TAILIEUCHUNG - Simple Molecular Orbitals - Sigma and Pi Bonds in Molecules

. government agency bonds and securities are issued by agencies that are owned, backed, or sponsored by the . some of those bonds and securities are backed by the full faith and credit of the government, others carry less formal most common agency securities are mortgage pass-through securities such as those issued by the Government National Mortgage Association (GNMA, or “Ginnie Mae”), the Federal National Mortgage Association (FNMA, or “Fannie Mae”), and the Federal Home Loan Mortgage Corporation (FHLMC, or “Freddie Mac”). Mortgage pass-through securities are backed by home mortgage loans. By purchasing mortgage pass-through securities, investors are making mortgage loans to homeowners through intermediary make monthly. | 1 Lecture 2 Simple Molecular Orbitals - Sigma and Pi Bonds in Molecules An atomic orbital is located on a single atom. When two or more atomic orbitals overlap to make a bond we can change our perspective to include all of the bonded atoms and their overlapping orbitals. Since more than one atom is involved we refer to these orbitals as molecular orbitals. Quantum mechanics uses higher mathematics to describe this mixing but we can use symbolic arithmetic and descriptive pictures of the mathematical predictions. The total number of atomic orbitals mixed is always the same as the number of molecular orbitals generated. At this point we just want to show how to create the two most common types of bonds used in our discussions sigma bonds and pi bonds. You very likely remember these bonds from your earlier chemistry course but it s usually good to take a quick review. The first covalent bond between two atoms is always a sigma bond. We will use hydrogen as our first example because of its simplicity. Later we will use this approach to generate a sigma bond between any two atoms. Recall our earlier picture of two hydrogen atoms forming a bond becoming molecular diatomic hydrogen. H H H -H Two hydrogen atoms join together to attain the helium Noble gas configuration by sharing electrons and form a molecule. f Two electron pure covalent bond Each hydrogen atom brings a single electron in its 1s atomic orbital to share electron density thus acquiring two electrons in its valence shell. This shared electron density lies directly between the bonding atoms along the bonding axis. The interaction of the two bonded atoms with the bonding electrons produces a more stable arrangement for the atoms than when they are separated and the potential energy is lowered by an amount referred to as the bond energy lower potential energy is more stable . Using our simplistic mathematics we will indicate this by adding the two atomic 1s orbitals together to produce a sigma molecular orbital

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