Molecular Compounds (a.k.a. Covalent Compounds)

Ionic Compounds Review

• Ionic compounds are the combination of ions. E.g. Na+ + Cl- NaCl.

• This formula represents the Ratio of sodium ions and chloride ions not the exact number of them.

• They are composed of a Metal and a Non-Metal and form a crystal structure (e.g. NaCl and CaCl2 which are both salts)

Molecular Compounds

• Consist of atoms covalently bonded together

• The elements involved are all Non-Metals

Some examples

• N2O Nitrous Oxide= Gas used at the dentist to relax patients

• NO2 Nitrogen Dioxide= poisonous toxin emitted from car exhaust

More Examples

• H2O Water

• CO2 Carbon Dioxide= Gas

exhaled and created during

combustion reactions

• CO Carbon Monoxide= Lethal gas created during incomplete combustion

Recall

• In ionic compounds, the non-metal rips away the electron(s) from the metal because the metals loosely hold their electrons

• This creates a positive and negative ion which attract each other making them stick together (opposites attract)

Molecular Compounds

• made up of a non-metal

and a non-metal.

• also called Covalent Compounds

Molecular Compounds

• Share electrons (e-) to form a stable arrangement.

• Neg. e- are attracted to the Pos. nuclei of both atoms

Molecular CompoundsMolecular compounds are formed

when non metals share electrons to fill their outer electron orbit (shell).

If we draw the Lewis Structure for Fluorine,

We can see that it needs one more electron to become stable

It wants to share one electron with another atom.

If it bonds with another fluorine atom

We draw the shared pair of electrons between the two atoms.

F F

F F

Shared electron pair both now have 8 electrons in outer orbit.

Molecular CompoundsMany molecular compounds are

predictable. How will Oxygen bond with

Hydrogen to form a molecule.Draw the Lewis Structure of both

Atoms.Oxygen wants to share two

electrons and hydrogen wants to share one.

Clearly we need another Hydrogen atom

Putting the shared electrons between the atoms.

The molecule is H2O

H OH

H O

H

**The Oxygen atom has 8 valence electrons (full) and the Hydrogen has 2 valence electrons (full)

Molecular CompoundsNot all molecular compounds are as

predictable.How will Oxygen bond with

Carbon?Draw the Lewis Structures of each

atom. Carbon wants to share 4.Oxygen wants to share 2

If we add one more Oxygen, each oxygen can share two.

However, if Oxygen will share two and donate one of its other electrons pairs, Oxygen can bond with just on Carbon atom.

OC

OCO

C O

Carbon Dioxide

Carbon Monoxide

Types of Bonds

OCO

Carbon Monoxide – Triple Bond because three pairs of electrons are shared

H O

H

Water – Single bond because One pair of electrons are shared

Carbon Dioxide – Double bond because two pairs of electrons are shared

C O

Molecular Compounds – NamingSo Oxygen can form CO or CO2

We need a more flexible naming system for covalent compounds to reflect the many different bonding possibilities.

To name a covalent compound,

1. Starting with the atom that is to the left in the periodic table, (or lower)

2. Write the name of the atoms with the prefix indicating the number of that atom in the compound.

3. Change the ending of the last atom to – ide.

The prefixes are,

1 Mono

2 Di

3 Tri

4 Tetra

5 Penta

6 Hexa

7 Hepta

Note, the mono is omitted on the first atom.

CO Carbon Monoxide

CO2 Carbon Dioxide

Memorize me!

Number Prefix Number Prefix

1 mono 6 Hexa

2 Di 7 hepta

3 tri 8 octa

4 Tetra 9 nona

5 penta 10 deca

Naming Example

• P4O10

Phosphorus - use prefix tetra (4)

Oxygen – change ending – Oxide

Oxide – use prefix deca (10)

= Tetraphosphorus Decaoxide

Molecular Compounds - Naming

Name the following compounds

CF4

Carbon tetrafluoride

H2O

Dihydrogen Monoxide

PF5

Phosphorus Pentafluoride

Write the chemical formula for,

Sulphur Dioxide

SO2

DiCarbon Tetrahydride

C2H4

Molecular Compounds – Diatomic Gases

Molecular compounds show an incredible variety in structure, physical and chemical properties.

There are categories that further divide covalent compounds into categories with common properties.

Diatomic Gases – When two atoms of the same type form a covalent molecule, the result is often a colorless, odorless gas.

The Halogens, Oxygen, Nitrogen and Hydrogen form diatomic gases.

H2 is called Hydrogen Gas not Dihydrogen.

Writing Formulas

left-most element goes 1st

Carbon monoxide

- Carbon (C) no prefix therefore = C- Monoxide – Mono = 1 therefore, 1 oxygen

atom = O= CO

Writing Formulas

• Nitrogen dioxide

- Nitrogen (N) no prefix therefore = N

- Dioxide – Di = 2 therefore, 2 oxygen atoms = O2

= NO2

Writing Formulas

DiCarbon Tetrahydride

- Dicarbon (C) Di = 2 therefore = C2

- Tetrahydride (H) Tetra = 4 therefore, 4 Hydrogen atoms = H4

= C2H4

Eg. N2 - Nitrogen Gas

O2 - Oxygen Gas

Air

Exceptions that need to be Memorized!

CH4 – Methane

H2O2 – Hydrogen peroxide

H2O – Water

These Too!

NH3 – Ammonia

O3 – Ozone