Unit 3: Bonding and Nomenclature
Content Outline: Molecular Geometry and VESPR Theory (3.5)
I. Structure = Function
A. The 3 dimensional shape of atoms in molecules helps determine the chemical properties of that molecule.
1. Properties such as:
a. Polarity and charge
b. Attraction or Repulsion with other molecules (Intermolecular forces).
c. Re-activity or inactivity with other molecules.
II. VESPR Theory
A. This stands for: Valence-Shell, Electron-Pair Repulsion.
B. The theory states “Repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible.”
1. “Shared pairs” of electrons are as far apart as possible.
2. “Lone pairs” (unshared) electrons occupy space around the central atom.
a. Some times they are shown as “. .” but they may not be all the time, it is just understood that they are present in the molecule.
b. They have greater repulsion strengths than “paired” electrons.
3. Sigma (σ) bonds (Head to head overlap)
a. These are the primary (first) bonds formed by the overlapped orbitals of valence shells.
i. Because of this overlap, the electron density in that region increases.
4. Pi (π) bonds (Side by side overlap)
a. These are the second (double) or third (triple) bonds, in a multiple bond set.
i. These are from over lapping “p” orbitals
C. There are 8 geometric shapes of molecules:
1. Linear (Line in one plane)
a. Number of atoms bonded to the central atom = 2.
b. The molecule has a bond angle of 180O.
c. It has a basic molecular formula of: AB2 (“A” is one element; “B” is the other element)
For example: Be F2
2. Trigonal-Planer (3 point pyramid in one plane “flat”)
a. Number of atoms bonded to the central atom = 3.
b. The molecule has bond angles of 120O.
c. It has a basic molecular formula of: AB3.
For example: BF3
3. Bent or angular planar (Boomerang shape in one plane “flat”)
a. Number of atoms bonded to the central atom = 2.
b. The molecule has bond angles of greater than 120O.
c. It has a basic molecular formula of: AB2E (E = number of Lone pair electrons…on 1 point)
For example: ONF
4. Bent or angular pyramidal (Boomerang shape of atoms but 4 point 3 side pyramid whole)
a. Number of atoms bonded to the central atom = 2.
b. The molecule has bond angles of 104.5O.
c. It has a basic molecular formula of: AB2E2. (Lone pair on top of pyramid and 1 base point)
For example: H2O
5. Tetrahedral (3 sided 4 point pyramid with flat bottom)
a. Number of atoms bonded to the central atom = 4.
b. The molecule has bond angles of 109.5O.
c. It has a basic molecular formula of: AB4.
For example: CH4
6. Trigonal-Pyramidal (3 sided, 4 point pyramid with flat bottom)
a. Number of atoms bonded to the central atom = 3.
b. The molecule has bond angles of 107O.
c. It has a basic molecular formula of: AB3E. (Lone pair on top of pyramid)
For example: NH3
7. Trigonal-Bipyramidal (2- 3 sided, 4 point pyramids with flat bases put base to base)
a. Number of atoms bonded to the central atom = 5.
b. The molecule has internal vertical bond angles of 90O and horizontal (base) angles of 120O.
c. It has a basic molecular formula of: AB5.
For example: PCl5
8. Octahedral (2 – 4 sided, 5 point pyramids with flat bases put base to base)
a. Number of atoms bonded to the central atom = 6.
b. The molecule has bond vertical and horizontal internal angles of 90O.
c. It has a basic molecular formula of: AB6.
For example: SF6
III. Hybridization of electron orbitals during bonding
A. “Hybrid” means “two combined as one”.
B. These are electron configuration models that are helpful to explain the rearranging of electron orbitals when covalent bonds occur.
C. It is the mixing of two or more atomic orbitals of similar energies on the same atom to produce new hybrid atomic orbitals of equal energies.
1. Carbon is a great example with its tetravalence (4 valence shell electrons).
a. Step 1: 1 of the “s” electrons elevates out to the open “p” suborbital.
b. Step 2: Now all four orbitals “mix” so that each (the 1 “s” and 3 “p”) possesses equal energies.
c. Step 3: Each hybridized orbital now enters into a covalent bond with another atom.
For example: CH4 (Methane Gas) sp3
D. Hybridized orbitals are represented as: sp, sp2, sp3
1. The superscript number on the “p” + 1 equals the number of hybrid orbitals.
For example: sp = 2; sp2 = 3; sp3 (above) = 4.