Lesson overview
Ions and ionic bonding: When does salt conduct electricity, and why?
Salt water lights a bulb, but what carries the current: the salt or the water? Predict which samples will light it, test pure water and salt dry, dissolved in water and melted, and look at each sample through the magnifier for the particles that carry the current.
Suggested sequence
- Lesson 1 · Salt and the bulb. Ions, the ionic bond and the properties of ionic compounds; Supplement: the giant lattice and why it explains them. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Dots and crosses. Dot-and-cross diagrams for Group I and Group VII compounds; Supplement: other metals with non-metals. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Salt and the bulb
- Describe the formation of positive ions, known as cations, and negative ions, known as anions
- State that an ionic bond is a strong electrostatic attraction between oppositely charged ions
- Describe the properties of ionic compounds: high melting points and boiling points, good electrical conductivity when aqueous or molten and poor when solid
- Describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions
- Explain in terms of structure and bonding the properties of ionic compounds: high melting points and boiling points, good electrical conductivity when aqueous or molten and poor when solid
Part 2: Dots and crosses
- Describe the formation of ionic bonds between elements from Group I and Group VII, including the use of dot-and-cross diagrams
- Describe the formation of ionic bonds between ions of metallic and non-metallic elements, including the use of dot-and-cross diagrams
Misconceptions to listen for
- “Dry crystals of lithium chloride light the bulb.” — The crystals are made of ions, but in a solid the ions are held in place. They cannot move to carry the charge.
- “The dry powder lights the bulb, because it is broken up.” — A powder is still tiny solid grains. The ions inside each grain are still held in place.
- “The solution conducts because water conducts.” — Pure water leaves the bulb dark, and molten salt conducts with no water at all. The moving ions carry the current.
- “It loses 1 electron and becomes K−.” — An electron is negative. Losing one leaves more protons than electrons, so the ion is positive: K+.
- “It gains 1 proton and becomes K+.” — The nucleus does not change in a reaction. K+ is positive because it has one electron fewer than protons.
- “It gains 7 electrons to fill its outer shell.” — Losing the 1 outer electron is far easier than gaining 7. Metal atoms lose electrons.
- “A weak electrostatic attraction between oppositely charged ions.” — The attraction is strong: that is why ionic compounds need a lot of energy to melt.
- “A pair of electrons shared between two oppositely charged ions.” — Ions do not share electrons. A shared pair is a covalent bond; in an ionic compound the electron has moved over completely.
- “The electron transfer that turns two atoms into oppositely charged ions.” — The electron transfer makes the ions. The bond is the strong electrostatic attraction between the oppositely charged ions afterwards.
- “A cation has extra electrons.” — A cation is positive: it has fewer electrons than protons. An anion has extra electrons.
Scope
Ions and ionic bonds (2.4 Core 1–4, Supplement 5–7): cations and anions, the ionic bond as a strong electrostatic attraction between oppositely charged ions, dot-and-cross diagrams for Group I with Group VII compounds and, in the Supplement, for other metallic and non-metallic elements, the high melting and boiling points of ionic compounds and their conduction when molten or aqueous but not when solid, the giant lattice of alternating ions, and how that structure explains the properties. Covalent bonds, giant covalent and metallic structures, writing formulae and naming compounds in general, and electrolysis are left to other modules.