Lewis Structures for Molecular
Compounds
These are
NOT Cornell
Notes
Learning Objectives
• Use a systematic procedure to draw Lewis
structures that represent the formation of
molecular compounds and polyatomic ions.
Molecular Structures
• Several models can be
used to represent a
molecule
–
–
–
–
–
Molecular formula
Structural formula
Space-filling model
Ball-and-stick model
Lewis structure
PH3
Molecular Structure
• One of the most useful
molecular models is the
structural formula,
which uses letter
symbols and bonds to
show relative positions
of atoms.
• You can predict the
structural formula for
many molecules by
drawing the Lewis
structure.
Lewis Structures
• Bonding electrons –
pairs of valence
electrons that are
shared between atoms
– bonding electrons are
represented by dashes
• Nonbonding electrons
– pairs of valence
electrons that are not
involved in a bond
bonding
electrons
Systematic Procedure for Drawing
Lewis Structures
1. Calculate the total number of valence
electrons available.
2. Predict the location of atoms and write a
skeletal structure. The atom that has the
lowest electronegativity (see notes on page
128) will be the central atom.
Lewis Structures
• How do I know which atom is the central
atom?
– Common binary compound - look at molecular
formula. Central atom – atom that appears once
in formula.
• Ex. SO2 – S is central atom
– Molecular compounds that have hydrogen,
oxygen, and an additional element - additional
element is central atom
• Ex: HNO3 – N is central atom
Lewis Structures
– Carbon is the central atom in nearly all carboncontaining compounds
 CH4 – C is central atom
– Neither hydrogen nor fluorine is ever the central
atom.
Lewis Structures
3. Add nonbonding electron pairs to the
structure such that each atom bonded to the
central atom has an octet.
4. Place any remaining electrons on the central
atom.
Lewis Structures
5. If there are not enough electrons to give the
central atom an octet, use one more pair of
nonbonding electrons on the atoms bonded to
the central atom to form double and triple
bonds.
6. Count the total number of electrons in the
completed Lewis structure to make sure it is
equal to the total number of valence electrons
available.
Example 1
Copy this onto page 166 in your notebook
Draw a Lewis structure for PF3.
Example 2
Copy this onto page 167 in your notebook
Draw a Lewis structure for HCN.
Practice 1
Complete this on page 168 of your notebook.
Show ALL steps.
Draw a Lewis structure for SiCl4.
Practice 2
Complete this on page 169 of your notebook.
Show ALL steps.
Draw a Lewis structure for H2CO.
Lewis Structures for Polyatomic Ions
• Use same procedure as for molecular
compounds, with the accommodation that the
total number of electrons used in the
structure must be adjusted to take into
account ion charge.
Lewis Structures for Polyatomic Ions
• In the Lewis structure for an ionic compound
that contains a polyatomic ion, the positive
and negative ions are treated separately to
show that they are individual ions not linked
by covalent bonds.
Example 3
Copy this onto page 170 of your notebook.
Draw a Lewis structure for SO42-.
Practice #3
Complete this on page 171 of your notebook.
Show all steps.
Draw a Lewis structure for BrO3-.
Summary
Different models can be used to represent
molecules.
A systematic approach can be used when
drawing Lewis structures for molecular
compounds and polyatomic ions.