CH 101 1st Edition Lecture 14 Outline of Last Lecture I Bond Order in Complex Molecules II Pi and Sigma Bonds III Introduction to Hybridization Outline of Current Lecture I Molecular Orbital Theory MO Theory A HOMO and LUMO II Bonding MO s III Antibonding MO s IV Nonbonding MO s A Acceptable and Unacceptable MO bonding Current Lecture I Molecular Orbital Theory MO Theory Molecular Orbital Theory shows us the different possible bonds between the electrons in a molecule this is also known as delocalized bonding MO theory tells us that when electrons combine they could create a bonding orbital an antibonding orbital or a nonbonding orbital Paramagnetic An element is paramagnetic if it has unpaired electrons Di magnetic If the element fills all of the orbitals like Ne it is di magnetic Bond Order To find the bond order through the MO theory you take the number of electrons that are bonding subtract the number of electrons that are antibonding then divide by two Here is the MO diagram some call it a staircase you will use to determine the bond order if the element is paramagnetic di magnetic and where the LUMO and HOMO is The sigma and pi bonds with a star next to them represent antibonding A HOMO and LUMO HOMO The acronym homo stands for highest occupied molecular orbital If you visualize an electron configuration chart the HOMO is the last orbital that is fully occupied with electrons LUMO This stands for lowest unoccupied molecular orbital This orbital would be the orbital that has no electrons in it completely unoccupied Ex Fill out the MO diagram for O2 Determine the HOMO and LUMO the bond order and decide if it is paramagnetic or diamagnetic Valence electrons for O2 12 First look at the periodic table and determine the valence electrons remember it is O2 so the valence electrons are doubled Take the 12 valence electrons and distribute them into the orbitals of your MO diagram Because there are 2 lone pairs O2 is paramagnetic Bonding electrons 8 Antibonding electrons 4 8 4 2 2 therefore the bond order is 2 II Bonding MO s Also called constructive MO s bonding MO s occur when the electron waves are the same so when they come together they form a bond These bonds form molecular orbitals by combining in phase through constructive overlap In pictures combining electrons that are the same color are going to bond and and and Sigma Bonding Pi bonding III Antibonding MO s Antibonding or deconstructive bonding occurs when electrons with opposite spins meet and the electron waves cancel each other out think of it like the graphs of sin and cos Antibonding creates nodal planes between electrons that are combined out of phase These bonds are higher in energy than the bonding MO s In pictures antibonding occurs when electrons of different colors combine A nodal plane forms between the electrons that are antibonding hence the number of antibonding lectrons is equal to the number of nodal planes and Pi antibonding and Sigma antibonding IV Nonbonding MO s Nonbonding MO s occur when electrons bond because they are separated by a node In nonbonding there is no energy change A Acceptable and Unacceptable MO bonding It is important to understand what types of MO bonding is allowed it all depends on the placement of the nodes Here are the accepted bonds from lowest to greatest energy the energy increases with the number of nodes The black dots in the pictures count as a node because those electrons are nonbonding and the area between each antibonding electron pair is also a node Sigma Bonds 4 nodes high energy 3 nodes 2 nodes 1 node 0 nodes low energy Pi Bonds 4 nodes high energy 3 nodes 2 nodes 1 node 0 nodes low energy There are new arrangements of electron orbitals as the amount of nodes increases but if you are asked to identify an MO that is not correct check the number of nodes and see if it matches these pictures above