O-Level Chemistry · Secondary 3 and 4
Chemical Bonding
Chemical bonding explains why sodium chloride is a hard crystal that melts at a high temperature, why water is a liquid and why copper conducts electricity. In O-Level Chemistry you learn three types of bond, ionic, covalent and metallic, and how each one leads to a particular structure and set of properties. This topic underpins almost everything that follows, from writing formulae and predicting electrolysis products to understanding why organic molecules behave as they do.
What the syllabus expects
- Describe ionic bonding as the transfer of electrons to form ions held by strong electrostatic attraction
- Draw dot-and-cross diagrams for ionic compounds and for simple covalent molecules, including double and triple bonds
- Describe covalent bonding as the sharing of pairs of electrons between non-metal atoms
- Describe metallic bonding as a lattice of positive ions in a sea of delocalised electrons
- Relate melting point, boiling point, electrical conductivity and hardness to the type of structure
- Describe the giant covalent structures of diamond, graphite and silicon dioxide
Why atoms bond
Atoms bond to achieve a more stable electronic configuration, usually a full outer shell like that of a noble gas. Metal atoms have few outer electrons and tend to lose them, forming positive ions. Non-metal atoms have nearly full outer shells and tend to gain electrons, forming negative ions, or share electrons with other non-metal atoms. So a metal combined with a non-metal usually gives an ionic compound, two or more non-metals give a covalent substance, and a metal on its own is held together by metallic bonding. Knowing the electronic configuration, such as Na 2.8.1 or Cl 2.8.7, lets you predict how many electrons each atom loses, gains or shares.
Ionic bonding and giant ionic lattices
In sodium chloride, a sodium atom (2.8.1) transfers its single outer electron to a chlorine atom (2.8.7). This forms Na⁺ (2.8) and Cl⁻ (2.8.8). The ionic bond is the strong electrostatic attraction between oppositely charged ions. In a dot-and-cross diagram, show each ion in square brackets with its charge at the top right, and show the outer shell of the negative ion with both dots and crosses. The formula comes from balancing charges: Al³⁺ and O²⁻ combine as Al₂O₃ because 2 × (+3) = +6 balances 3 × (−2) = −6. Ionic compounds form giant lattices in which every ion is surrounded by ions of opposite charge. A lot of energy is needed to overcome the many strong attractions, so they have high melting and boiling points. They conduct electricity when molten or dissolved in water because the ions are then free to move, but not when solid, because the ions are held in fixed positions.
Covalent bonding and simple molecules
A covalent bond is a shared pair of electrons between two non-metal atoms. Hydrogen, H₂, and chlorine, Cl₂, each have one shared pair. In water, oxygen shares one pair with each hydrogen atom and keeps two lone pairs. In ammonia, nitrogen forms three bond pairs and has one lone pair. Carbon dioxide has two double bonds, O=C=O, with each double bond made of two shared pairs, and nitrogen, N₂, has a triple bond. Substances made of small molecules are simple molecular substances. The covalent bonds inside each molecule are strong, but the intermolecular forces between molecules are weak. Melting or boiling only overcomes these weak intermolecular forces, so simple molecular substances have low melting and boiling points and are often gases or liquids at room temperature. They do not conduct electricity because they have no mobile ions or electrons.
Giant covalent structures
Some non-metals form giant covalent structures in which every atom is joined to its neighbours by strong covalent bonds. In diamond, each carbon atom is bonded to four others in a rigid three-dimensional network. Melting means breaking many strong bonds, so diamond has a very high melting point and is very hard, which is why it is used in cutting tools. It does not conduct electricity because all outer electrons are used in bonding. In graphite, each carbon atom is bonded to three others in flat hexagonal layers, leaving one delocalised electron per atom that can move along the layers, so graphite conducts electricity. The forces between layers are weak, so layers slide over each other, making graphite soft and useful as a lubricant and in pencils. Silicon dioxide has each silicon atom bonded to four oxygen atoms and each oxygen to two silicon atoms, giving a hard solid with a high melting point.
Metallic bonding
In a metal, the atoms lose their outer electrons to form a lattice of positive ions surrounded by a sea of delocalised electrons. Metallic bonding is the strong electrostatic attraction between the positive ions and these delocalised electrons. This explains the typical properties of metals. They conduct electricity as solids and as liquids because the delocalised electrons can move through the structure. They usually have high melting points because the attraction is strong. They are malleable and ductile because the layers of ions can slide over each other while the sea of electrons keeps holding them together. Alloys such as steel are harder than the pure metal because atoms of a different size disrupt the regular layers, making it harder for them to slide.
Worked example
Magnesium reacts with chlorine to form magnesium chloride. Describe the dot-and-cross diagram for magnesium chloride and explain why it has a high melting point and conducts electricity only when molten or in aqueous solution.
- Write the electronic configurations: Mg is 2.8.2 and Cl is 2.8.7.
- Magnesium loses its two outer electrons, one to each of two chlorine atoms, so the formula is MgCl₂.
- Draw Mg²⁺ in square brackets with the 2.8 configuration and a 2+ charge at the top right.
- Draw two Cl⁻ ions, each in square brackets with eight outer electrons, seven dots of its own and one cross from magnesium, and a minus charge.
- High melting point: MgCl₂ is a giant ionic lattice, and a lot of energy is needed to overcome the strong electrostatic attraction between the Mg²⁺ and Cl⁻ ions.
- Conductivity: in the solid the ions are held in fixed positions, but when molten or dissolved the ions are free to move and carry charge.
Answer: MgCl₂ contains one Mg²⁺ ion and two Cl⁻ ions in a giant ionic lattice. It melts at a high temperature because of the strong attraction between oppositely charged ions, and it conducts only when molten or aqueous because only then are the ions mobile.
Common mistakes
- Saying ionic compounds conduct because of mobile electrons. Molten or aqueous ionic compounds conduct because their ions move.
- Writing that melting a simple molecular substance breaks covalent bonds. It only overcomes the weak intermolecular forces, and the molecules stay intact.
- Leaving out the lone pairs on oxygen in water or on nitrogen in ammonia when drawing dot-and-cross diagrams.
- Drawing shared electrons between the ions in an ionic compound. Ionic diagrams show separate ions in brackets with charges, not shared pairs.
- Explaining the conductivity of graphite with mobile ions. Graphite conducts because of delocalised electrons that move along the layers.
- Describing metallic bonding as attraction between metal atoms. It is the attraction between positive ions and delocalised electrons.
Exam tips
- In structure and properties answers, always name the structure (giant ionic, simple molecular, giant covalent or metallic), the particles and the force being overcome. Marks are usually given for each.
- Count the outer electrons in every dot-and-cross diagram before moving on. Each atom in a covalent molecule should end with a full outer shell, two for hydrogen.
- For conductivity, always name the mobile charged particle: ions for ionic compounds, delocalised electrons for metals and graphite.
Practice questions
1. Easy: Aluminium forms Al³⁺ ions and oxygen forms O²⁻ ions. Write the formula of aluminium oxide.
Show answer
Al₂O₃. Two Al³⁺ ions give +6 and three O²⁻ ions give −6, so the charges balance.
2. Medium: Explain why diamond and graphite both have very high melting points, but only graphite conducts electricity.
Show answer
Both are giant covalent structures, so melting means breaking many strong covalent bonds, which needs a lot of energy. In diamond each carbon uses all four outer electrons in bonding, so there are no mobile charged particles. In graphite each carbon bonds to only three others, so one electron per atom is delocalised and can move along the layers, carrying current.
3. Harder: State the number of bonding pairs and lone pairs around the nitrogen atom in ammonia, NH₃, and explain why ammonia is a gas at room temperature.
Show answer
Three bonding pairs and one lone pair. Nitrogen (2.5) shares one electron with each of three hydrogen atoms, and its remaining two outer electrons form a lone pair. Ammonia is a simple molecular substance, so only weak intermolecular forces hold the molecules together. Little energy is needed to overcome them, so its boiling point is below room temperature.
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