some valence electrons shared between atoms Most bonds are somewhere in between. 3 Essentially complete electron transfer from an element of low IE (metal) to an element of high affinity for electrons (nonmetal) 2 a(s) + l 2 (g) ---> l - 2 a Therefore, ionic compds. exist primarily between metals at left of periodic table (Grps( 1A and 2A and transition metals) and nonmetals at right ( and Ionic Bonds 4 ovalent Bonding The bond arises from the mutual attraction of 2 nuclei for the same electrons. Electron sharing results. (creen 9.9) A + B A B Bond is a balance of attractive and repulsive forces. 5 hemical Bonding bjectives bjectives are to understand: 1. valence e- distribution in molecules and ions. 2. molecular structures 3. bond properties and their effect on molecular properties. 6 Page 1

2 Electron Distribution in Molecules G.. Lewis Electron distribution is depicted with Lewis electron dot structures Valence electrons are distributed as shared or BD PAIR and unshared or LE PAIR. 7 Bond and Lone Pairs Valence electrons are distributed as shared or BD PAIR and unshared or unshared or LE PAIR. l shared or bond pair lone pair (LP) This is called a LEWI ELETR DT structure. 8 Bond Formation A bond can result from a head-to- head overlap of atomic orbitals on neighboring atoms. + l l verlap of (1s) and l (2p) ote that each atom has a single, unpaired electron. 9 Valence Electrons core and Electrons are divided between core and valence electrons B 1s 2 2s 2 2p 1 ore = [e] and valence = 2s 2 2p 1 Br [Ar[ Ar] ] 3d 10 4s 2 4p 5 ore = [Ar[ Ar] ] 3d 10 and valence = 4s 2 4p 5 10 Rules of the Game o. 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.. o. 11 Rules of the Game o. 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. o. Except for (and sometimes atoms of 3rd and higher periods), BP s + LP s = 4 This observation is called the TET RULE 12 Page 2

3 Building a Dot tructure Ammonia, 3 1. Decide on the central atom; never. entral atom is atom of lowest affinity for electrons. Therefore, is central 2. ount valence electrons = 1 and = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs 13 Building a Dot tructure 3. Form a single bond between the central atom and each surrounding atom 4. Remaining electrons form LE PAIR to complete octet as needed. 3 BD PAIR and 1 LE ote PAIR. that has a share in 4 pairs (8 electrons), while shares 1 pair. 14 ulfite ion, tep 1. entral atom = tep 2. ount valence electrons = 6 3 x = 3 x 6 = 18 egative charge = 2 TTAL = 26 e- or 13 pairs tep 3. Form bonds 10 pairs of of electrons are now left. 15 ulfite ion, arbon Dioxide, 2 17 arbon Dioxide, 2 18 Remaining pairs become lone pairs, first on outside atoms and then on central atom. 1. entral atom = 2. Valence electrons = or pairs 3. Form bonds. 4. Place lone pairs on outer atoms. Each atom is surrounded by an octet of electrons. 4. Place lone pairs on outer atoms. This leaves 6 pairs. 5. o that has an octet, we shall form DUBLE BD between and. The second bonding pair forms a pi (π) bond. Page 3

4 Double and even triple bonds are commonly observed for,, P,, and 2 F ulfur Dioxide, 2 1. entral atom = 2. Valence electrons = 18 or 9 pairs 3. Form double bond so that has an octet but note that there are two ways of doing this. bring in R bring in left pair right pair 20 ulfur Dioxide, 2 bring in left pair This leads to the following structures. R bring in right pair These equivalent structures are called REAE TRUTURE. The true electronic structure is a YBRID of the two Urea, ( 2 ) 2 Urea, ( 2 ) 2 Urea, ( 2 ) 2 1. umber of valence electrons = 24 e- 2. Draw sigma bonds. 3. Place remaining electron pairs in the molecule. Page 4

5 Urea, ( 2 ) 2 4. omplete atom octet with double bond. 25 Violations of the ctet Rule Usually occurs with B and elements of higher periods. 26 Boron Trifluoride entral atom = Valence electrons = or electron pairs = Assemble dot structure 27 BF 3 F 4 F F B F The B atom has a share in only 6 pairs of electrons (or 3 pairs). B atom in many molecules is electron deficient. 28 ulfur Tetrafluoride,, F 4 entral atom = Valence electrons = or pairs. Form sigma bonds and distribute electron pairs. F F F F 5 pairs around the atom. A common occurrence outside the 2nd period. Page 5