Chapter 10: The Shape of Molecules

Lewis structure procedure

• count total number of valence electrons, including negative or positive charges

• draw skeleton structure; usually the unique atom or the atom written first in the formula is central (or the least electronegative?)

• fill in structure; if you are short, make multiple bonds, and if you have too many electrons, stick them on the central atom (must be period 3 or higher to violate the octet rule)

Formal charge

• = # valence electrons - # nonbonding electrons - # bonds

• sum on a molecule or ion must add up to its total charge

• structure with the smallest formal charges is usually the best; same nonzero formal charges on adjacent atoms are not preferred; most plausible structures place more negative formal charges on the more electronegative atoms

Exceptions to the octet rule

• incomplete octets; B or Be

• Molecules with an odd number of electrons (free radicals—dangerous)

• expanded octets (elements in third period or higher; can access d orbitals)

VSEPR

• valence shell electron pair repulsion

• since electron pairs will repel each other, they arrange themselves as far apart as possible to minimize this repulsion energy

• lone pairs occupy more space than do bonding electrons

Electron group arrangement

• defined by the bonding groups (shared electron pairs) and nonbonding electron groups (lone pairs) around the central atom

Molecular shape

• defined only by the relative positions of the nuclei (bonding groups only)

Linear MS (no lone pairs)

• AB2

• 180 degree angle

Trigonal planar MS

• AB3

• 120 degree angle

Bent MS (one lone pair)

• AB2E

• less than 120

Tetrahedral MS

• AB4

• 109.5 angle

Trigonal pyramidal MS

• AB3E

• Less than 109.5

Bent MS (two lone pairs)

• AB2E2

• less than 109.5

Trigonal bipyramidal MS

• AB5

• 90, 120

Seesaw MS

• AB4E

• less than 90, less than 120

T-shaped MS

• AB3E2

• less than 90

Linear MS (3 lone pairs)

• AB2E3

• 180

Octahedral MS

• AB6

• 90

Square pyramidal MS

• AB5E

• Less than 90

Square planar MS

• AB4E2

• 90