Resonance, Bond Energy, VSPER Theory and Polarity

Chapter 8, chem 1211

Describe bond order

• a way of counting the number of electron pairs that contribute to bonding ( single = 1 pair, double = 2 pairs, etc.)

• when electrons are resonate and aren’t exclusively between a certain pair of atoms, we solve for bond order like so → bond order = # of bonds in bonding regions/ # of bonding regions

Describe Resonance Contribution

• the actual, true structure of a compound is an average of the resonance structures (the actual bonds themselves won’t strictly be single or double bonds, but somewhere in the middle)

• we determine this average using formal charges; structures that follow our rules for “best structures” using formal charges usually contribute more than structures that don’t

describe bond energy

• the average bond energy is the average amount of energy (in kJ/mol) that must be put into a specific type of bond to break it

• calculated as an average over a large number of compounds that contain that specific type of bond

• because we are putting energy into it, breaking a bond must be endothermic

• by extension, forming a bond must be exothermic

How do we determine the ethalphy of reaction using average bond energies?

• add up the energy of all the bonds broken (endothermic process) and then subtract the energy of all the bonds formed (exothermic process)

• BE of reactants - BE of products

what is electron geometry?

the geometry of all the atoms and electrons in a compound

What is molecular geometry?

the geometry of just the atoms in a compound

How do you determine the electron and molecular geometries?

The Valence Shell Electron Pair Repulsion (VSEPR) Theory

• lewis structures show electrons organized into Electron Pair Domains or “electron groups”; a lone pair or any covalent bond counts as one electron group, no matter how many electrons are present

• electrons are negatively charged, so electron groups should want to get as far away from one another as possible

Describe how to determine geometry with VSEPR Theory

1. draw the lewis structure of the compound

2. determine how many electron groups are around the central atom; in a larger structure, the electron energy MUST be determined for each individual central atom

3. match that number of electron groups to one of the possible electron geometries

Describe the table of electron geometries given # of electron groups

• 2 electron groups = linear

• 3 electron groups = trigonal planar

• 4 electron groups = tetrahedral

• 5 electron groups = trigonal bipyramidal

• 6 electron groups - octahedral

How do you determine molecular geometries?

• once you have determined the electron geometry of a compound, count the number of lone pairs vs. bonding groups in your electron groups; this determines your molecular geometry

What kind of molecular geometry can Linear molecules have?

linear ( 0 lone pairs, 180 angle)

What kind of molecular geometry can trigonal planar molecules have?

1. trigonal planar ( 0 lone pairs, 120 angle)

2. bent ( 1 lone pair, less than 120 angle)

What kind of molecular geometry can tetrahedral molecules have?

1. tetrahedral ( 0 lone pairs, 109.5 angle)

2. trigonal pyramidal (1 lone pair, less than 109.5 angle)

3. bent (2 lone pairs, less than 109.5 angle)

What kind of molecular geometry can trigonal bipyramidal molecules have?

1. trigonal bipyramidal ( 0 lone paira 120 or 90 angle)

2. seesaw (1 lone pair, less than 120 or less than 90 angle)

3. T-shaped (2 lone pairs, greater than 180 or less than 90)

4. linear (3 lone pairs, 180 angle)

What kind of molecular geometry can octahedral molecules have?

1. octahedral (0 lone pairs, 90 angle)

2. square pyramidal (1 lone pair, less than 90 angle)

3. square planar (2 lone pairs, 90 angle)

What do dashes and wedges mean on drawn molecules?

• dashes - bond is going away from you

• wedges - bond is going toward you

What is true about electronegativity regarding shared electrons in a molecule?

when atoms have different electronegativity values, the shared electrons “lean” more toward the more electronegative element

What is dipole moment

the unequal sharing of electrons based on differences in electronegativity resulting in a distribution of charge over some difference; the larger the difference in electronegativity, the larger the dipole moment

How are dipole moments represented?

by an arrow with a “plus” at the tail

• the head of the arrow points to the negative part of the bond/molecule

• the tail of the arrow indicates the positive part of the bond/molecule

Describe the Bonding Continuum

• small ( 0-0.4) electronegativity difference → covalent bond

• intermediate (0.4-1.8) difference → polar covalent

• large (1.8+) difference → ionic

Which symmetrical molecular geometries in molecules with identical terminal atoms tend to be non-polar?

• linear

• trigonal planar

• tetrahedral

• trigonal bipyrimidal

• octahedral

• square planar

• if we change a terminal atom on a non-polar molecules, we will break the symmetry, causing a dipole to be re-introduced