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Bonding and Molecular Structure

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

Orbital Overlap

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


means the system will be more stable

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

Hybrid Orbitals

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

Bond Formation

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

Double Bond

always consists of a sigma bond and pi bond

Triple bond

always consists of a sigma bond and 2 pi bonds


compounds that have the same formula but different structures

Molecular Orbitals

(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

Stabilized; Destabilized

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)

Bond Order Formula

1/2(number of electrons in bonding MOs-number of electrons in antibonding MOs)

Bond Order

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 ___________.

Heteronuclear Diatomic Molecules

containing two different elements; examples CO, NO, CIF

Homonuclear Diatomic Molecules

Molecules such as H2, L2, N2, in which two identical atoms are bonded

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