Covalent bonding When two nonmetal atoms join to

Covalent bonding: • When two non-metal atoms join to one another they tend not to form ions. Why do you think this is? • Clue: What happens to non-metals when they form ions?

Covalent bonding: • Instead of losing and gaining electrons, the non-metal atoms share electrons.

Covalent bonding: Definition: A covalent bond is a shared pair of electrons

Covalent Bonding Cl Cl • How many electrons are there in the outer shell of and atom of 17 Cl? • If two atoms of chlorine form one covalent bond, each of the atoms would then have eight electrons in its outer shell.

• If an atom needs to share more than one electron to obtain a Nobel gas configuration, it can form more than one covalent bond. • These bonds could be between the same two atoms, giving rise to double or triple bonds • Or they could be between several atoms • For GCSE purposes you can assume that each atom involved in a covalent bond provides one electron. O O H H C H H

Molecules • The word “molecule” is one which is often misused. What does it mean? Definition: A molecule is two or more atoms joined by covalent bonds.

Draw dot and cross diagrams for the following molecules. Show outer shell electrons only. 1. Bromine (Br 2) 2. Hydrogen (H 2) 3. Nitrogen (N 2) 4. Water (clue: put the oxygen in the middle) 5. Carbon dioxide (clue: put the carbon in the middle)

Top tips for covalent dot and cross diagrams: 1. Work out number of outer shell electrons 2. Work out the number of covalent bonds needed to get a noble gas configuration. 3. Draw those covalent bonds 4. Fill in any left-over electrons in the outer shells not involved in bonding.

Simple covalent molecules • Most molecules are made of a finite number of atoms. • For example H 2, NH 3, or C 10 H 22 • These are described as simple molecules.

Examples of simple covalent molecules Cl Cl Chlorine (Cl 2) O O Oxygen (O 2) H H C H H Methane (CH 4)

Simple covalent molecules • The bonds within the molecules are covalent. • The bonds between the molecules are much weaker bonds often referred to as intermolecular bonds. • Because these intermolecular bonds are weak, simple covalent substances will have low melting and boiling points.

Giant covalent molecules • Some molecules are very large and, in theory, could contain any number of atoms. • These are called giant molecules.

Examples of giant covalent molecules Diamond Graphite

Sort the following properties into the table below: Properties of simple covalent structures Finite number of atoms Properties of giant covalent structures 1. Molecules are very large and could contain any number of atoms. 2. All the atoms are interconnected by strong covalent bonds 3. Bonds between the molecules are much weaker bonds often referred to as intermolecular bonds. 4. Have high melting points and boiling points. 5. Have low melting and boiling points.

Properties of giant covalent molecules: • In giant covalent molecules all the atoms are interconnected by strong covalent bonds. • This means that giant covalent structures will generally have very high boiling points (also known as fixed points)

Conductivity of giant covalent molecules • Most giant covalent structures do not have charged particles which are free to move. • Therefore most giant covalent substances will not conduct electricity. • One exception to this rule that you should know is graphite.

Conductivity of graphite • Graphite consists of layers of covalently bonded carbon atoms, resembling a honeycomb structure • Between these layers are delocalised (free) electrons. • These electrons are free to move through the graphite. • Therefore graphite can conduct electricity. Electrical current

Graphite is also slippery The covalent bonds between the carbon atoms in graphite are very strong, giving it a high melting point. However, between the layers of graphite there are weak intermolecular forces. These intermolecular forces can break and reform relatively easily allowing the layers to slide over one another. This makes graphite soft and slippery.

6 Mark Question Explain how you would prove whether a compound is ionic rather than giant covalent or simple molecular. You may use diagrams in your answer.

6 Mark Question – Mark Scheme SM will have low boiling point SM was be a gas at room temp GC and Ionic will have high boiling points GC insoluble in water (or reverse) GC and Ionic will not conduct electricity when solid Ionic will conduct electricity with molten or in solution • Graphite conducts electricity • • •