GCSE Bonding, Structure and Properties of Matter (Chemistry) — Questions and Answers
Question 1: What type of bonding is found in sodium chloride (NaCl)?
- Covalent
- Metallic
- Ionic (Correct answer)
- Hydrogen
Correct answer: Ionic
Sodium chloride is formed from a metal (sodium) and a non-metal (chlorine), so it contains ionic bonds between Na⁺ and Cl⁻ ions.
Ionic bonding occurs between metals and non-metals. The sodium atom loses one electron to form a Na⁺ ion and the chlorine atom gains that electron to form a Cl⁻ ion. The oppositely charged ions are held together by strong electrostatic forces of attraction acting in all directions, forming a giant ionic lattice. Covalent bonding, by contrast, involves sharing electrons between non-metal atoms.
Question 2: Why do ionic compounds have high melting points?
- They contain weak intermolecular forces
- There are strong electrostatic forces between oppositely charged ions that need a lot of energy to overcome (Correct answer)
- They contain delocalised electrons
- Their molecules are very large
Correct answer: There are strong electrostatic forces between oppositely charged ions that need a lot of energy to overcome
In a giant ionic lattice, many strong electrostatic attractions between ions must be broken, requiring lots of energy.
Ionic compounds form giant lattice structures where every ion is surrounded by ions of opposite charge. The strong electrostatic forces act in all directions throughout the lattice. Melting the compound means breaking many of these strong forces, which needs a large amount of energy, so the melting and boiling points are high. Sodium chloride, for example, melts at 801 °C.
Question 3: Which statement explains why ionic compounds conduct electricity when molten or dissolved in water, but not when solid?
- The ions are free to move when molten or dissolved, but fixed in place in the solid (Correct answer)
- Electrons become delocalised when the solid melts
- Water reacts with the compound to produce free electrons
- The solid has no charged particles
Correct answer: The ions are free to move when molten or dissolved, but fixed in place in the solid
Conduction requires charged particles that can move. In the solid the ions are locked in the lattice; when molten or in solution they are free to carry charge.
An electric current is a flow of charge. Ionic solids contain charged ions, but they are held in fixed positions in the lattice so they cannot move and the solid does not conduct. When the compound is melted or dissolved in water the lattice breaks down and the ions become free to move, so they can carry the charge through the liquid or solution. Ionic compounds do not have delocalised electrons; that is a feature of metals and graphite.
Question 4: How many covalent bonds does a single carbon atom form in a diamond structure?
- 2
- 3
- 4 (Correct answer)
- 6
Correct answer: 4
In diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid giant covalent structure.
Diamond is a giant covalent structure made only of carbon atoms. Each carbon forms four strong covalent bonds to four neighbouring carbon atoms, producing a rigid three-dimensional tetrahedral network. This is why diamond is extremely hard and has a very high melting point. Graphite, another form of carbon, has each atom bonded to only three others in layers, leaving one electron per atom delocalised.
Question 5: Which property of graphite makes it useful as a lubricant?
- It has a very high melting point
- Its layers can slide over each other because there are only weak forces between them (Correct answer)
- It conducts electricity
- Each carbon atom is bonded to four others
Correct answer: Its layers can slide over each other because there are only weak forces between them
Graphite's carbon atoms are arranged in layers held together by weak intermolecular forces, so the layers slide easily.
In graphite each carbon atom is covalently bonded to three others, forming flat hexagonal layers. There are no covalent bonds between the layers, only weak intermolecular forces, so the layers can slide over one another. This makes graphite soft and slippery, useful as a dry lubricant and in pencils. The fourth outer electron of each carbon is delocalised, which is why graphite also conducts electricity.
Question 6: Why do simple molecular substances such as water and methane have low melting and boiling points?
- Their covalent bonds are weak and easily broken
- Only the weak intermolecular forces between molecules need to be overcome, not the covalent bonds (Correct answer)
- They contain free ions
- Their molecules are held in a giant lattice
Correct answer: Only the weak intermolecular forces between molecules need to be overcome, not the covalent bonds
Melting or boiling a simple molecular substance only breaks the weak intermolecular forces between molecules; the strong covalent bonds within the molecules stay intact.
Simple molecular substances consist of small molecules in which the atoms are joined by strong covalent bonds. However, the forces between separate molecules (intermolecular forces) are weak. When the substance melts or boils, it is these weak intermolecular forces that are overcome, which needs relatively little energy, so melting and boiling points are low. A frequent exam error is to say the covalent bonds break; they do not.
GCSE Bonding, Structure and Properties of Matter (Chemistry)
What type of bonding is found in sodium chloride (NaCl)?