Cl h h cl overlap of h 1s and cl 2p note that each

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Cl H H Cl •• •• •• •• + Overlap of H (1s) and Cl (2p) Note that each atom has a single, unpaired electron.
162 Chemical Bonding: Objectives Objectives are to understand: 1. valence e- distribution in molecules and ions. 2. molecular structures 3. bond properties and their effect on molecular properties.
163 Electron Distribution in Molecules Electron distribution is depicted with Lewis electron dot structures Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. G. N. Lewis 1875 - 1946
164
165 Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS . •• •• H Cl lone pair (LP) shared or bond pair This is called a LEWIS ELECTRON DOT structure.
166 Valence Electrons Electrons are divided between core and valence electrons B 1s 2 2s 2 2p 1 Core = [He] , valence = 2s 2 2p 1 Br [Ar] 3d 10 4s 2 4p 5 Core = [Ar] 3d 10 , valence = 4s 2 4p 5
167 Rules of the Game No. of valence electrons of a main group atom = Group number •For Groups 1A -4A, no. of bond pairs = group number. • For Groups 5A - 7A, BP’s = 8 - Grp. No.
168 Rules of the Game •No. of valence electrons of an atom = Group number •For Groups 1A -4A, no. of bond pairs = group number For Groups 5A - 7A, BP’s = 8 - Grp. No. •Except for H (and sometimes atoms of 3rd and higher periods), BP’s + LP’s = 4 This observation is called the OCTET RULE
169 Building a Dot Structure Ammonia, NH 3 1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. Therefore, N is central 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs
170 10 pairs of electrons are now left. Sulfite ion, SO 3 2- Step 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 O O O S
171 Sulfite ion, SO 3 2- Remaining pairs become lone pairs, first on outside atoms and then on central atom. O O O S •• •• •• •• •• •• Each atom is surrounded by an octet of electrons.
172 Carbon Dioxide, CO 2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs 3. Form bonds. 4. Place lone pairs on outer atoms. This leaves 6 pairs.
173 Carbon Dioxide, CO 2 4. Place lone pairs on outer atoms. The second bonding pair forms a pi (π) bond. 5. So that C has an octet, we shall form DOUBLE BONDS between C and O.
174 Double and even triple bonds are commonly observed for C, N, P, O, and S H 2 CO SO 3 C 2 F 4
175 Sulfur Dioxide, SO 2 1. Central atom = S 2. Valence electrons = 18 or 9 pairs bring in left pair 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. O S •• •• •• •• •• O
176 Sulfur Dioxide, SO 2 This leads to the following structures. These equivalent structures are called RESONANCE STRUCTURES . The true electronic structure is a HYBRID of the two.
177 Urea, (NH 2 ) 2 CO
178 Urea, (NH 2 ) 2 CO 1. Number of valence electrons = 24 e- 2. Draw sigma bonds.

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