Covalent Bonds and Bond Orders
Now that you can draw Lewis electron-dot structures, understanding what a covalent bond "looks like" on the molecular level should be much easier. In this section, we'll discuss the different types of covalent bonds and how to name covalent compounds.
We've already mentioned covalent bonds in which more than one pair of electrons is shared. An example of this is the carbon dioxide molecule that we electron-dotted on the previous page; each oxygen shares two pairs of electrons with the carbon atom. Covalent bonds in which one pair of electrons is shared are called first-order bonds. H2O and Cl2 employ only first-order bonds. Two pairs shared, as in CO2 and O2, results in a second-order bond. In elemental nitrogen, N2, three pairs of electrons are shared, creating a third-order bond.
The sharing of multiple pairs of electrons gives some unique characteristics to these bonds. First, they are stronger than first-order bonds, releasing more energy when formed and requiring more energy to break. Second, the bonding atoms move closer together with each successive electron pair shared. Third, they can "lock" the molecule into a particular shape; atoms are free to rotate around a first-order bond, as can two styrofoam balls with a toothpick joining them, but two or more shared pairs prevents rotation around the bond.
Just for fun, let's look at the Lewis electron-dot structure for sulphur trioxide, SO3. Okay, it's not just for fun. In this molecule, as shown by the Lewis diagram below, two oxygen bonds are first-order, while the other is second order:
However, there is no reason to favor putting the double-bond on any particular oxygen atom; formal charge is the same no matter what. Further, experiments have concluded that each bond has identical strength and energy. Therefore, the sulphur-oxygen bonds in this molecule aren't first or second order. Rather, the order is found by taking the total number of bonds (2 single plus 1 double = 4) and dividing by the number of atom-atom connections (3 sulphur-oxygen connections) to get a bond order of 4/3. So, each bond in this molecule is a 4/3-order covalent bond!
Two or more Lewis structures with equally-favorable formal charges are called resonance structures, and the real configuration is like an average of all possible resonance structures.
Another compound exemplifying resonance structures is benzene, a ring of six carbon atoms with every other carbon-carbon bond being second-order. The molecule is given below, at left, with the six attached hydrogen atoms. The symbol at right represents the 3/2 bond order of all the carbon-carbon bonds in benzene.
Naming "binary" covalent substances (with just two elements involved) is easy, but certain prefixes must be used to indicate how many atoms of each element are present. These prefixes are:
# of atoms Prefix 1 Mono 2 Di 3 Tri 4 Tetra 5 Penta 10 Deca The first element in the compound uses its normal elemental name, preceded by on of the above prefixes if necessary. The second element uses a numerical prefix, sometimes even if only one is present (as in CO, carbon monoxide), and its "ium," "ine," or "ogen" is replaced with an "ide." For example, water can be called dihydrogen monoxide, As4O10 is tetrarsenic decaoxide, and N2O4 is dinitrogen tetroxide. There are no special rules or exceptions to this naming system, making it very simple and much easier to remember.
Compounds with more than two elements are given their own names, and there is no system for naming them. If you do not know the name of such a compound, using the molecular formula is acceptable.
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