Lewis Structures of Covalent CompoundsLearning Target:I can represent the structure of a molecule by drawing bonds and unshared pairs.

Criteria for Success:I can construct electron dot structures to illustrate covalent bonds.

Lewis Structures of Covalent Compounds

A. Lewis Structures are formulas used to model what atoms look like in a compound that contains atoms that are covalently bonded together.

Lewis Structures of Covalent Compounds

Remember that in a covalent bond, nonmetal atoms share electrons in order to obtain a full valence shell. For hydrogen, a full valence shell will have two electrons. For all other nonmetals, a full valence shell will have eight.

Lewis Structures of Covalent Compounds

The idea that nonmetals will share electrons to get eight valence electrons and be stable is part of the octet rule.

What’s in a Lewis Structure

1. Element symbols represent the atom’s core. 2. Valence electrons are then represented using either dot-pairs or dashes.

What’s in a Lewis Structure

a. If the valence electrons are not involved in bonding they are represented using a dot. These represent unshared pairs of electrons.

What’s in a Lewis Structure

b. If the valence electrons are involved in bonding they are represented using a dash between two atomic symbols. These represent shared electron pairs in covalent bonds.

What’s in a Lewis Structurei. Remember that covalent bonds are formed

when nonmetals share valence electrons in order to mutually achieve full valence shells (to be stable like noble gases).

ii. Only valence electrons that are already unpaired will be shared from one atom to another.

Unpaired Electron Sharing Example 1: CH4

Unpaired Electron Sharing Example 2: NH3

Unpaired Electron Sharing Example 3: OF2

Unpaired Electron Sharing Example 4: FOCN

How do I Lewis

Notice that a pattern shows up for covalent bonds with certain atoms. Certain atoms are most stable with a specific number of bonds and a specific number of unshared (lone) pairs of electrons.

Patterns in bonding--Atoms in the same groups as C, N, and O will tend to follow the same pattern, but they may not always.

Element # bonds # lone pairs

H 1 0

C 4 0

N 3 1 O 2 2

All Halogens 1 3

All Noble Gases 0 4

B. Steps to writing a Lewis structure of a neutral covalent compound1. Choose a central atom.

a. If carbon is in the molecule, it will be the central atom. b. Otherwise, the atom that is capable of having the most bonds will most likely be the central atom.

B. Steps to writing a Lewis structure of a neutral covalent compoundc. It is possible for all atoms in a molecule to be bonded in a straight line, like Example 4 earlier. In this case, there is not one sole central atom. The formula for the molecule will generally be written by listing the elements in the order they are chained in the atom.

Ex: HSCN, CH3COOH

Draw dat Lewis structure

2. Write the symbol for the central atom. Then write the symbols for the other atoms around the central atom. 3. Draw the Lewis structures for the individual atoms.4. Figure out how to “connect the dots.”

Draaaaaaawwwwwiiiiiiinnnnng5. Draw dashes between atoms to show bonding pairs of electrons. Be sure to show the lone pairs around atoms that have them.6. Double check that each atom has a full octet by counting both bonds and lone pairs as two electrons each belonging to the atom.

C. Multiple bonds, expanded octets, and other exceptions

1. Only certain atoms can form double and triple bonds. Generally, they are the “PCONS,” although others can exist.

C. Multiple bonds, expanded octets, and other exceptions

2. Some atoms in the third and higher energy levels can hold electrons in their otherwise empty d orbitals. This allows them to form more than 4 bonds and have more than a full octet.

C. Multiple bonds, expanded octets, and other exceptions

2. This is most common with P and S, although any nonmetal that has access to d orbitals (so, nonmetals in the third period and below) are capable of doing it. These can have 8, 10, or 12 valence electrons in a molecule.

C. Multiple bonds, expanded octets, and other exceptions

2. If you are ever drawing a molecule that has lone pairs and unsure of where else to put them, the lone pairs should go on the expanded octet atom.

C. Multiple bonds, expanded octets, and other exceptions

3. Boron is stable with only six electrons, rather than a full octet of eight. This means it can be stable with three bonds and zero lone pairs.

C. Multiple bonds, expanded octets, and other exceptions

4. Some noble gases, under extreme laboratory conditions, can be forced to form bonds. This has only been shown to occur with the largest nonradioactive noble gases (Xe and Kr) and the most electronegative atom (F).

C. Multiple bonds, expanded octets, and other exceptions

4. Fluorine is able to so strongly attract electrons to itself that it can attract valence electrons in noble gases that have a lot of shielding.

C. Multiple bonds, expanded octets, and other exceptions

5. Lewis structures for polyatomic ions can be drawn using generally the same process as covalent molecules.

C. Multiple bonds, expanded octets, and other exceptions

5. However, the final structure must have square brackets drawn around it and the charge must be written outside the brackets as a superscript in the upper right corner.

D. How to tell if you’re dealing with an exception—NAS charts

1. If a molecule is forming more bonds than what you would predict it will then it is an exception