2.2 - How Bonding & Structure are Related to the Properties of Substances

What is ionic bonding?

The electrostatic attraction between positive and negative ions. It’s a relatively strong attraction.

How are ionic compounds held together?

• They are held together in a giant lattice.

• It’s a regular structure that extends in all directions in a substance.

• Electrostatic attraction between positive and negative ions holds the structure together.

State properties of ionic substances.

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

• Don’t conduct electricity when solid (ions in fixed positions).

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

Give 5 examples of positive ions and 5 examples of negative ions.

• Positive: Na⁺, Mg²⁺, Al³⁺, Ca²⁺, Rb⁺

• Negative: Cl^-, Br^-, SO₄^2-, NO₃^-, OH^- (chloride, bromide, sulfate, nitrate, hydroxide).

What is important when working out a formula of an ionic compound?

Ionic compounds are electrically neutral, e.g. positive and negative charges balance each other.

How are ionic compounds formed? Explain in terms of MgO case?

• Reaction of a metal with a non-metal.

• Electron transfer occurs - metal gives away its outer shell electrons to non-metal.

• Mg is in Group 2 so has 2 available outer shell electrons, while O is in Group 6. so can accept 2 electrons to get a full outer shell configuration.

• Mg becomes Mg²⁺ and O^2- becomes O (oxide).

What is a covalent bond?

A shared pair of electrons between two atoms.

Describe the structure and properties of simple molecular covalent substances.

• Don’t conduct electricity (no ions).

• Small molecules.

• Weak intermolecular forces, therefore:

• Low melting and boiling points.

What is metallic bonding?

Forces of attraction between delocalized electrons and nuclei of metal ions.

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?

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

What are thermosoftening polymers?

• Special type of polymers; they melt/soften when heated.

• There are no bonds between polymers chains. Strong intermolecular forces ensure that the structure is solid at room temperature.

• These forces are overcome with heating - polymer melts.

What are giant covalent substances? Give examples.

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

• High melting/boiling points - strong covalent bonds.

• Mostly don’t conduct electricity (no delocalized electrons).

• Diamond, graphite, silicon dioxide.

Describe properties of metals.

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

• Good conductors of heat and electricity (delocalized electrons).

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

What are alloys? Why are they harder than pure metals?

• Mixtures of metal with other elements, usually metals.

• Different sizes of atoms distorts the layers, so they can’t slide over each other, therefore alloys are harder than pure metals.

What are the limitations of the simple model?

There are no forces between spheres and atoms, molecules and ions are solid spheres - this is not true.

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 nature of the particles involved depends on the type of bonding and the structure of the substance.

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

A pure substance will melt or boil at what? What about the mixture?

• A fixed temperature.

• A mixture will melt over a range of temperatures.

What are the three states of matter?

solid, liquid, and gas