Bonding + Structure: Relation to Properties of Substances

State properties of ionic substances

• High melting and boiling point (due to strong electrostatic forces between oppositely charged ions).

• Do not conduct electricity when solid (ions are in fixed positions).

• Conduct when molten or dissolved in water (ions are free to move).

Describe the structure and properties of simple molecular covalent substances

• Do not conduct electricity (no ions or delocalised electrons)

• Small molecules

• Weak intermolecular forces, therefore: low melting and boiling points

How do intermolecular forces change as the mass/size of the molecule increases?

They increase. That causes melting/boiling points to increase as well (more energy is needed to overcome these forces).

What are polymers?

Polymers are very large molecules (>100s,1000s of atoms) with atoms linked by covalent bonds.

What are thermosoftening polymers?

Thermosoftening polymers are polymers that melt/soften when heated because there are no bonds between the polymer chains; the weak intermolecular forces are overcome by heating.

What are giant covalent substances?

Solids, with atoms covalently bonded together in a giant lattice.

Do giant covalent substances conduct electricity?Explain your answer.

They mostly don’t conduct electricity because they have no delocalised electrons that can carry charge through the structure. There are exceptions to this (e.g. graphite and graphene)

Do giant covalent substances have high melting/boiling points? Explain your answer.

Yes, they have high melting/boiling points due to strong covalent bonds that require a lot of ev=nergy to overcome.

What are some examples of giant covalent substances?

Diamond, graphite, silicon dioxide.

Describe properties of metals

• High melting/boiling points (due to strong forces of attraction)

• Good conductors of heat and electricity (due to delocalised electrons)

• Malleable, soft (layers of atoms can slide over each other whilst maintaining the attraction forces)

What are alloys?

Alloys are mixtures of a metal with other elements (usually metals).

Why are alloys harder than pure metals?

They are harder than pure metals because the different sizes of the atoms distort the layers, preventing them from easily sliding over each other.

What are the limitations of the simple model?

The simple model suggests there are no forces between spheres and that atoms, molecules and ions are solid spheres – this is not true (e.g., intermolecular forces exist).

What does the amount of energy needed to change state from solid to liquid or liquid to gas depend on?

The strength of the forces between the particles of the substance. The stronger the forces, the higher the melting point and boiling point.

A pure substance will melt or boil at…?

A pure substance will melt or boil at a fixed temperature.

A mixture will melt over a…?

A mixture will melt over a range of temperatures.

What are the three states of matter?

Solid, liquid and gas

Why do simple covalent substances have low boiling/melting points?

Because of the weak intermolecular forces between the molecules, which require little energy to overcome.

Why don't simple covalent substances conduct electricity when solid or molten?

Because they have no free-moving ions or delocalised electrons to carry a charge.

What is the general description of simple covalent substances?

Mostly gases and liquids.

Why do ionic compounds have high boiling/melting points?

Because they form a giant lattice with strong electrostatic forces between oppositely charged ions, which requires a lot of energy to break.

Why don't ionic compounds conduct electricity when solid?

Because the ions are held in fixed positions within the lattice and are not free to move.

Why do ionic compounds conduct electricity when molten or dissolved?

Because the ions are free to move and carry a charge.

What is the general description of ionic compounds?

Crystalline solids.

Why do giant covalent structures have high boiling/melting points?

Because of the many strong covalent bonds between atoms in the giant structure, which require a lot of energy to break.

Why do giant covalent structures like diamond and sand not conduct electricity?

Because their electrons are held in fixed covalent bonds and cannot move.

Why does graphite conduct electricity?

It has free delocalised electrons between its layers that are free to move and carry a charge.

What is the general description of giant covalent structures?

Solids.

Why do metals have high boiling/melting points?

Because of the strong electrostatic forces between the positive metal ions and the delocalised electrons.

Why do metals conduct electricity when solid or molten?

They have delocalised electrons that are free to move throughout the structure and carry a charge.

What is the general description of metals?

Shiny metal solids.