BOND THEORIES VALENCE BOND THEORY Electron dot structures and the VSEPR model allow us to predict molecular shape and electron distribution But what about the electronic nature of a bond A quantum mechanical model called valence bond theory can be used to describe and visualize bonds using orbital pictures Uses spatial overlap of orbitals to explain bond formation Covalent bonds are formed by overlap of atomic orbitals each of which contains one electron of opposite spin To get a bonding interaction in the valence bond model the overlapping orbitals must be of the same phase Important for p and higher orbitals The greater the overlap the stronger the bond Bond formation by head on overlap of orbitals gives sigma bonds How can valence bond theory explain molecular shapes How can the bonding in CH4 be explained Carbon ground state configuration 2s 2 2p 2 Carbon excited state configuration 2s 1 2p 3 Linus Pauling showed that QM wave functions for s and p orbitals can be mathematically combined to form a new set of equivalent wave functions called hybrid atomic orbitals spy hybridization is also possible when lone pairs are involved Some of the sp3 orbitals simply don t overlap with another bonding partner Molecules with 5 or 6 charge clouds obviously must involve other orbitals but recent QM studies have shown that bonding is much more complex than originally assumed Orbital names reflect number of orbitals used sp sp2 sp3 CONTINUED HYBRID ORBITALS Atoms with three charge clouds form sp2 hybrid orbitals from one s and two p orbitals 120 degrees apart One p orbital stays unchanged and oriented at 90 degrees to hybrid orbitals For sp2 hybridized atoms the unhybridized p orbitals can also form a bond in a sideways fashion Electrons are shared in areas above and below the line connecting the nuclei A pi bond is the sideways overlap of p orbitals Atoms with 2 charge clouds form sp hybrid orbitals from one s and one p orbitals 180 degrees apart Two p orbitals stay unchanged and oriented at 90 degrees to each other and the sp hybrid orbitals For sp hybridized atoms the two unhybridized p orbitals can form two pi bonds Either one triple or two double bonds MOLECULAR ORBITAL THEORY The concepts introduced so far are easy to visualize and can explain most molecules However some properties of molecules cannot be satisfactorily explained with valence bond theory We need a better description in such cases Molecular orbital theory is more complex and less visual but can give a better description of certain molecules than valence bond theory This includes simple molecules like O2 as we will see later MO theory extends the QM concepts introduced for atoms Atomic orbital A wave function whose square gives the probability of finding an electron in a given region of space in an atom Molecular orbital A wave function whose square gives the probability of finding an electron in a given region of space in a molecule MO has many similarities to atomic orbitals Specific energy levels Can be occupied by a maximum of two electrons with opposite spins Molecular orbitals form through additive or subtractive combination of atomic orbital wave functions Molecular orbitals can be represented with energy diagrams similar to what we used for atomic orbitals BOND ORDER bonding electrons antibonding electrons 2 MOs are to molecules what atomic orbitals are to atoms MOs are formed by combining atomic orbitals on different atoms The number of MOs is the same as the starting number of AOs MOs that are lower in E than the starting AOs are called bonding MOs that are higher in E are called anti bonding Electrons occupy MOs beginning with the lowest E Two electrons per orbital which are spin paired Bond orders are calculated by subtracting the number of electrons in anti bonding MOs from the number in bonding MOs and dividing by 2 Equal orbitals interact e g 2s 2s 2p 2p