Forming ionic compounds

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

To conduct electricity, charged particles must be able to move. In a solid state, the ions are not able to move because they are held in the ionic lattice.

An ionic compound formed between metal X and non-metal Y. It has a structure where each ion of X is surrounded by eight ions of Y. Also, each ion of Y is surrounded by four ions of X.

What is its formula?

• X₄Y₈

• X₈Y₄

• 
  X₂Y

• XY₂

Answer explanation

Twice as many Y ions are present compared to X ions. Since it is an ionic compound, the formula must be empirical (i.e. the simplest ratio of ions in the lattice).

What is the formula for aluminium sulphide?

• AlS

• AlS_1.5

• Al₂S₃

• Al₄S₆

Answer explanation

To find the formula of aluminium sulfide:

Identify the cation: Al³⁺.

Identify the anion: S^2–.

Balance the charges: Al₂S₃.

Ions with variable charge are most often found in which section of the periodic table?

• Second period

• The transition metals

• Group 1

• Group 18

Answer explanation

Elements in the transition metals section (d block) have more complex electron arrangements, with valence electrons in both an s subshell and a d subshell, and may be able to form more than one type of ion.

What is the formula of the compound ammonium phosphate?

• (NH₄)₃PO₄

• 
  NH₃P

• NH₄PO₄

• (NH₄)₂PO₃

Answer explanation

To find the formula of ammonium phosphate:

Identify the cation: NH4+

Identify the anion: PO43−

Balance the charges: (NH₄)₃PO₄

What is the compound CuO called?

• Copper oxide

• Copper dioxide

• Copper(II) oxide

• Copper(I) oxide

Answer explanation

To find the name of CuO:

Identify the cation: Cu²⁺, copper (II)

Identify the anion: O²⁻, oxide

Balance the charges: CuO

Hence, the name is copper (II) oxide.

What are the blue crystals of CuSO₄ ⋅ 5H₂O called?

• Copper sulfate pentahydrate

• Copper sulfate hexahydrate

• Copper(II) sulfate pentahydrate

• 
  Copper(I) sulfate hexahydrate

Answer explanation

To find the name of CuSO₄ ⋅ 5H₂O:

Identify the cation: Cu2+, copper (II)

Identify the anion: SO42−, sulfate

Five water molecules is pentahydrate.

Hence, the name is copper(II) sulfate pentahydrate.