Molecular Orbital Theory basics

It involves the bond between a diatomic molecule, such as H2, N2, CN-, C2, etc. (hence the MOLECULAR orbital theory)

H Hydrogen has 1 electron in its valence shell. This electron exists in the 1s orbital. This can be seen on the picture

That one electron That electron is in the 1s orbital, as we said. This orbital is around the Hydrogen ATOM. Therefore this is an ATOMIC orbital.

H2 When we put 2 Hydrogens together, they become a molecule. That’s why it is called molecular hydrogen . Now that the electrons from each hydrogen are interacting, they are no longer in an atomic orbital, but rather a MOLECULAR orbital.

Atomic orbitals vs molecular orbitals (using Carbon vs C2 as an example) Molecular orbital diagram for C2 Electron diagram of 1 carbon atom: VS. These are atomic orbitals. We learned that 1s is closest to the atom and then 2s is next, and then 2p, and then 3s, etc. The middle boxes are molecular orbitals, so you simply learn their order (sigma, sigma*, pi, sigma, etc) like you learned the atomic orbital order (1s, 2s, 2p, 3s, etc). Next slide will talk more about this.

Explaining this diagram. The grey boxes represent the valence electrons of eachof the 2 Carbons. The * symbol means that it is an antibonding orbital. If there is no * next to it, then it is a bonding orbital. The next slide talks about what bonding vs. antibonding means in terms of energy. Notice how there are dashed lines connecting the boxes. Each 2s box connects only to the sigma and sigma* because those are the 2s molecular orbitals. The 2p boxes, however, connect to a pi, sigma, pi*, and sigma* because those are all of the 2p molecular orbitals. You guys will have to memorize the order from bottom to top of the molecular orbitals (so: sigma, sigma*, pi, sigma, pi*, sigma* You can connect any two elements this way and see how they would make a bond.

Quick concept about Energy The energy of the electrons in a BONDING molecular orbital is low. It is lower than the energy that the electrons have in their respective ATOMIC orbital. EX. Looking at the previous picture, the sigma and pi BONDING (no *) molecular orbitals for the 2p atomic orbital are lower than the 2p atomic orbitals. Now look at the pi* and sigma* molecular NONBONDING orbitals. They are HIGHER than the 2p atomic orbitals, meaning they are higher in energy and are therefore more unstable. This concept is so you understand that antibonding orbitals are higher in energy than bonding orbitals for each respective atomic orbital.

There is one last thing you need to know about the diagrams: BOND ORDER: It is calculated using the following formula: A bond order of 0 or lower means that the bond does not occur. A bond order greater than 0 means that it does occur. The higher the bond order the stronger the bond! Now calculate the bond order for the diagram on the right.

Okay just 1 Last thing Here we can see that C2 is diamagnetic because the last molecular orbital filled has only paired electrons. When you see a diagram where the final molecular orbital filled has unpaired electrons, then you would call it paramagnetic. If you do not know what those are, look it up.