Bonding

Chemical Bonding:Basic Concepts : 

Chemical Bonding:Basic Concepts

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9.1 Valence electrons are the outer shell electrons of an atom. The valence electrons are the electrons that particpate in chemical bonding.

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9.1

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9.2 The Ionic Bond 1s22s1 1s22s22p5 1s2 1s22s22p6 [He] [Ne]

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9.3 Lattice energy (E) increases as Q increases and/or as r decreases. r F < r Cl Electrostatic (Lattice) Energy Q+ is the charge on the cation Q- is the charge on the anion r is the distance between the ions Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.

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9.3 Born-Haber Cycle for Determining Lattice Energy

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9.3

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Chemistry In Action: Sodium Chloride Mining Salt Solar Evaporation for Salt

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A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Lewis structure of F2 9.4

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+ + Lewis structure of water Double bond – two atoms share two pairs of electrons or Triple bond – two atoms share three pairs of electrons or 9.4

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Lengths of Covalent Bonds Bond Lengths Triple bond < Double Bond < Single Bond 9.4

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9.4

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Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms electron rich region electron poor region e- rich e- poor d+ d- 9.5

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Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond. Electron Affinity - measurable, Cl is highest Electronegativity - relative, F is highest 9.5

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9.5

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9.5

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Classification of bonds by difference in electronegativity Difference Bond Type 0 Covalent ? 2 Ionic 0 < and <2 Polar Covalent 9.5

Bonding Triangle : 

Bonding Triangle You will NOT see this on the AP Exam.

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Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3 Ionic H – 2.1 S – 2.5 2.5 – 2.1 = 0.4 Polar Covalent N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent 9.5

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Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center. Count total number of valence e-. Add 1 for each negative charge. Subtract 1 for each positive charge. Complete an octet for all atoms except hydrogen If structure contains too many electrons, form double and triple bonds on central atom as needed. Writing Lewis Structures 9.6

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Step 1 – N is less electronegative than F, put N in center Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5) 5 + (3 x 7) = 26 valence electrons Step 3 – Draw single bonds between N and F atoms and complete octets on N and F atoms. Step 4 - Check, are # of e- in structure equal to number of valence e- ? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons 9.6

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Step 1 – C is less electronegative than O, put C in center Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4) -2 charge – 2e- 4 + (3 x 6) + 2 = 24 valence electrons Step 3 – Draw single bonds between C and O atoms and complete octet on C and O atoms. Step 4 - Check, are # of e- in structure equal to number of valence e- ? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons 9.6 Step 5 - Too many electrons, form double bond and re-check # of e-

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9.7 Two possible skeletal structures of formaldehyde (CH2O) An atom’s formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure. The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion.

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formal charge on C = 4 - 2 - ½ x 6 = -1 formal charge on O = 6 - 2 - ½ x 6 = +1 -1 +1 9.7

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formal charge on C = 4 - 0 - ½ x 8 = 0 formal charge on O = 6 - 4 - ½ x 4 = 0 0 0 9.7

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Formal Charge and Lewis Structures 9.7 For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present. Lewis structures with large formal charges are less plausible than those with small formal charges. Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms.

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A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure (after formal charge has been determined!). The true structure is an AVERAGE of all the possible structures. More possible structures gives the overall structure more validity. 9.8

Violations of the Octet Rule : 

Violations of the Octet Rule Usually occurs with B and elements of higher periods and most nonmetals. Common exceptions are: Be, B, P, S, Xe, Cl, Br, and As. How do you know if it’s an EXPANDED octet? More than 4 bonds Formal Charge doesn’t work out with just 8 Be: 4 B: 6 P: 8 OR 10 S: 8, 10, OR 12 Xe: 8, 10, OR 12

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Exceptions to the Octet Rule The Incomplete Octet BeH2 BF3 9.9

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Exceptions to the Octet Rule Odd-Electron Molecules NO The Expanded Octet (central atom with principal quantum number n > 2) SF6 9.9

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The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond energy. Bond Energy 9.10

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Bond Energies (BE) and Enthalpy changes in reactions DH0 = total energy input – total energy released = SBE(reactants) – SBE(products) Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products. 9.10

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9.10

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DH0 = SBE(reactants) – SBE(products) DH0 = 436.4 + 156.9 – 2 x 568.2 = -543.1 kJ 9.10