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Valence Bond (VB) Theory
qualitative, visual picture; the idea that bonds are formed by the overlap of atomic orbitals
Molecular Orbital (MO) Theory
quantitative description of bonding in which pure atomic orbitals combine to produce molecular orbitals delocalized over two or more atoms
when the electron clouds on two atoms interpenetrate; increases the probability of finding the bonding electrons in the region of space between the two nuclei; bases for valence bond theory
Sigma (σ) Bond
Bond in which electon density is greatest along the axis of the bond; covalent bond that arises from the overlap of two s orbitals
Orbital Hybridization Theory (Lin Pauling)
suggested that a new set of orbitals, called hybrid orbitals, could be created by mixing the s, p, and (when required) d atomic orbitals on an atom
an orbital formed by mixing two or more atomic orbitals; is always equal to the number of atomic orbitals mixed to create set; built by combining an s orbital with as many p or d orbitals to accommodate the bond and lone pairs on the central atom; directed toward terminal atoms
According to valence bond theory, ___________ requires that two orbitals on addjacent atoms overlap
Pi (π) Bond
the overlap region is above and below the internuclear axis, and the electron density is above and below the bond axis
Pi (π) Bond Formation
can only form if there are unhybridized p orbital on adjacent (bonded) atoms and p orbitals are perpendiculat ro the plane of the molecule and parallel to one another
(according to MO theory) pure atomic orbitals of the atoms in the molecule combine to produce orbitals that are spread out, delocalized, over several atoms or even over an entire molecule
First Principle of MO
the total number of molecular orbitals is always equal to the total number of atomic orbitals contributed by the atoms that have combined
Bonding Molecular Orbital
one molecular orbital results from the addition of the 1s atomic orbital wave functions, leading to an increased probability that electrons will reside in the bond region between the two nuclei; also a sigma orbital because the region of the electron probability lies directly along the bond axis
Antibonding Molecular Orbital
subtracting one atomic orbital wave funcion from the other; reduces probability of finding and electron between the nuclei in the molecular orbital; probability of finding it in other regions is higher
Second Principle of MO
the bonding molecular orbital is lower in energy than the parent orbitals, and the antibonding orbital is higher in energy
According to 2nd Principle of MO, the system is _________ by chemical bond formation; system is ___________ when electrons occupy antibonding orbitals because the energy is higher than atoms
Third Principle of MO
electrons of the molecule are assigned to orbitals of successively higher energy (according to the Pauli Exclusion Principle and Hund's Rule)
The net number of bonding electron pairs linking a pair of atoms; *fractional values are possible
Fourth Principle of MO
atomic orbitals combine to form molecular orbitals most effectively when the atomic orbitals are of similar energy
Bond Order; Decreases
as the ________ between a pair of atoms increases, the energy required to break the bond increases and the bond distance ___________.
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