VSEPR

• Valence shell electron pair repulsion

• Repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible.

Two atoms

• Linear• Electron pairs spread out as far as possible

to minimize repulsive forces• Draw picture of example

AB2

• Be is used, it is an exception to the octet rule.

• No unshared pairs of electrons.• Linear shape

AB3

• Equilateral triangle with all atoms on one plane

• Trigonal planar• Angles 120 degrees apart• Example -- GaF3

• No unshared pairs

AB4

• Tetrahedron shape– Tetrahedral shape

• No unshared pairs• Example CH4

Now, with unshared electron pairs

• An unshared pair of electrons is associated with the central atom.

• It is like an electron cloud shaped like a pear with one end attached to the nucleus.

• FYI (a shared pair moves between two nuclei and therefore forms a more slender, stretched pear shaped cloud.)

continued

• the unshared pair does not literally occupy space.

• The unshared pair shows greater repulsion than shared electron pairs.

• For geometry purposes only, we think of the unshared electron pair as occupying “space” around the nucleus.

AB3E

• E represents the unshared pair.• Trigonal Pyramidal

– Base forms a triangle, the unshared electron pair forms the top of the pyramid.

AB2E2

• Water --H2O

• Bent, angular shape• 105 degree between the H atoms

Table on page 186

Hybridization

• Carbon – Only has 2 “free” p electrons, but bonds with 4

atoms. – Forms tetrahedral shaped molecules

sp3

• The s orbital merges with the p orbitals to make a sp3 orbital

• Hybrid orbitals• All 4 are identical

Other types of hybridization

Hybridization explains the geometry of many group 15 and 16 elements

sp linear• Sp2 trigonal planar• Sp3 tetrahedral

• Table 6.6 page 189

Polarity

• In a covalent bond the more electronegative atom will pull on the electron more.

• So, the electron will spend more time around the more electronegative atom’s nuclei.

• With geometry this creates a polar molecule.

Polar molecule

• If you can draw a line through it making a partial negative and a partial positive side.

• Polar molecule – uneven distribution of charge.

• Polar molecules cause dipoles.

Intermolecular forces

• Force of attraction between two moleculestypes:

dipole - dipoleinduced dipolehydrogen bondingLondon dispersion

Intermolecular forces

• Forces of attraction between two molecules.

• Measured by boiling point- energy required for a molecule to break away from the other molecules.

• Higher boiling point, stronger the attraction

Polar molecules

• Strongest intermolecular forces• Form dipoles

– Dipole– Arrow indicates the direction of the dipole– Positive to negative pole– Indicated on the bonds

Dipole-dipole forces

• The forces of attraction between polar molecules

• Short range, acts on nearby molecules• Larger the dipole dipole attraction, stronger

the intermolecular forces

Induced dipoles

• Short range• Weaker than dipole- dipole between polar

molecules

• Why some non-polar substances dissolve in polar water

Induced continued

• A polar molecule comes into contact with a non-polar molecule.The partial charge either attracts or repels the electrons of the non-polar molecule. Thus creating a temporary dipole.

Hydrogen bonding

• Some hydrogen containing compounds have unusually high boiling points.

• Explained by a strong dipole dipole force called hydrogen bonding.

• Represented by dashed lines----• Intermolecular force

Hydrogen Bonding definition

• The intermolecular forces in which a small hydrogen atom, that is bonded to a highly electronegative atom, is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.

London dispersion forces

• Electrons are in continuous motion.• At any given instant, the physical

distribution of electrons could be uneven. • Momentary imbalance can cause a positive

and negative pole.• This can then induce a dipole in a

neighboring molecule!

London dispersion forces

• The intermolecular attraction resulting from the constant motion of electrons and the creation of instantaneous dipoles.

• Weak• Intermolecular• More electrons more possibility – stronger

they are.• More electrons generally means more mass.