The last part of Lecture 11 introduces bond order. Bond order is defined as the number of bonds divided by the number of bonding regions. Bonding regions are defined as an area of electron density between two atoms. Here are some Lewis structures of diatomic gases.
For these structures, bond order is very easy to determine. I see one bond. within one bonding region for hydrogen. So the bond order is 1. This is also called a single bond.
For oxygen, I see two lines within the bonding region, so this would be known as bond order 2 or a double bond. For nitrogen, I see three bonds within the bonding region, so this would be known as a triple bond with bond order 3. While many bond orders are integers, soon you'll find examples where the bond order might be a fraction. In order to introduce some other concepts about bond order, I'd like to give you a series of compounds with carbon-carbon bonds.
The first one is ethane, and if you'd like to check later, you can check my math and make sure I've done this correctly. The important point is that there is a single bond between the two carbons. This would be for C2H6.
If I take away two hydrogens, the bond order between the carbons is 2. If I take away two more hydrogens to C2H2, the bond order between the carbons is 3. Here are all three structures where the carbon-carbon bond order goes from 1 to C2H6. to 2 to 3. There are other things that change. The carbon-carbon bond length for a single bond is 1.54 angstroms.
For a double bond, it is shorter at 1.33 angstroms, and a triple bond is shorter still at 1.20 angstroms. There's also the bond energy change. Bond energy is the energy required to break a bond. The bond energy for a carbon-carbon single bond is 347 kilojoules per mole.
For a double bond, it is 612 kilojoules per mole. Notice 612 is not double of 347. Soon you'll learn about sigma and pi bonds and realize why this is so. A carbon-carbon triple bond requires 820 kilojoules per mole to break that bond. So here is the pattern.
Within a series of bonds between these same atoms, the higher the bond order, the shorter the bond, the stronger the bond, the higher the bond energy. So this is a demo I do in person in class where two unfortunate students are called to the front to be carbon atoms. So here are two students holding a rope between them. Right now, the rope is like a single bond.
The connection is weak between the two students, and the distance is long. But if we move to bond order two, where we... double the rope by bending it back on itself, notice that the connection is stronger and the distance is shorter.
And if we double up the rope again, the connection is the strongest and the distance is the shortest. So hopefully this reminds you of single, double, and triple bonds. Single bonds are the weakest.
Triple bonds are the strongest. are the strongest, single bonds are the longest, and triple bonds are the shortest. So now a question. Here are a series of compounds with different nitrogen to nitrogen bond order. Please choose the structure with the longest nitrogen to nitrogen bond and the strongest nitrogen to nitrogen bond.