Drawing Lewis Structures

1
Drawing Lewis Structures valence dot diagrams

• The valence shell holds up to 8 electrons.
• 0. Determine the number of valence electrons.
• Write the elements symbol.
• Add one electron to each side.
• Then double up the electrons as necessary.
• Any single electrons are available for bonding.

How many unpaired electrons?
2
Practicing Lewis Structures

• Draw
• Chlorine, Chlorine Ion
• Sodium,
• Sodium ion
• Oxygen, Oxygen ion
• Neon

3
Draw Lewis dot diagrams for

• Oxygen and Sodium
• (determine valence electrons, distribute dots
  appropriately)

4
CH 6 Covalent Compounds

• Covalent compounds are also known as
• Molecular Compounds

5
Molecular Compounds

• Consist of two or more non-metals
• 1.
• 2.

6
Naming Covalent Compounds(molecular compounds)

• Ionic charge does NOT dictate ratio of atoms.
• The name of the compound must indicate the
  number of each element .

7
Compounds of Carbon and Oxygen

• Carbon and Oxygen combine to form the following
  molecular compounds
• CO
• CO2

• Both are non-metals
• Combine in multiple ratios
• carbon oxide
• does not provide enough information to give the
  correct ratio of C and O.

8
Use of Numeric Prefixes1st element if more than
12nd element always

• CO
• CO2

9
Read Naming Covalent Compounds Pgs. 206-207

• Then complete the following chart

Formula Ionic or Covalent? Name
CO
CO2
N2O
KCl
PCl5
MgCl2
P4O6
10
Number Prefixes 1-10

• Mono
• Di
• Tri
• Tetra
• Penta
• Hexa
• Hepta
• Octa
• Nona
• Deca

• BF3
• N2O

11
Covalent Bonding

• The more civilized way to form a relationship

12
Comparing Ionic to Covalent

• Ionic
• Electrons are stolen or move to another atom to
  complete octets
• A net charge is created.
• The ions are attracted to opposite charge forming
  an electrically neutral salt
• Usually a metal and non-metal like Sodium and
  ChlorineNaCl

• Covalent
• Atomic Orbitals valence overlap and hybridize
  to form molecular orbitals.
• Electrons are shared to complete octets.
• 2 non-metals like Carbon and Oxygen.CO2

13
Shared Electrons

• Sharing occurs to form a stable gas configuration
  (full octet)
• It takes two electrons to form a bond
• These bonding electrons are called a shared
  pair
• The pair counts toward a full octet for each atom.

Draw out the valence dot diagrams for Fluorine
and Chlorine
14
How many electrons do each need for a full octet?
F
Cl
1e- needed
1e- needed
Why not share a pair??
15
The line represents a bond, consisting of 2
electrons. This shared pair counts for both Cl
and F
F
Cl
Shared pairs vs. unshared pairs.
16
Showing overlapping P-orbitals similar to what F
and Cl would do.
17
Bonding Tendencies
Family Number of valence electrons Electrons needed to make an octet Number of bonds formed
Halogens 7 1 1
Oxygen
Nitrogen
Carbon
Hydrogen 1 For H and He, an octet is only 2 electrons
18
Drawing Lewis Structures Valence Dot Diagrams

• Count the total number of valence electrons.
• Connect each atom using single bonds.
• The first atom is usually central to the
  structure
• Halogens are usually terminal (end atoms)
• Hydrogen is always terminal.
• Add lone pairs to each atom in order to get a
  full octet.

• CCl4 H2O NH3

19
Multiple bonds double, triple

• If there are not enough electrons to make every
  atom single bonded, you will need to use double
  or triple bonds.
• For every 2 electrons you are short, you will
  need one more bond.

• H2CO

20
Double and Triple bonds

• Double bonds use 2 shared pairs
• Triple bonds use 3 shared pairs

• Carbon Dioxide CO2
• Nitrogen (gas) N2

21

• Draw Lewis structures for
• Oxygen gas
• NI3

22
Lewis structuresresonance

• Draw 03 (ozone)

• Resonance helps explain the true structure when 2
  or more equally valid structures can be drawn for
  a molecule.
• Resonance structures have identical arrangements
  of atoms, they differ in distribution of
  electrons.

23
What type of bonds does ozone have?
Type of bond Length in picometers
O-O oxygen-oxygen single bond 148
OO oxygen-oxygen double bond 121
Oxygen-Oxygen bond in OZONE 128
The bond length shows that there the ozone bonds
are neither the single nor the double bond
length. The length is in-between single and
double This tells us that it is a hybrid, 1 ½
bond Drawing resonance structures is the
chemists way to show this.
24
Resonance explained
25
Draw Lewis Structures for

• Count up available electrons.
• Identify the central atom, then single bond
  everything.
• Assign lone pairs so all atoms have 8 electrons
• Compare amount used to amount available.
• If you used 2 many, retry with multiple bonds.
• Determine if resonance is necessary.

• Carbonate ion, CO32-

26
Covalent Bondsshared electrons

• Non-polar covalent, (Pure covalent)

27
Electronegativity values
28
Polar Covalent
29
Polar Covalent
Find the electronegativity differences in the
following pairs of atoms. Check pg 194 Figure 6

• 2 different atoms are covalently bonded.
• The bond is a Polar Bond
• The more electronegative atom pulls the shared
  electrons closer to its nucleus.

• H and Cl
• H and C
• F and Cl

30
Water

• Find the Electronegativity difference for H and O
• Draw Water
• Label the partial and partial charge

• The polar molecules cause special properties
• Dipole interactions
• A molecule that is polar is said to be a polar
  molecule
• DIPOLE
• Hydrogen Bonding
• Occurs w/ water.
• The partial positive H is attracted to the lone
  pairs of oxygen.

Use d (delta)
31
Molecular Shapes

• Ideal Geometries of molecules with a Central
  Atom.
• Determine the number of bonds to the central
  atom.
• Determine the number of lone pairs around the
  central atom.

• Put this information into the AXE formula to
  help categorize the molecule.
• A Represents the Central Atom
• X Bonded atoms to central atom
• E Lone pairs around central atom

32
X E 4
AXE formula Shape Example
AX4 Methane CH4
AX3E Ammonia NH3
AX4 Water H2O

33
VSEPR Valence Shell Electron Pair Repulsion Theory

• Bonding Angles for a tetrahedral are ______
• Example Methane , CH4 AX4

• Draw Ammonia, NH3
• AXE
• Bond angles
• Methane
• Ammonia

34

• Draw Water (lewis dot)

What effect do unshared pairs have?
35
X E lt 4
AXE formula Shape Example
AX2 CO2
AX2E GeF2
AX3 BeF3
Molecules with 2 atoms are linear Molecules with 2 atoms are linear Molecules with 2 atoms are linear
36
Warm UP

• What are the bond angles for the following
  molecules
• Methane
• Ammonia
• Water
• Carbon Dioxide

37
Exceptions to the octet rule Odd of electrons

• If the total number of valence electrons is odd
  you end up with a free radical. This unpaired
  electron is extremely reactive.
• Examples NO NO2