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 (  - ), hydrogen is (  + ) Ex 2: CO 2 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 CO 2 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 H 2 O 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  -  +

Tetrahedral Four atoms shared with a single central atom Carbon and silicon most notable to have this arrangement Ex 1: Methane (CH 4 ) 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 CH 3 Cl 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: NH 3 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: BF 3 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