Test 2 - Ch 4 (VSEPR and Molecular Polarity)

Electron group arrangement

3D geometric shape the bonding and non-bonding e- occupy

Molecular shape

The 3D shape that only the bonded atoms occupy, i.e. what the molecule “looks like” (based on bond angles effect on bonds)

VSEPR notation

AX_mE_n

A - central atom, X - bonded atom, E - L pair

m and n - how many of each you have

X and E are e- groups

Steric #

m + n: total # of “e- groups”

Note: bond order between A and X does not matter. X counts as 1e- group

AX_2

AXE

Note: bond angle N/A bc only 2 atoms (all diatomic are linear)

AX_3

Trigonal planar

angle: 120

AX_2E

Bent/V-shaped

Angle: <120

AX_4

Tetrahedral

Angle: 109.5

AX_3E

Trigonal pyramidal

Angle: <109.5

AX_2E_2

Bent/V-shaped

Angle: <109.5

AX_5

Trigonal bipyramidal

Angle: 90, 120

AX_4E

Seesaw

<90, <120

AX_3E_2

T-shaped

Angle: <90

AX_2E_3

Trigonal bipyramidal

Angle: 180

AX_6

Octahedral

Angle: 90

AX_5E

Square pyramidal

Angle: <90

AX_4E_2

Square planar

Angle: 90

Groups that cause bond angle

• Lone pairs

• Double/triple bonds

• Lone pair - lone pair repulsion is STRONGEST

Molecular polarity

• An uneven distribution of charge over a whole molecule or large portion

• Units: Debye (D) = (3.34E-30) x c x m

• Dipole moment abbreviated as M(mu)

Must have polar bonds (Predicting molecular polarity)

Homoatomic molecules (N2, P4, S8, etc.) cannot be polar, but O3 (ozone) is an exception

Geometry is key (Predicting molecular polarity)

The presence of polar bonds does not guarantee that a molecule will be polar, geometry is key

Structures w/ no lone pairs on central atoms (AX_m) (Predicting molecular polarity)

• All X’s are the same type of outer atom. All nonpolar

• Structures w/ more than 1 type of outer atom (AXX’E_n). Almost always polar

Structures w/ 1 lone pair on central atom (AX_mE_1) (Predicting molecular polarity)

Always polar

-All bent molecules are polar

Structures w/ more than 1 lone pair (Predicting molecular polarity)

May or may not be polar