Chapter 9- Molecular Geometry and Bonding Theories

VSEPR theory

a model used to predict the shape of the molecules from the electron pairs that surround the central atoms of the molecule

Bonding electrons

electrons participating in a bond

nonbonding electrons

electrons that are not involved in any covalent bonding

How do you determine the electron domains around a central atom?

draw the Lewis structure and count the bonding electrons domains and nonbonding electrons domains

What is the generalization for a molecule with no lone pairs of electrons on the central atom?

the edg is the same as the molecular geometry; AB_n

When is the electron domain geometry and the molecular geometry are the same for a molecule?

when there are no no bonding

What atoms are capable of expanding their octet capacity?

sulfur, phosphorus, silicon, chlorine, and atoms in n=3 and below

When a bond is formed between two atoms, which orbitals overlap?

sigma bonds; s-s, s-p, s-d, p-d

Valence bond theory

• Bonds form when orbitals overlap

• The greater the overlap, the stronger the bond

• Two electrons of opposite spin in the orbital overlap

• This accounts for shapes of molecules

Hybridization of orbitals

determined by the electron domain geometry to create bonds of equal energy

How many hybrid orbitals form when atomic orbitals are mixed?

equal to the number of atomic orbitals that were combined to produce the set

Sigma bonds

head to head overlap; every single bond; electron density is between the nuclei

Pi bonds

electron density is above and below the plane of the nuclei; side by side overlap; in double and beyond bonds

How many sigma and pi bonds does a double bond have?

1 sigma and 1 pi bond

How many sigma and pi bonds does a triple bond have?

1 sigma and 2 pi bonds; the p-orbitals involved in pi-bonding come from unhybridized orbitals

Localized electrons

sigma and pi electrons are associated totally with the two atoms that form the bond; found at a specific spot

Delocalized electrons

sigma and pi bonds are not associated with a single atom or covalent bond and can be anywhere on the molecule; resonance structures

How to find sigma and pi bonds?

1. draw the Lewis structure

2. count number of single (1 sigma)

3. count the number of double bonds (1 sigma and 1 pi)

4. count the number of triple bonds (1 sigma and 2 pi)