Drawing Lewis Structures
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1One Page Lesson: Drawing Lewis Structures
The atoms that need the most electrons to achieve a noble gas electron configuration (and thus form the most covalent
bonds) should be used as central atoms. The atoms that need very few electrons to achieve a noble gas electron
configuration (and thus form very few bonds) should be used as peripheral atoms.
Examples of central atoms:
Carbon has FOUR valence electrons. It needs FOUR more electrons to achieve an octet. It gets those four electrons
•
by sharing: by forming FOUR covalent bonds. CARBON’S octet: FOUR BONDS and NO LONE PAIR.
Nitrogen has FIVE valence electrons. It needs THREE more electrons to achieve an octet. It gets those three
•
electrons by sharing: by forming THREE covalent bonds. NITROGEN’S octet: THREE BONDS and ONE LONE
PAIR.
Oxygen has SIX valence electrons. It needs TWO more electrons to achieve an octet. It gets those two electrons by
•
sharing: by forming TWO covalent bonds. OXYGEN’S octet: TWO BONDS and TWO LONE PAIRS.
Examples of peripheral atoms:
Fluorine has SEVEN valence electrons. It needs ONE more electron to achieve an octet. It gets that one electron by
•
sharing: by forming ONE covalent bond. FLUORINE’S octet (and that of the other halogens): ONE BOND and
THREE LONE PAIRS.
Hydrogen has ONE valence electron. It needs ONE more electron to achieve a duet. It gets that one electron by
•
sharing: by forming ONE covalent bond. HYDROGEN’S duet: ONE BOND.
Indicate the covalent bonds between atoms with lines. Indicate lone pairs of electrons (unshared electrons) with dots.
Arrange the bonds and lone pairs so that each atom achieves its noble gas electron configuration (octet, or duet for
hydrogen). Be sure to draw NO MORE and NO FEWER electrons than the TOTAL number of valence electrons that the
atoms bring with them to the molecule.
A “single bond” is one pair of electrons shared between two atoms, and is represented in the Lewis structure by a
•
single line.
A “double bond” is two pairs of electrons shared between two atoms, and is represented by drawing two lines between
•
the bonding atoms. Double bonds are most common between atoms of carbon, nitrogen, oxygen, and sulfur. (In
organic and biological molecules, there are many examples of carbon-carbon, carbon-nitrogen, and carbon-oxygen
double bonds.)
A “triple bond” is three pairs of electrons shared between two atoms, and is represented by drawing three lines
•
between the bonding atoms. Triple bonds are most common between carbon and nitrogen atoms.
Example Lewis Structure: Formaldehyde, CH
O
2
The TOTAL number of valence electrons the atoms bring with them to the molecule:
1 carbon atom with 4 valence electrons
4
2 hydrogen atoms, each with 1 valence electron
2(1)
1 oxygen atom with 6 valence electrons
6
Total available electrons
12
Consider the bonding patterns of our component atoms:
Carbon is always a central atom because it forms four bonds.
Oxygen tends to form two bonds and two lone pairs.
Hydrogen atoms form one bond.
Since carbon forms the most bonds, we’ll place it at the center and attach the oxygen and hydrogen atoms
O
O
to it. Since the carbon needs four bonds, and the oxygen two bonds, we’ll place a double bond between
them. Two lone pairs of electrons are added to the oxygen to complete its octet. The total number of
C
C
valence electrons the atoms brought into the molecule is 12. This Lewis structure accounts for ALL 12 of
H
H
H
H
those electrons: two electrons in each of the four bonds (eight bonding electrons), plus two lone pairs
(four unshared/non-bonding electrons).
C. Graham Brittain
10/6/2007
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