Drawing Lewis Structures “

valence dot diagrams”

The valence shell holds up to 8 electrons.

0. Determine the number of valence electrons.

1.

2.

Write the element’s symbol.

Add one electron to each side.

3.

4.

Then double up the electrons as necessary.

Any single electrons are available for bonding.

How many unpaired electrons?

Practicing Lewis Structures

Draw:  Chlorine, Chlorine Ion  Sodium,  Sodium ion  Oxygen, Oxygen ion  Neon

Draw Lewis dot diagrams for

Oxygen and Sodium

( determine valence electrons, distribute dots appropriately)

CH 6 Covalent Compounds

Covalent compounds are also known as: Molecular Compounds

Molecular Compounds

Consist of two or more non-metals 1.

2.

5 Ch 5 Chemical Names and Formulas

Naming Covalent Compounds

(molecular compounds)

Ionic charge does NOT dictate ratio of atoms.

The name of the compound

must

the number of each element .

indicate 6 Ch 5 Chemical Names and Formulas

Compounds of Carbon and Oxygen  Both are non-metals  Combine in multiple ratios  “carbon oxide” does not provide enough information to give the correct ratio of “C” and “O”.

Carbon and Oxygen combine to form the following molecular compounds:   CO CO 2 7 Ch 5 Chemical Names and Formulas

 CO  CO 2

Use of Numeric Prefixes

1

st

element: if more than 1 2

nd

element: always

8 Ch 5 Chemical Names and Formulas

Read “Naming Covalent Compounds” Pgs. 206-207 Then complete the following chart:

Formula Ionic or Covalent?

Name

CO CO 2 N 2 O KCl PCl 5 MgCl 2 P 4 O 6

          Mono Di Tri Tetra Penta Hexa Hepta Octa Nona Deca BF N Number Prefixes 1-10 2 3 O 10 Ch 5 Chemical Names and Formulas

Covalent Bonding

The more civilized way to form a relationship

Comparing Ionic to Covalent

Ionic

 Electrons are “

stolen

complete octets ” or move to another atom to  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 Chlorine…NaCl

Covalent

 Atomic Orbitals [valence] overlap and

hybridize

to form molecular orbitals.

 Electrons are “

shared

” to complete octets.  2 non-metals like Carbon and Oxygen….CO

2

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

How many electrons do each need for a full octet?

1e- needed

Why not share a pair??

1e- needed

The line represents a bond, consisting of 2 electrons. This shared pair counts for both Cl and F Shared pairs vs. unshared pairs.

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

Drawing “Lewis Structures”

Valence Dot Diagrams

1.

2.

 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.

3.

Count the total number of valence electrons.

Add lone pairs to each atom in order to get a

full octet.

CCl 4 H 2 O NH 3

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.

H 2 CO For every 2 electrons you are “short”, you will need one more bond.

Double and Triple bonds

 Double bonds use 2 shared pairs  Triple bonds use 3 shared pairs Carbon Dioxide CO 2 Nitrogen (gas) N 2

Draw Lewis structures for:  Oxygen gas  NI 3

Lewis structures…resonance

Draw 0 3 (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.

What type of bonds does ozone have?

Type of bond Length in picometers

O-O oxygen-oxygen single bond O=O oxygen-oxygen double bond Oxygen-Oxygen bond in OZONE 148 121 128 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.

Resonance explained

Draw Lewis Structures for:

Carbonate ion, CO 3 2-

1.

2.

3.

4.

5.

6.

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.

Covalent Bonds…shared electrons

Non-polar covalent, (Pure covalent)

Electronegativity values

Polar Covalent

Polar Covalent

 2 different atoms are covalently bonded.

The bond is a

Polar Bond

Find the electronegativity differences in the following pairs of atoms. Check pg 194 Figure 6  H and Cl  H and C  The more electronegative atom pulls the shared electrons closer to it’s nucleus.

 F and Cl

 Find the Electronegativity difference for H and O Water The polar molecules cause special properties  Draw Water 

Dipole interactions:

A molecule that is polar is said to be a polar molecule… DIPOLE  Label the partial – and partial + charge

Use : δ + (delta)



Hydrogen Bonding:

Occurs w/ water. The partial positive H is attracted to the lone pairs of oxygen.

Molecular Shapes

Ideal Geometries of molecules with a Central Atom.

Put this information into the “AXE” formula to help categorize the molecule.

1.

2.

Determine the number of bonds to the central atom.

Determine the number of lone pairs around the central atom.

A = Represents the Central Atom X # = Bonded atoms to central atom E # = Lone pairs around central atom

X + E = 4

AXE formula

AX 4

Shape

AX 3 E AX 4

Example

Methane CH 4 Ammonia NH 3 Water H 2 O

VSEPR

Valence Shell Electron Pair Repulsion Theory Bonding Angles for a tetrahedral are ______°  Draw Ammonia, NH 3 AXE: Example: Methane , CH 4 [AX 4 ]  Bond angles   Methane: Ammonia:

 Draw Water (lewis dot) What effect do unshared pairs have?

X + E < 4

AXE formula

AX 2 AX 2 E AX 3

Shape Example

CO 2 GeF 2 BeF 3 Molecules with 2 atoms are linear

Warm UP

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

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 NO 2