Chem Exam 2: Lewis Structures and Bonding

Lewis Structures

Models that estimate the distribution of the valence electrons, but do not predict the shape of the molecule

Octet Rule

Main-group atoms in the second period and below prefer to be surrounded by eight valence electrons (shared or unshared)

Formal Charge

A rough estimate of the charge distribution in the molecule, calculated by: (# valence electrons in the atom) – (# electrons obtained from the equal division of the bond)

Resonance Structures

Can be drawn when more than one valid Lewis structure for a given molecule or molecular ion can be drawn

Equivalent Resonance Structures

Lewis Structures that include the same information and have the same number of formal charges and multiple bonds between the same kind of atoms

Non-equivalent Resonance Structures

Lewis Structures that have several different ways to push electrons to reach an octet that result in different bond configurations and different formal charges

Bond Order (BO)

The number of bonding electron pairs participating in resonance divided by the number of equivalent bonds; for whole numbers, BO = 1 is a single bond, BO = 2 is a double bond, and BO = 3 is a triple bond

Bond Strength and Bond Length

As bond order increases, the strength of the bond increases and the length of the bond decreases

Apparent Octet Expansion

When some molecules appear to have more than an octet around the center atom, possibly occurring for elements with Z > 12 with a steric number > 4

Steric number

The number of bonded outer atoms + number of lone pairs on the central ato