Covalent network lattices and covalent layer lattice

3 Different types of Carbon

• Charcoal, diamond and graphite are different physical forms of carbon ,

these are called allotropes of carbon.

Covalent Network Lattice• The carbon in Diamond is bonded very

strongly in a 3D lattice making it hard and crystalline with a very high melting point.

Other examples of covalent network lattices

• Silicon dioxide also called silica is found in quartz

Covalent lattices• Graphite is hard in two dimensions but

slippery in another. The carbon in graphite is bonded in 2D layers.

Weak intermolecular bonds

Revision

• How are carbon dioxide molecules bonded together?

• With Dispersion forces

• Are Dispersion forces weak or strong?

• Weak

Properties of covalent molecular, network and layer lattices

• You can use the covalent molecular, network and lattice models to explain properties of non- metal compounds.

Melting and boiling points• When a substance melts the particles break

out of their position (the intermolecular forces have been overcome).

• For small molecules, the intermolecular forces are weak and therefore its does not take a lot of energy to overcome these forces, making melting and boiling points low

• Network and layer lattices have high m.p. and b.p. because they covalent bonding throughout the lattices are very strong

c

Electrical conductivity

• In Covalent molecular substances and covalent network lattices the electrons are localised and therefore cannot move

• Graphite has one electron from each carbon delocalised and therefore these are able to carry and electric current

Chemical reactivity

• Chemical reactivity is how well a substance reacts with other substances

• Covalent network lattices and covalent layer lattices are very unreactive because they have such strong bonds

• Small molecular substances are more reactive because their bonds are not as strong.

Hardness and softness• Covalent network lattices are very hard

because they have strong covalent bonds and atoms are held in fixed positions

• In covalent layer lattices the forces between layers are weak and layers can slide over one another

• The intermolecular forces in Covalent molecular substances determines whether they will be hard or not

• Because intermolecular forces are quite weak covalent molecular substances tend to be gases and liquids

• Explain the trend in the following melting points.

• The trend is the increase in melting points of halogens down a group.

• There is an increase in the melting points because the strength of intermolecular forces increases as you go down the halogen group. Iodine is heavier and has more electrons and therefore the London forces between iodine molecules are stronger than those between the smaller fluorine molecule. Stronger intermolecular forces result in a higher melting point

Melting point (C)

Fluorine (F2)

-220

Chlorine (Cl2)

-101

Bromine (Br2)

-7

Iodine (I2)

114

• The mass of the HF molecule is similar to the mass of a neon atom, yet there boiling points is very different. HF’s b.p. is 19.5C and Ne’s b.p is -246C. Explain this difference.

• At room temperature , CCl4 is a liquid whereas CH4 is a gas. Why is this the case?

• Both are tetrahedral, non polar covalent molecules so London forces occur between the molecules. The London forces between CCl4 are stronger than CH4. This is due to CCl4 being a larger or heavier molecule making the london forces stronger making it a liquid at room temperature.

As a group create a concept map using the following words

Polar Bond Non-polar bond Dispersion Forces

Ionic Bonding Covalent Bonding Metallic Bonding

Intermolecular Forces Intramolecular Forces

Polar Molecule Non-polar Molecule

Partial Positive Partial Negative

Dipole Dipole Forces Covalent network lattice

Hydrogen Bonding Covalent layer lattices

• How is hydrogen bonding different from a dipole dipole interaction?

• Pg 127 Question 13 and 15