H Represents a pair of shared electrons (a single covalent bond) Sec. 8.2: The Nature of Covalent Bonding In forming covalent bonds, electron sharing occurs so that atoms attain the electron configurations of noble gases. Electron dot structure

The picture to the left shows the model of a fluorine molecule as well as the configuration of each atom when bonded. Try writing the electron dot structure for the covalent bonding of two fluorine atoms. FF + FF FF or Sec. 8.2: The Nature of Covalent Bonding

Draw electron dot structures for each molecule below: O 2H + OH H 3H+NNH H H Sec. 8.2: The Nature of Covalent Bonding

Stuctural formulas and the “Have-Want-Need” Method: Example Question: Draw a structural formula for nitrogen trifluoride (NF 3 ). Valence Electrons: Have = ____ Want = ____ Need = ____ # of Bonds = ________ N=5 F=7 F=7 F=726 N=8 F=8 F=8 F=8 32 (Want – Have) = 6 6 ∕ 2 = 3 There will be three covalent bonds in NF 3 Sec. 8.2: The Nature of Covalent Bonding

* After you have determined the number of bonds in the molecule you can start constructing the structural formula. Step #1: The central atom will be the least electronegative element. (Hydrogen and Fluorine will never be central atoms!) N Step #2: Draw the single covalent bonds between the central atom and each of the surrounding atoms. F F F Step #3: Draw in any “un-bonded” valence electrons. Sec. 8.2: The Nature of Covalent Bonding

Problem: Draw a structural formula for Carbon Dioxide (CO 2 ) Step #1: Determine the number of bonds in the molecule. Have:____ Want:____ Need:____ Bonds:____ C = 4 O = 6 O = C = 8 O = 8 O = 8 Step #2: Determine the central atom. C Step #3: Draw in bonds between central atom and surrounding atoms. OO Step #4: Draw in any un-bonded valence electrons. Sec. 8.2: The Nature of Covalent Bonding

Some molecules, in order to fulfill the octet rule, will exhibit two or even three pairs of shared electrons between atoms. Sec. 8.2: The Nature of Covalent Bonding

Double bonds are stronger than single bonds. Triple bonds are the strongest of all three types of covalent bonds. Sec. 8.2: The Nature of Covalent Bonding

* Sometimes covalent bonds are formed by the sharing of two electrons donated solely from one atom. The result is a coordinate covalent bond. Sec. 8.2: The Nature of Covalent Bonding

Some molecules are NOT adequately described by one Lewis structure. Some molecules can be described as being a BLEND or AVERAGE of two or more structural formulas. For example, nitrate (NO 3 -1 ) has three equivalent structural formulas that can be drawn: N OO O N OO O N OO O These three structures together show the proper electron distribution of Nitrate. The double-headed arrow indicates that the structures shown are resonance forms. Sec. 8.2: The Nature of Covalent Bonding

Not all compounds obey the octet rule. Some compounds, like boron trifluoride, have fewer electrons than a complete octet. Other compounds, like sulfur hexafluoride and phosphorus pentachloride have more electrons than a complete octet. Sec. 8.2: The Nature of Covalent Bonding sulfur hexafluoride boron trifluoride phosphorus pentachloride