Lewis Dot Structures
• Dots are arranged around an element’s
symbol to indicate the valence electrons
• Reminder: valence shell only has s & p
orbitals  maximum of 8 e−s
• Place the electrons around the four sides
of the symbol, in pairs.
Lewis Dot Structures - Atoms
• example for Cl
5 “p”
electrons
Cl
2 “s”
electrons
Lewis Dot Structures - Atoms
• example for B
1 “p”
electron
B
2 “s”
electrons
Lewis Dot Structures - Compounds
• Octet Rule (only for each atom in a
compound)
– Each atom in a compound must see eight
electrons.
– These electrons will be on the outside of the
atom OR in the bond to another atom
(shared)
Lewis Dot Structures - Compounds
• Compound Structures
– must have ONE central atom – must look like
O
spokes on a wheel
O
S
O
– NO rings (unless told to do so)
O
O
O
S
O
Lewis Dot Structures - Compounds
• Bonds are a pair of electrons:
• We represent the pair of electrons in a
bond with a short line segment:
Lewis Dot Structures - Compounds
• For compounds, count the total number of
valence electrons for the atoms in the
compound
– ex: PCl3
1 P  5 valence electrons=5
3 Cl  3•(7 valence e−s)=21
Total = 26 electrons to share
• These electrons DO NOT belong to each
atom anymore. They are shared to make
the compound
LDS – Compounds (cont.)
• Place either the single atom of an element
in compound at the center OR the least
electronegative element (if not one single)
• For PCl3:
Cl
P
Cl
Cl
LDS – Compounds (cont.)
• Connect each terminal (outside) atom to
the central (center) atom with a single
bond (shared pair of electrons)
Cl
P
Cl
Cl
LDS – Compounds (cont.)
• Subtract number of electrons used in
bonds from total and distribute around the
atoms, in pairs
• make sure the terminal atoms have 8
electrons, including the 2 atoms in the
bond
LDS – Compounds (cont.)
• (26 total e−s) – (6 bonding e−s) = 20 e−s
• 20 e−s go around the atoms, terminal first
Cl
P
Cl
Cl
LDS – Compounds (cont.)
• Count to make sure you have:
– 8 e−s around each atom including all e−s in
the bonds to that atom (shared e−s)
– each atom has a bond (pair of e−s) to another
atom
– there are no single e−s anywhere in the
structure
• If all checks have passed, you have a
proper lewis dot structure!
Lewis Dot Structures - Ions
• Count the number of electrons in the
original valence shell AFTER adding or
taking e−s to make the ion.
• Place them around the symbol (or not, if
there are none)
• Then, place square brackets ([ ]) around
the LDS for the ion, with the charge
outside the brackets
Lewis Dot Structures - Ions
• Ex: Cl1−
Cl
[ ]
Cl
Neutral chlorine has
7 valence electrons
1−
Chloride has 8
valence electrons
the extra
electron added
to make charge
of 1−
Lewis Dot Structures - Ions
• Ex: Mg2+
Neutral magnesium has
2 valence electrons
Mg
[ ]
Mg
2+
The magnesium ion has no
valence electrons in its original
valence shell
2 electrons removed to
make charge of 2+
LDS – Polyatomic Ions
• Count valence e−s form atoms
• Add electrons that make charge
• construct the LDS the same way as for
compounds, using the extra e−s for the
charge
• Place brackets around entire compound
and place charge outside
LDS – Polyatomic Ions
• Example: SO42−
2−
S: 6 electrons
O
O: 4•6 = 24 electrons
Neutral atom valence electrons
= 30
2 more electrons for the 2−
charge = 32 total electrons
O
S
O
O
Note: sulfate cannot be made without adding 2 electrons to its structure. Try it!
LDS Exceptions
• Hydrogen and Helium
– Their valence shell will only hold 2 e─s
– So, they may only have 2 electrons around
them in a LDS for a compound
– This mean all of their electrons will be shared
in the bond
LDS Exceptions
• Multiple bonds
– You can ONLY have single bonds, double bonds, or
triple bonds in a compound
• Sometimes, after distributing the electrons
around the compound’s structure, not all of the
atoms see 8 electrons
• To fix this: take a pair of e─s from the TERMINAL
atom and place them in the bond – now the
terminal atom and the central atom see both e
─s
LDS Exceptions
• Multiple Bonds
– do not overbond – this means do not put more
multiple bonds than is necessary in an atom
• Ex
SO2: First distribution of e─s
O
S
• Fixed structure: O S O
• All atoms, through sharing, see 8 e─s
O