Unit 4 Unit 4 Covalent Covalent BondingBonding

Fructose

Carbon Dioxide

Ammonia

Molecules and Molecular Molecules and Molecular CompoundsCompounds

Compounds that are NOT held together Compounds that are NOT held together by an electrical attraction, but instead by an electrical attraction, but instead by a sharing of electrons.by a sharing of electrons.

Atoms held together by a sharing of Atoms held together by a sharing of electrons are joined by a electrons are joined by a covalent covalent bondbond..

HH•• ••HH

Molecules and Molecular Molecules and Molecular CompoundsCompounds

A A moleculemolecule is a neutral group of is a neutral group of atoms joined together covalent atoms joined together covalent bonds. A compound composed of bonds. A compound composed of molecules is called a molecules is called a molecular molecular compoundcompound. .

The chemical formula for a The chemical formula for a molecule is called the molecule is called the molecular formulamolecular formula..

Properties of Molecular Properties of Molecular CompoundsCompounds

Composed of two or more Composed of two or more nonmetalsnonmetals..

Usually gases or liquids at room Usually gases or liquids at room temperature.temperature.

Properties of Molecular Properties of Molecular CompoundsCompounds

Molecular compounds tend to have a Molecular compounds tend to have a relatively lower melting and boiling relatively lower melting and boiling point than ionic compounds.point than ionic compounds. Reason: There are no (or few and Reason: There are no (or few and

weak) bonds holding the molecules weak) bonds holding the molecules together in molecular compounds.together in molecular compounds.

Ionic CompoundMolecular Compound

Properties of Molecular Properties of Molecular CompoundsCompounds

Do not conduct electricity. They Do not conduct electricity. They form form nonelectrolytesnonelectrolytes in solution. in solution. Reason: Molecular compounds do not Reason: Molecular compounds do not

break apart into ions in solutions.break apart into ions in solutions.

Covalent Bonding and the Octet Covalent Bonding and the Octet RuleRule

Compounds that are NOT held together Compounds that are NOT held together by an electrical attraction, but instead by by an electrical attraction, but instead by a a sharingsharing of electrons. of electrons.

Atoms attain an Atoms attain an octetoctet (also called noble (also called noble gas electron configurations) by sharing gas electron configurations) by sharing electrons.electrons.

The bonds that form from this sharing can The bonds that form from this sharing can be single, double or triple. Triple bonds be single, double or triple. Triple bonds are shorter and stronger than double are shorter and stronger than double bonds, which are shorter and stronger bonds, which are shorter and stronger than single bonds.than single bonds.

To draw Lewis structures for covalent To draw Lewis structures for covalent bonds, use the NAS method:bonds, use the NAS method:

NN (Needed): Find the number of electrons (Needed): Find the number of electrons needed to form full octets for all elements needed to form full octets for all elements involved. For most nonmetals, the number involved. For most nonmetals, the number needed is 8. Hydrogen is the exception, it needs needed is 8. Hydrogen is the exception, it needs only 2.only 2.

AA (Available): Find the number of electrons (Available): Find the number of electrons available by adding up all of the valence available by adding up all of the valence electrons for all elements involved.electrons for all elements involved.

SS (Shared): Subtract the two numbers. S= N-A (Shared): Subtract the two numbers. S= N-A A bond is formed with two electrons, so divide A bond is formed with two electrons, so divide by two to tell you how many bonds to draw by two to tell you how many bonds to draw between the elements.between the elements.

Draw the molecule. Put first atom in the center. Draw the molecule. Put first atom in the center. H’s are always outside. Draw in the bonds, then H’s are always outside. Draw in the bonds, then fill in the rest of the electrons.fill in the rest of the electrons.

Check to ensure all atoms have a full octet.Check to ensure all atoms have a full octet.

Example #1: CHExample #1: CH44

Example #2: COExample #2: CO22

Example #3: NExample #3: N22