Chemistry Chapter 8 - Bond Theories and Molecular Geometry

Valence Bond Theory (Covalent)

an orbital on one atom that overlaps an orbital on a second atom; the single electrons in each orbital combine to form an electron pair

Sigma Bond

involves the s orbital electrons and occurs when orbitals overlap on the bond axis to create a single bond

Bond Axis

a region of high electron density forms on a line that connects the two nuclei

Pi Bond

involve p orbitals, can be found in double and triple bonds, weaker than sigma bonds

How do Pi bonds form?

double and triple bonds form when more than one set of orbitals from the p sublevel overlap

Where do Pi bonds form?

the lobes overlap above and below the bond axis to form two regions of high electron density parallel to the bond axis

Molecular Resonance

no single Lewis structure can completely describe the distribution of electrons

Resonance

a molecule that does not oscillate between two (or more) possible electron configurations

Intermediate Character

average between single and double bond possibilities

Octet Exceptions

some molecules have an odd number of valence electrons to share

Free Radicals

unpaired electrons due to an odd number of valence electrons to share

Electron Deficiency

some elements are content with fewer than four pairs of electrons

Hypervalent Molecules

some molecules contain a central atom that has more than eight valence electrons

Where do hypervalent molecules occur?

only in period three or higher because elements need to fill d and f sublevels 

Molecular Orbital Theory

the orbitals of an atom combine when a molecule is formed; create a lower energy molecule; electrons fill low to high orbitals; best if similar energy levels

Valence Shell Electron Pair Repulsion Theory (VSEPR)

electrons repel each other until they are as far apart as geometrically possible; determines the shape of molecules and will result in a bond angle

Linear

two atoms or more with no lone pairs

Bent

depends on the number of lone pairs

Trigonal Pyramidal

has a lone pair with three bonding pairs

Trigonal Planar

central atom surrounded by three atoms with no lone pairs

Tetrahedral

four atoms around the central atom

Trigonal Bipyramidal

one atom at the center with five or more surrounding it