Ch 6 · Giant Covalent Structures · IGCSE Chemistry 0620
Topic 6 of 7 in Atoms, Elements and Compounds — IGCSE Chemistry 0620 — 48 lessons.
Introduction
Diamond and graphite are both pure carbon, yet one is the hardest natural substance and the other is soft enough to write with. The difference lies entirely in how the atoms are bonded, which makes this topic the perfect test of whether you can link structure to properties. Extended candidates also meet silicon(IV) oxide, the main constituent of sand, whose structure mirrors diamond.
Key ideas — What a giant covalent structure is
In a giant covalent structure (also called a macromolecule) a very large number of atoms are joined to one another by strong covalent bonds in a continuous network that extends throughout the whole crystal. There are no separate molecules. Because breaking up the structure means breaking many strong covalent bonds, giant covalent substances have very high melting and boiling points, are usually hard, and are insoluble in water. Diamond, graphite and silicon(IV) oxide are the examples required.
🧑🏫 Teacher's explanation
Imagine a single LEGO brick. That is a molecule. Now, imagine millions of LEGO bricks all snapped together in one massive, endless block. That is a giant covalent structure. Instead of having a collection of separate little pieces, you have one single, enormous object where every atom is tightly locked to its neighbors.
📖 Definitions
🔍 Worked example — Easy
Explain why diamond has a very high melting point.
🔍 Worked example — Standard
Compare the electrical conductivity of graphite and diamond.
🔍 Worked example — Exam-style
Silicon(IV) oxide (SiO_2) is used in abrasives. Explain why its properties are similar to diamond.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Diamond
In diamond each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, producing a rigid three-dimensional network.
🧑🏫 Teacher's explanation
Imagine a giant, interconnected jungle gym where every single joint is welded to four other joints in a perfect pyramid shape. Because every single connection is a strong weld, you cannot bend or break the structure easily; this is like diamond.
📖 Definitions
🔍 Worked example — Easy
Explain why diamond has a very high melting point.
🔍 Worked example — Standard
Compare the electrical conductivity of diamond and graphite.
🔍 Worked example — Exam-style
Silicon(IV) oxide (SiO_2) is a giant covalent structure similar to diamond. Explain why SiO_2 has a high melting point.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why diamond is much harder than graphite. [2] (b) Explain why graphite can conduct electricity. [2] [4 marks]
Key ideas — Graphite
In graphite each carbon atom is covalently bonded to only three other carbon atoms, forming flat layers of hexagons. The layers are held together only by weak forces between them.
🧑🏫 Teacher's explanation
Imagine a large stack of thin, slippery sheets of paper. If you push the top sheet, it slides easily over the others because there is nothing holding the sheets together except very weak friction. This is exactly how graphite works; it is made of layers of carbon atoms, but the layers aren't 'glued' together strongly, allowing them to slide.
📖 Definitions
🔍 Worked example — Easy
State the number of covalent bonds formed by each carbon atom in graphite and explain why it is a good conductor of electricity.
🔍 Worked example — Standard
Compare the hardness of graphite and diamond based on their structures.
🔍 Worked example — Exam-style
Silicon(IV) oxide (SiO_2) has a structure similar to diamond. Explain why SiO_2 has a high melting point.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Silicon(IV) oxide
🧑🏫 Teacher's explanation
Imagine a massive, interconnected jungle gym where every single joint is welded shut. In a small molecule like water (H_2O), the molecules are like individual LEGO bricks that can slide around each other easily. But in a giant covalent structure like silicon(IV) oxide, the entire crystal is one single, massive 'super-molecule'.
📖 Definitions
🔍 Worked example — Easy
Explain why silicon(IV) oxide has a high melting point.
🔍 Worked example — Standard
Compare the electrical conductivity of diamond and silicon(IV) oxide.
🔍 Worked example — Exam-style
Calculate the molar mass of silicon(IV) oxide, SiO_2, given A_r(Si) = 28 and A_r(O) = 16.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the structure of silicon(IV) oxide. [2] (b) Explain why silicon(IV) oxide has a high melting point. [2] [4 marks]
Worked example
Question. Diamond and graphite are both forms of carbon.
🔍 Worked example — Easy
State the number of covalent bonds formed by a single carbon atom in diamond and in graphite.
🔍 Worked example — Standard
Explain why silicon(IV) oxide (SiO_2) has a high melting point, similar to diamond.
🔍 Worked example — Exam-style
A sample of graphite is used as an electrode. Explain why it conducts electricity while diamond does not.
⚠️ Common errors
Common mistakes
Practice
Answers Giant covalent structure in which each carbon atom is bonded to four others [B1] in a tetrahedral / three-dimensional arrangement [B1]; a large amount of energy is needed to break the many strong covalent bonds [B1]. In diamond all four outer electrons of each carbon are used in covalent bonds, so there are no free electrons [B1]; in graphite each carbon uses only three electrons for bonding [B1]; the fourth electron is delocalised and free to move along the layers, carrying charge [B1]. Lubricant, because the layers slide over one another owing to weak forces between them [B1 + B1]; or electrode, because delocalised electrons allow it to conduct and it has a high melting point [B1 + B1]. Giant covalent structure in which each silicon atom is bonded to four oxygen atoms and each oxygen to two silicon atoms [B1]; tetrahedral arrangement like diamond [B1]; both have many strong covalent bonds to break and no free electrons, so both are hard, have very high melting points and do not conduct [B1].