Chapter 8- Covalent Bonding
Section 3- Molecular Structures
Section 3 Learning Objectives:
1. List the basic steps to draw Lewis structures.
2. Explain why resonance occurs and identify resonance structures.
3. Identify three exceptions to the octet rule and name molecules in which these exceptions occur.
VOCABULARY
Structural formula, resonance, coordinate covalent compound
Models of Molecules
- There are five different models to represent a molecule:
1. Molecular formula- shows the symbols and subscripts- tells you type/number of each atom present.
2. Lewis structure- shows bonding and non-bonding pairs of valence electrons.
3. Structural formula- uses lines to represent bonding pairs of electrons. Do not show lone pairs.
4. Ball-and-stick model- uses balls to represent each atom and sticks to illustrate bonds and positions
of the atoms in their true 3D arrangement.
5. Space-filling model- represents the amount of space each atom’s electron cloud occupies.
Figure 8.13- Representation of phosphorus trihydride
Drawing Lewis Structures
1. Predict the location of certain atoms.
- Atoms that have the least attraction for electrons are usually in the middle (center).
- Carbon is usually a central atom.
- Hydrogen is called a terminal atom (end atom)- it only shares one electron.
2. Determine the number of electrons available for bonding.
- Equals number of valence electrons.
3. Determine the number of bonding pairs.
- Equals the number of valence electrons divided by two.
4. Place the bonding pairs.
- Place a bonding pair around the central atom and each terminal atom.
5. Determine the number of bonding pairs remaining.
6. Determine whether the central atom satisfies the octet rule.
Lewis Structure for a Covalent Compound with Multiple Bonds
Carbon dioxide is a product of cellular respiration. Draw the Lewis structure for carbon dioxide (CO 2).
- Carbon has a lower attraction for electrons than oxygen, it is the central atom.
- Carbon and oxygen have a total of 16 electrons available for bonding (4 from carbon and 6 from
each oxygen).
- This means there are 8 pairs of electrons (bonding and non-bonding).
- If one bonding pair is attached to each oxygen from the carbon, we notice a problem: carbon does
not have an octet and all pairs of electrons are used. This is incorrect.
- If two pairs are used on each side, we see carbon and oxygen now have stable octets and no pairs
of electrons are left unused.
Practice
C2H4
CS2
SeO3
Lewis Structures for Polyatomic Ions
- Polyatomic ions act as ions, but are bonded together covalently.
- To draw the structure of the ion, you must know the charge and add that number (or subtract if
positive) to the total number of available electrons.
Draw the Lewis structure for phosphate (PO4 3-).
- Phosphorus is the central atom and has 5 valence electrons. Each oxygen contributes 6 electrons
(24 total) and 3 come from the negative charge = 32 total electrons or 16 pairs.
- Phosphorus achieves its octet by using 4 pairs (one with each oxygen), leaving 12 left over.
- Each oxygen takes 3 pairs (lone pairs) and already has one shared with phosphorus for its octet.
Practice
NH4+
CO3 2-
ClO4 1-
Resonance Structures
- When more than one Lewis structure can be written for a molecule or ion, it is called a resonance
structure. It occurs when there is more than one correct placement of single and double bonds.
Ex. NO3 1-
Practice
NO2 1-
SO2
O3
SO3 2-
Exceptions to the Octet Rule
- Most elements achieve stability by having an octet. There are a few exceptions to this rule.
Odd Number of Valence Electrons
- Some elements have an odd number of valence electrons and are unable to form an octet with the
atoms around them. NO2 is such a molecule.
Practice
ClO2
NO
Suboctets and coordinate covalent bonds
- In rare occasions, compounds form between molecules with fewer than 8 valence electrons.
BH3 is such a compound.
- When an atom or group of atoms donate electrons to be shared with another atom or ion to form a
stable electron configuration, it is called a coordinate covalent bond.
Ex. BH3 and NH3
In this reaction between boron trihydride (BH3) and ammonia (NH3), the nitrogen donates both
electrons that are shared by boron and ammonia, forming the covalent bond.
Expanded octets
- A third group of atoms that do not follow the octet rule has central atoms that contain more than
eight valance electrons.
- This is called an expanded octet. Expanded octets rarely occur with elements, but when they do, the
d orbital is where the bond occurs.
Ex. PCl5
Ex. SF6
Ex. Xenon is a noble gas that will form a few compounds with nonmetals that strongly attract
electrons. Draw the Lewis structure of xenon tetrafluoride (XeF4).