Ch 4 · Ions and Ionic Bonds · IGCSE Chemistry 0620
Topic 4 of 7 in Atoms, Elements and Compounds — IGCSE Chemistry 0620 — 48 lessons.
Introduction
When a metal reacts with a non-metal, electrons are transferred and charged particles called ions form. The attraction between these ions is the ionic bond, and it explains why salts are hard crystalline solids with very high melting points that conduct electricity only when melted or dissolved. Dot-and-cross diagrams for ionic compounds are a standard exam question, and every candidate should be able to draw sodium chloride from memory.
Key ideas — How ions form
Atoms lose or gain electrons to obtain a full outer shell, the stable electronic configuration of a noble gas.
🧑🏫 Teacher's explanation
Imagine you are at a party where everyone wants to be 'stable' by having a full set of trading cards. Some people have one extra card they don't want (metals), and others are missing exactly one or two cards to complete their set (non-metals). To reach stability, the person with the extra card gives it away, becoming 'positive' because they lost something negative. The person who receives the card becomes 'negative' because they gained something negative.
📖 Definitions
🔍 Worked example — Easy
A Magnesium atom has the electronic configuration (2, 8, 2). Describe the formation of the Mg^2+ ion.
🔍 Worked example — Standard
A Chlorine atom has the electronic configuration (2, 8, 7). Show the formation of the Cl^- ion.
🔍 Worked example — Exam-style
Predict the formula of the ionic compound formed between Calcium (2, 8, 2) and Oxygen (2, 6).
⚠️ Common errors
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🎯 Exam question
Key ideas — The ionic bond and dot-and-cross diagrams
Group I with Group VII: sodium chloride. The diagram shows the outer shells only. A sodium atom (one dot in its outer shell) transfers that electron to a chlorine atom (seven crosses in its outer shell). The result is a sodium ion, drawn with an empty outer shell (or the full inner shell of eight) inside square brackets with a + sign, and a chloride ion drawn with seven crosses and one dot, inside square brackets with a – sign. Each ion now has a full outer shell. Lithium fluoride, potassium bromide and every other Group I with Group VII compound follow the same 1 : 1 pattern.
🧑🏫 Teacher's explanation
Imagine two people playing a game of tug-of-war with a single ball. In ionic bonding, the 'metal' is like a person who finds the ball too heavy and just lets it go, becoming 'positive' because they lost something negative (the electron). The 'non-metal' is the person who catches it, becoming 'negative' because they gained something negative. Once they both have their new identities, they are drawn together by a powerful magnetic-like force. This force is the electrostatic attraction between the positive and negative charges.
📖 Definitions
🔍 Worked example — Easy
Draw the dot-and-cross diagram for the formation of Lithium Fluoride (LiF). Use dots for Li electrons and crosses for F electrons.
🔍 Worked example — Standard
Calculate the formula of the compound formed between Magnesium (Group II) and Nitrogen (Group V). Assume they form an ionic bond.
🔍 Worked example — Exam-style
Explain why Magnesium Oxide (MgO) has a much higher melting point than Sodium Chloride (NaCl).
⚠️ Common errors
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🎯 Exam question
(a) Write the formula for the magnesium ion and the oxide ion. [2] (b) Explain why MgO has a higher melting point than NaCl. [2] [4 marks]
Key ideas — The giant ionic lattice
Ionic compounds do not exist as separate pairs of ions. The ions are packed into a giant lattice: a regular, repeating arrangement of alternating positive and negative ions that extends in three dimensions throughout the crystal. In sodium chloride every Na^+ ion is surrounded by six Cl^- ions and every Cl^- ion by six Na^+ ions, held together by strong electrostatic attractions acting in all directions.
🧑🏫 Teacher's explanation
Imagine a massive, perfectly organized dance floor where every person wearing a red shirt must be surrounded by people wearing blue shirts, and every blue shirt must be surrounded by red shirts. No one is standing alone in a pair; instead, everyone is locked into a rigid, repeating grid. This is exactly how an ionic compound looks.
📖 Definitions
🔍 Worked example — Easy
Explain why sodium chloride, NaCl, has a high melting point.
🔍 Worked example — Standard
A sample of magnesium oxide, MgO, has a melting point of 2852 ^circC. A sample of sodium chloride, NaCl, has a melting point of 801 ^circC. Explain why MgO has a significantly higher melting point than NaCl. (Use Mg^2+ and O^2- vs Na^+ and Cl^-)
🔍 Worked example — Exam-style
Predict the state of CaCl_2 at 25 ^circC and explain its electrical conductivity in this state.
⚠️ Common errors
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🎯 Exam question
Key ideas — Properties of ionic compounds
🧑🏫 Teacher's explanation
Imagine a giant, perfectly organized crowd of people where every person is wearing a bright red shirt and every second person is wearing a bright blue shirt. If the red people are all magnets and the blue people are all magnets, they will snap together into a rigid, repeating grid. This grid is incredibly hard to break apart because every single person is being pulled by many others simultaneously.
📖 Definitions
🔍 Worked example — Easy
Explain why sodium chloride, NaCl, has a high melting point.
🔍 Worked example — Standard
A solid sample of magnesium oxide, MgO, does not conduct electricity. However, when melted, it conducts electricity well. Explain this observation.
🔍 Worked example — Exam-style
Calculate the number of moles in 11.7 g of NaCl. (A_r: Na=23, Cl=35.5)
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. Magnesium reacts with chlorine to form magnesium chloride, MgCl_2.
🔍 Worked example — Easy
A sodium atom (group I) reacts with a fluorine atom (group VII). Write the formula of the ionic compound formed.
🔍 Worked example — Standard
Calcium reacts with oxygen to form calcium oxide. Show the electron transfer using the concept of ions.
🔍 Worked example — Exam-style
Explain why magnesium oxide (MgO) has a much higher melting point than sodium chloride (NaCl).
⚠️ Common errors
Common mistakes
Practice
Answers Potassium loses one electron to form K^+, configuration 2,8,8 [B1]; fluorine gains one electron to form F^-, configuration 2,8 [B1]; the electron is transferred from potassium to fluorine [B1]. A strong electrostatic attraction between oppositely charged ions [B1]. A giant lattice / regular arrangement [B1] of alternating positive sodium ions and negative chloride ions [B1]. mathrmCa^2+ shown with an empty outer shell (or a full shell of eight) in square brackets with a 2+ charge [B1]; mathrmO^2- shown with eight outer electrons, two of them marked as coming from calcium, in square brackets with a 2– charge [B1]. Strong electrostatic attractions between oppositely charged ions throughout the giant lattice [B1]; a large amount of energy is needed to overcome them, and the 2+ and 2– charges make the attraction stronger still [B1].