## Ionic Bonding

2019 O Level, MCQ 4
Sodium chloride, NaCl, is an ionic compound. Which statement sodium chloride is not correct?

A. It contains positive and negative ions.
B. It dissolves in water.
C. It has a low melting point.
D. The ions are arranged in a giant crystal lattice.

• A is correct as sodium chloride is an ionic compound made up of sodium positive ions and chloride negative ions. The oppositely charged ions attract each other.
• B is correct as most ionic compounds can dissolve in water to form aqueous solutions.
• C is wrong. Ionic compounds have a high melting point, because the electrostatic forces of attraction between the oppositely charged ions are strong. Therefore, a large amount of energy is required to overcome them.
• D is correct, as many oppositely charged ions attract one another to form a giant ionic crystal lattice.
2019 O Level, MCQ 6
Which ions are present in solid chromium sulfate, Cr2(SO4)3?

A. Cr2+ and SO42-
B. Cr3+ and SO42-
C. Cr2+ and SO43-
D. Cr3+ and SO43-

STEP 1: Recall and state the charge of the common ion
Sulfate anion has a charge of 2- and hence is SO42-.

STEP 2: Calculate the charge of the unfamiliar ion
A formula unit has 3 sulfate ions with a combined negative charge of 6-. To balance it, the combined positive charge must be 6+. Since there are only 2 chromium ions, each chromium ion has a positive charge of 6+ ÷ 2 = 3+.

This is a harder inference question. To do it, you need to have memorised the charges of sulfate ion, which is an ion commonly tested in the O Level. Then, you infer the charge of the unknown chromium ion.

There is no way to directly tell the charge of the chromium ion, because it is a transition metal ion that can take on different charges in different compounds.

2017 O Level, MCQ 14
Sodium chloride is an ionic compound. Which statement about sodium chloride is not correct?

A. Sodium ions and chloride ions are oppositely charged.
B. Sodium chloride has a high melting point.
C. Sodium chloride solid conducts electricity.
D. Sodium chloride solid exists as a lattice.

• A is correct. Sodium is a metallic element that loses electron to form a positive ion. Chlorine is a non-metallic element that gains electron to form a negative ion.
• B is correct. The strong electrostatic forces of attraction between sodium cations and chloride anions require a large amount of energy at high temperature to overcome.
• C is wrong. In the solid state, the oppositely charged ions are held in fixed position by the strong electrostatic forces of attraction between them. Therefore, the ions are not mobile to conduct electricity. Instead, ionic compounds only conduct electricity in the liquid and aqueous states.
• D is correct. Sodium chloride has a giant ionic lattice structure comprising many oppositely charged ions arranged in a regular pattern.

## Covalent Bonding

2019 O Level, MCQ 5
Two elements, A and B, form a compound of formula AB3. Which statement is not correct?

A. The molecule contains a total of eight electrons.
B. A and B are both non-metals.
C. A is in Group V of the Periodic Table.
D. B is hydrogen.

• A is wrong because only the outer shell electrons are shown. Element A has at least one inner shell with 2 more electrons, as its outer shell contains 5 electrons.
• B is correct. The ‘dot-and-cross’ diagram shows a covalent compound whereby atoms share electrons. And most covalent compounds are formed between non-metallic atoms.
• C is correct. Atom A has 5 valence electrons represented by the 5 dots. Therefore, it must be from Group V.
• D is correct. Atom B has 1 valence electron represented by the 1 cross. The only non-metal with 1 valence electron is hydrogen.
2018 O Level, MCQ 4
Which statement describes the formation of a covalent bond?

A. Electrons are shared between metallic atoms.
B. Electrons are shared between non-metallic atoms.
C. Electrons are transferred from a metallic atom to a non-metallic atom.
D. Electrons are transferred from a non-metallic atom to a metallic atom.

In a covalent bond, two atoms share electrons. The atoms are non-metallic.

## Dot and Cross Diagrams

2019 O Level, P3, Section B, Q10(b)
Sodium oxide and hydrogen sulfide are both compounds but with very different properties. Draw ‘dot and cross’ diagrams for sodium oxide and hydrogen sulfide, showing only the outer shells of electrons.

[Proton numbers: H, 1: O, 8; S, 16; Na, 11]

The most important step to answering this question is to first consider if the compound is covalent or ionic. This is hinted by the question. It tells us that sodium oxide and hydrogen sulfide have very different properties, suggesting that their structure and bonding are different.

Sodium oxide is ionic as it comprises a metallic element and a non-metallic element. In an ionic compound, the metallic element has lost its valence electrons to form positive ions while the non-metallic element has gained electrons to form negative ions. The ions formed have the noble gas electronic configuration.

However, hydrogen sulfide is covalent, as it is formed between non-metallic elements. In a covalent compound, the atoms share electrons to achieve the noble gas electronic configuration. In hydrogen sulfide, hydrogen has 2 valence electrons upon sharing, just like the noble gas helium. Sulfur has 8 valence electrons after sharing, like in the noble gas argon.

2018 O Level, P3, Section B, Q11(b)

Carbon dioxide is a gas at room temperature and pressure. Draw a ‘dot and cross’ diagram to show the arrangement of the outer shell electrons in the molecule of carbon dioxide.

[Proton number: O, 6; O, 8]

## Structure and Properties of Materials

2019 O Level, P3, Section B, Q10(c)
Explain, in terms of structure and bonding, why both solid sodium oxide and gaseous hydrogen sulfide do not conduct electricity. [3 marks]

Sodium oxide has a giant ionic lattice structure. In the solid state, the ions held in fixed position are not mobile to conduct electricity.

Hydrogen sulfide has a simple molecular structure. In all states, there is no free-moving electrons or mobile ions to conduct electricity.

2019 O Level, P3, Section B, Q10(c)
Explain, in terms of structure and bonding, why molten sodium oxide will conduct electricity. [1 mark]

Sodium oxide has a giant ionic lattice structure. Upon melting, the strong electrostatic forces of attraction between oppositely charged ions are overcome, allowing the ions to be free-moving.

When something is molten, it has been melted at high temperature to form a liquid.

2018 O Level, P3, Section B, Q10(b)
Carbon dioxide can be liquefied and has a low boiling point. Explain, with reference to the structure of carbon dioxide, why the boiling point is low. [2 marks]

Carbon dioxide has a simple molecular structure. The intermolecular forces of attraction between carbon dioxide molecules are weak, such they can be overcome with a small amount of energy at a low temperature.