Molecular Shapes and Polarity 2

Valence Shell Electron Pair Repulsion Theory• Also known as VSEPR• A theory based on the repulsive forces between

valence electrons• Valence electrons

occur in pairs• Electrons are negative,

so push against each other to make room

• The shape of a molecule is determined by the number of shared and unshared electron pairs

– Unshared pairs (lone pairs) have MORE repulsive force than shared pairs (bonding pairs)

Linear Shape Ex 1: HCl (different electronegativities)• the H and Cl share one pair of electrons • But unequally, due to chlorine’s higher

electronegativity• Chlorine is (d - ), hydrogen is ( d + )

Ex 2: CO2

• Carbon shares 4 electrons with each of 2 oxygens• Thus there are two bonds between the carbon and

each oxygen • Each oxygen still has two

unshared pairs of electrons• These push the shared pairs so that CO2 becomes a

LINEAR molecule, and NONPOLAR

Bent or Angular shape• Forms from unshared pairs repelling

shared pairs of electrons• These unshared pairs have greater

repulsive forces than the shared ones

Ex: Water or H2O

• Each hydrogen shares a pair of electrons with oxygen

• But oxygen’s 2 unshared pairs have a greater repulsive force and “push” the shared electron pairs between O and H into the bent shape

• Making water a POLAR molecule

d -

d +

Tetrahedral • Four atoms shared with a single central atom• Carbon and silicon most notable to have this

arrangement

Ex 1: Methane (CH4)• Pairs of shared electrons have equal repulsion• C has four electrons to share• H each has 1 electron to share• A NONPOLAR molecule (symmetrical shape)

Ex 2: Chloromethane,or CH3Cl atoms • Cl pulls electrons toward it – higher

electronegativity• Each H only shares 1 pair of electrons, none left

over• Cl still has 3 pairs of unshared electrons• Is therefore a POLAR molecule (an

asymmetrical shape)

Pyramidal• Group 15 atoms such as nitrogen

and phosphorus form this the most due to five electrons in valence shell

• One unshared pair repels the other shared pairs

Ex: NH3 has 8 valence electrons– 2 unshared, – 6 shared in 3 bonds or between

N and the 3 H• The unshared pair repels the

shared pairs creating a pyramidal and POLAR shape

Trigonal Planar• Plane – a flat region in space• The trigonal planar shape is a three

spoked shape with a Group 13 atom at the center

• Boron and aluminum have 3 valence electrons

Ex: BF3 Boron triflouride

• The three shared pairs repel each other equally

• making them stay as far from each other as possible which is best done in a flat, planar shape

Polar shapes• in addition to London forces these molecules also

have dipole-dipole interactions• Increases their boiling / melting points

Nonpolar shapes• Only have London forces to hold the molecules

together • Tend to have lower boiling and melting points than

polar ones of same size