January 28, 2014
Introduction to Lewis Structures
January 28, 2014
• You will be able to draw Lewis
structures to show bonding
– Today: Activity/Rules
– Thursday: More Practice
Activity
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Identify what atoms and how many are present in a given molecule
Take out an element symbol card for each atom you need
Arrange (valence electrons) peas around each card
See how you can arrange the cards and electrons so all atoms have a full shell
(8 electrons)
– Note: Hydrogen is happy with only TWO electrons
Try to show bonding for:
– Cl2
CH3I
– CH2O
CN-
PF3
OH-
Lewis Structures
• Lewis structures are used to represent
molecules with covalent bonds (in which
electrons are shared.)
Lewis Structures:
Used to represent molecules with covalent bonds
(shared electrons)
1. Count the valence electrons from each atom
2. Add up the total number of valence electrons
What if the compound has an overall charge?
a) For anions (negative charge) add eb) For cations (positive charge) take away e-
Example: CNC:4 eN: 5 eCharge: 1 eTotal: 10 e-
3. Arrange atoms and connect with single
bonds.
a) Each bond represents two electrons
b) Nature likes symmetry!
c) Halogens (F, Cl, Br, I) are not usually
center atoms
d) Hydrogen is NEVER center
e) Carbon is always center, or the least
electronegative atom if no carbon
4. Use “leftover” electrons and add unshared
pairs (lone pairs) of electrons to atoms to
give full shells
a) This is called the octet rule because
everything needs 8 electrons for a full shell
b) Exception to the octet rule:
i. Hydrogen has a full shell when it has two
electrons
c) Do not draw too many electrons!
5. If there are not enough electrons to give
everything a full shell, try using multiple bonds.
- Double bond: four shared electrons
- Triple bond: six shared electrons
Check your work
• Have I accounted for all electrons?
• How many electrons does each atom have
around it?
EXAMPLE: Cl2
• Valence electrons:
– Cl – 7
– Cl – 7
– Total – 14
Each atom has 8
electrons surrounding
it, either in lone pairs
or in bonds
Electrons not
involved in bonding
are shown around
atoms in electron
pairs
Two shared electrons in a single
bond between the atoms
EXAMPLE: CH3I
• Valence electrons:
–C–4
–H–1
–H–1
–H–1
–I–7
– Total: 14
Electrons not
involved in bonding
are shown around
atoms in electron
pairs
Each atom has 8 electrons
surrounding it, either in lone pairs or
in bonds. Hydrogen only needs two
electrons to have a full valence shell.
Two shared
electrons in a
single bond
between the
atoms
EXAMPLE: PF3
• Valence electrons:
–P–5
–F–7
–F–7
–F–7
– Total: 26
Each atom has 8
electrons surrounding
it, either in lone pairs
or in bonds
Electrons not
involved in bonding
are shown around
atoms in electron
pairs
Two shared electrons in a single
bond between the atoms
EXAMPLE: CH2O
• Valence electrons:
–C–4
–H–1
–H–1
–O–6
– Total: 12
Each atom has 8 electrons
surrounding it, either in lone pairs or
in bonds. Hydrogen only needs two
electrons to have a full valence shell.
Electrons not
involved in bonding
are shown around
atoms in electron
pairs
Four shared electrons in a
double bond between the
atoms
CN- (Sorry it’s blurry)
• Valence electrons
–C–4
–N–5
– Neg. charge – 1
– Total: 10
Each atom has 8
electrons surrounding
it, either in lone pairs
or in bonds
Electrons not
involved in bonding
are shown around
atoms in electron
pairs
Brackets and charge
show that this is a
charged molecule
Six shared electrons in a triple
bond between the atoms
EXAMPLE: OH• Valence electrons
–O–6
–H–1
– Neg charge – 1
– Total: 8
Each atom has 8 electrons
surrounding it, either in lone pairs
or in bonds. Hydrogen only needs
two electrons to have a full
valence shell.
Electrons not
involved in bonding
are shown around
atoms in electron
pairs
Brackets and charge
show that this is a
charged molecule
Two shared electrons in a single
bond between the atoms