AOS1 KK4 Ionic Substances

What does the formation of an ionic compound from a reaction between atoms of two elements involve?

• 
  Sharing of pairs of electrons between atoms

• Donation of outer shell electrons to the entire crystal structure

• Transfer of electrons between atoms

• Ionisation of the atoms of some elements

Answer explanation

Ionic compounds form from the transfer of electrons between atoms so the compound achieves a group 18 electron structure.

What is the structure of solid calcium chloride best described as?

• A lattice consisting of diatomic chlorine molecules strongly bonded to calcium atoms.

• 
  A lattice of calcium and chloride ions, strongly bonded, in the ratio 1:2.

• Discrete molecules of calcium chloride with strong bonding within the molecule but weak bonding between molecules.

• An infinite lattice in which calcium and chloride are linked by strong covalent bonds.

Answer explanation

Calcium chloride (CaCl₂) is an ionic compound formed from ions and Ca²⁺. To balance these charges the ratio of calcium Ca²⁺ to chlorine Cl⁻ is 1:2.

Which of the following is not a property of ionic substances?

• Brittleness

• 
  Low melting temperatures

• Electrical conductivity in the molten (liquid) state

• Ability to form crystals

Answer explanation

Ionic substances have high melting temperatures because a large amount of energy is required to break the ionic bonds for the substance to melt. Ionic compounds generally have the properties of brittleness, ability to conduct electricity when molten and ability to form crystals.

Why do ionic compounds not conduct electricity in the solid state?

• 
  Only positive ions respond to the application of electricity.

• They do not contain ions.

• 
  The ions they contain are fixed in the lattice structure.

• 
  The charges on the positive and negative ions cancel each other out.

Answer explanation

The ions they contain are fixed in the lattice structure.

Calcium chloride is a crystalline substance at room temperature. Predict if the melting point is high or low, explain your prediction.

Answer explanation

Calcium chloride has a high melting point because it is an ionic compound with strong electrostatic attractive forces holding the ions together in a three-dimensional lattice.