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Lecture B1 Lewis Dot Structures and Covalent Bonding G.N

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Lecture B1 Lewis Dot Structures and Covalent Bonding G.N Lecture B1 Lewis Dot Structures and Covalent Bonding G.N. Lewis & Linus Pauling Two American Chemists G. N. Lewis Linus Pauling 1875-1946 1901-1994 The Covalent Bond 1. First proposed by G.N. Lewis in his seminal 1916 J. Am. Chem. Soc. publication (before QM was discovered!). 2. Lewis proposed that Covalent bonds consist of shared pairs of electrons. He created a powerful empirical formalism (Lewis dot structures) for understanding bonding in simple compounds. 3. Linus Pauling created a picture of covalent bonding that employed Quantum Mechanics (and won the 1954 Nobel Prize for it). G.N. Lewis, circa 1902 - the cubical atom. G.N. Lewis, Journal of the American Chemical Society 38 (1916) 762. G.N. Lewis, Journal of the American Chemical Society 38 (1916) 762. I2 molecule ...we still think about covalent bonds the way G.N. Lewis did. In contrast to ionic bond, electrons are shared, not transferred... ...if the atoms are identical, then electrons are equally shared - an example of a perfectly nonpolar covalent bond. In covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs. Covalent bonds are drawn as lines. Lone pairs are drawn as pairs of dots. I ⎯ I I2 molecule Lewis dot structures provide a simple, but extremely powerful, formalism for representing covalent bonding in molecules. a “single bond”, consisting of a pair of electrons. Lewis dot structures provide a simple, but extremely powerful, formalism for representing covalent bonding in molecules. a nonbonding pair of electrons: a“lone pair” Lewis dot structures provide a simple, but extremely powerful, formalism for representing covalent bonding in molecules. Note: no geometric information! Lecture B1 Systematic Method for Creating Lewis Dot Structures Lewis Dot Structures can be produced by following a sequence of steps. Let’s produce a Lewis Dot Structure for: + NH4 (the ammonium ion). Step 1: Count valence electrons: N = 5 4 x H = 4 x 1 = 4 “+” = -1 Total = 5+4-1= 8 electrons = 4 bonds and lone pairs. Lewis Dot Structures can be produced by following a sequence of steps. Let’s produce a Lewis Dot Structure for: + NH4 (the ammonium ion). Step 1: Count valence electrons: + N = 5 4 x H = 4 x 1 = 4 H “+” = -1 Total = 5+4-1= 8 electrons = 4 bonds and lone pairs. Step 2: Arrange the atoms (identify a central N H atom, if possible). H H Lewis Dot Structures can be produced by following a sequence of steps. Let’s produce a Lewis Dot Structure for: + NH4 (the ammonium ion). Step 1: Count valence electrons: + N = 5 4 x H = 4 x 1 = 4 H “+” = -1 Total = 5+4-1= 8 electrons = 4 bonds and lone pairs. Step 2: Arrange the atoms (identify a central N H atom, if possible). H Step 3: Place electron pairs on the atoms as either bonds or lone pairs. H Lewis Dot Structures can be produced by following a sequence of steps. Let’s produce a Lewis Dot Structure for: + NH4 (the ammonium ion). Step 1: Count valence electrons: + N = 5 4 x H = 4 x 1 = 4 H “+” = -1 Total = 5+4-1= 8 electrons = 4 bonds and lone pairs. Step 2: Arrange the atoms (identify a central N H atom, if possible). H Step 3: Place electron pairs on the atoms as either bonds or lone pairs. H Lewis Dot Structures can be produced by following a sequence of steps. Let’s produce a Lewis Dot Structure for: + NH4 (the ammonium ion). Step 1: Count valence electrons: + N = 5 4 x H = 4 x 1 = 4 H “+” = -1 Total = 5+4-1= 8 electrons = 4 bonds and lone pairs. Step 2: Arrange the atoms (identify a central N H atom, if possible). H Step 3: Place electron pairs on the atoms as either bonds or lone pairs. Step 4: Count electrons and compare with H known count. If octet rule is satisfied for all atoms and the electron count is right, you’re done. 8 valence electrons around N! Lecture B1 Lewis Dot Structures Multiple Bonds G.N. Lewis, Journal of the American Chemical Society 38 (1916) 762. ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 1: Count the valence electrons: 3 x C = 12 6 x H = 6 x 1 = 6 O = 6 Total = 12+6+6= 24 electrons = 12 bonds or lone pairs. ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 2: Arrange the atoms: Identify a central atom. If you have C or O to choose from, go with C. H O H H C C C H H H ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 3: Place 12 electron pairs on the atoms. H O H H C C C H H H ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 3: Place 12 electron pairs on the atoms. H O H H C C C H H H Nine Bonds, Three Lone Pairs ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 4: Check for the octet rule. oops -- only 6 e! H O H H C C C H H H Nine Bonds, Three Lone Pairs ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 4: Check for the octet rule. Better! H O H H C C C H H H Ten Bonds (one double bond), Two Lone Pairs Done! ...another example: Let’s produce a Lewis Dot structure for: acetone: (CH3)2CO. Step 4: Check for the octet rule. H O H H C C C H H H This takes practice, but then becomes tedious. Getting the polyatomic arrangement can be problematic. How about Double 07 (Nitrogen)? Step 1: 10 valence electrons/five bonds and lone pairs How about Double 07 (Nitrogen)? Step 1: 10 valence electrons/five bonds and lone pairs Step 2: Arrange atoms. N N How about Double 07 (Nitrogen)? Step 1: 10 valence electrons/five bonds and lone pairs Step 2: Arrange atoms. Step 3: Fill in electron pairs and bonds. N N How about Double 07 (Nitrogen)? Step 1: 10 valence electrons/five bonds and lone pairs Step 2: Arrange atoms. Step 3: Fill in electron pairs and bonds. N N How about Double 07 (Nitrogen)? Step 1: 10 valence electrons/five bonds and lone pairs Step 2: Arrange atoms. Step 3: Fill in electron pairs and bonds. Step 4: Octet rule obeyed! N N A Triple Bond! multiple bonds have higher bond dissociation energies... A quantitative comparison of CC bonds -1 N2 Bond Dissociation Energy: 941 kJ mol bond lengths are inversely correlated with bond energies - shorter bonds are stronger... Double 07 is very strong! Lecture B1 Lewis Dot Structures Formal Charge Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Step 2: Arrange the atoms. Go with Carbon in the center. S C S Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Step 2: Arrange the atoms. Go with Carbon in the center. S C S Step 3: Fill in the electron pairs. Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Step 2: Arrange the atoms. Go with Carbon in the center. S C S Step 3: Fill in the electron pairs. Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Step 2: Arrange the atoms. Go with Carbon in the center. S C S Step 3: Fill in the electron pairs. Step 4: Check for the octet rule. Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Step 2: Arrange the atoms. Go with Carbon in the center. S C S Step 3: Fill in the electron pairs. Step 4: Check for the octet rule. Good! Let’s produce a Lewis Dot structure for carbon disulfide, CS2. Step 1: Count valence electrons: C = 4 2 x S = 6 x 2 = 12 Total = 4 + 12 = 16 e/8 electron pairs. Step 2: Arrange the atoms. Go with Carbon in the center. S C S Step 3: Fill in the electron pairs. BUT wait, what about?? Step 4: Check for the octet rule. C S S To decide between these two candidate structures, we need to calculate the formal charges on the atoms. Less formal charge is better. Thus, SCS wins! SCS wins! Carbon dioxide, CO2, is “isoelectronic” with carbon disulfide, CS2. Nitrous oxide, N2O, is also isoelectronic with carbon disulfide, CS2. Lecture B1 Lewis Dot Structures Resonance When multiple Lewis Dot Structures exist that are differentiated only by the positions of the electrons (the positions of the atoms are identical), the ACTUAL structure is a weighted average of these “resonance” structures.
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