Lewis Structures
Covalent molecules form when two or more nonmetal atoms share valence electrons in order to complete their valence shell.
Example
Both hydrogen and chlorine need one extra electron to fill their valence shell.
Atoms
Molecule

Lewis Structures
In each covalent bond, two electrons are shared between the atoms.
Atoms can form single, double, and triple bonds with other atoms.

Lewis Structures
Lewis dot structures show the location of valence electrons in covalent molecules.
Remember, all elements (except for H and He) need 8 valence electrons to have a full shell.
Oxygen atoms can either form a double bond or two single bonds to fill its valence shell.

Lewis Structures
Example – Drawing the Lewis dot structure for NH
3
1. Draw the electron dot structure of each atom.

Lewis Structures
2. Match up all unpaired electrons. (You can move them to different sides, even multiple unpaired electrons to the same side)

Lewis Structures
3. Make sure each atom has a full valence shell.
(2 electrons for hydrogen, 8 electrons for all other atoms)

Lewis Structures
Covalent molecules are most often drawn using
structural formulas.
Single lines are used to represent each pair of bonding electrons.
Lewis Dot Structure Structural Formula

Examples
Lewis Structures

Lewis Structures
Number of Covalent Bonds Needed
One Bond Two Bonds Three Bonds Four Bonds
Hydrogen Oxygen
Fluorine Sulfur
Nitrogen
Phosphorus
Carbon
Silicon
Chlorine
Bromine
Iodine
Selenium
Tellurium
Arsenic
Remember HONC – 1, 2, 3, 4

Lewis Structures
Example – Drawing the structural formula for C
2
H
4
1. Determine the number of bonds each atom needs.

Lewis Structures
2. Draw a simplified structure of the molecule, placing the atoms that need the most bonds at the center of the molecule.

Lewis Structures
3. Add in double/triple bonds where needed so that each atom has the necessary number of bonds.

Lewis Structures
When multiple structures exist for a single chemical formula, they are called isomers.
C
2
H
6
O

Lewis Structures
Not all covalent compounds are electrically neutral. Polyatomic ions are charged covalent compounds.
Example – Hydroxide (OH ‒ )
The extra electron gives hydroxide its negative charge

Lewis Structures
The six polyatomic ions below are some of the more common polyatomic ions. These should be memorized.
Name Formula Electrons Molecule
Ammonium
Hydroxide
NH
4
+
OH ‒
1 less
1 extra
Nitrate
Carbonate
Sulfate
Phosphate
NO
3
‒
CO
3
2‒
SO
4
2‒
PO
4
3‒
1 extra
2 extra
2 extra
3 extra

Lewis Structures
The Lewis dot structure for polyatomic ions should give each atom a full valence shell, and must match the charge of the ion.
Examples – Carbonate (CO
3
2‒ ) and Ammonia (NH
4
+ )

Lewis Structures
1. In covalent bonds, the electrons are… a) Transferred from one atom to another b) Converted into neutrons c) Free to move from atom to atom d) Shared between two atoms

Lewis Structures
2. Which element needs two covalent bonds to complete its valence shell?
a) Hydrogen b) Oxygen c) Phosphorus d) Carbon

Lewis Structures
3. Which chemical below is not formed with covalent bonds?
a) SF
2 b) H
2
S c) HNO d) NaF

Lewis Structures
4. Draw the Lewis dot structure for SBr
2
.
Br S Br

b) c)
Lewis Structures
5. Which structure below correctly shows how carbon and hydrogen bond to form a compound?
a) d)

Polyatomic Ions
6. What is the formula for hydroxide?
a) HO
3
‒ b) H
2
O ‒ c) OH
4
‒ d) OH ‒