Year 8 Science · Unit 2 · Lesson 20
Foundation Worksheet
Learning Goals
Fill the gap
Choose the correct word from the word bank. Each word is used once at most. Two words will not be used.
A chemical uses element symbols and numbers to show which atoms are in a substance. The small number written just after a symbol is called a . If a symbol has no small number after it, count atom of that element. Oxygen gas, O₂, is a element because it naturally exists as two-atom molecules. A compound made of exactly two different elements is a compound, and LiCl is named lithium .
1. Write the formulas of all seven diatomic elements.
Put the formulas in order of size
Count the atoms of each element and add them up. Then number the formulas from 1 (fewest atoms) to 4 (most atoms). A number outside a bracket multiplies everything inside it.
| Formula | Atoms of each element | Total atoms | Order (1–4) |
|---|---|---|---|
| Na₂O | |||
| Mg(NO₃)₂ | |||
| SO₃ | |||
| LiI |
Read the data
Ava wrote down the formula of the main substance in five products at home (Table 1).
| Product | Formula | Name of the substance |
|---|---|---|
| Toothpaste | NaF | |
| Bath salts | MgSO₄ | magnesium sulfate |
| Zinc sunscreen cream | ZnO | |
| Antacid tablets | CaCO₃ | calcium carbonate |
| Salt substitute | KCl |
Real substances and formulas. Each product is simplified to its one main substance.
2. Use the -ide rule to fill in the three missing names in Table 1.
3. Count the atoms in MgSO₄ and in CaCO₃. Then use the formulas to explain why the -ide rule worked for NaF, ZnO and KCl but cannot name MgSO₄ or CaCO₃.
Scenario: the damp-absorber tub
A tub that soaks up moisture in a wardrobe lists its contents as CaCl₂. Your cousin asks: “Does that mean two calcium atoms and two chlorine atoms? And what is this stuff called?”
4. Write a reply to your cousin that explains how to read the formula CaCl₂ and how to name it.
Fill the gap
formula; subscript; one; diatomic; binary; chloride. Not used (2): zero, mixture.
1. H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ (any order; “HOFBrINCl”). [2 marks: all seven; 1 mark for five or six]
Put the formulas in order of size
Reading down the table as printed: 2, 4, 3, 1.
Counts: Na₂O = 2 Na + 1 O = 3; Mg(NO₃)₂ = 1 Mg + 2 N + 6 O = 9; SO₃ = 1 S + 3 O = 4; LiI = 1 Li + 1 I = 2.
Go Deeper
2. NaF: sodium fluoride. ZnO: zinc oxide. KCl: potassium chloride. [3 marks: 1 each]
3. Counts: MgSO₄ = 1 Mg + 1 S + 4 O = 6 atoms; CaCO₃ = 1 Ca + 1 C + 3 O = 5 atoms. NaF (sodium, fluorine), ZnO (zinc, oxygen) and KCl (potassium, chlorine) each contain exactly two elements, so they are binary compounds and the -ide rule applies. MgSO₄ (magnesium, sulfur, oxygen) and CaCO₃ (calcium, carbon, oxygen) each contain three elements, so they are not binary and the -ide rule cannot name them. [4 marks: 1 both counts, 1 NaF, ZnO and KCl have two elements, 1 MgSO₄ and CaCO₃ have three, 1 the rule is only for binary compounds]
Show What You Know
4. Marking guide (points expected): 1 each capital letter starts a new element symbol, so Ca is calcium and Cl is chlorine; 1 the small 2 is a subscript that counts only the Cl just before it, so there is 1 calcium atom and 2 chlorine atoms (3 atoms), not two of each; 1 CaCl₂ has two different elements joined, so it is a compound, and a binary compound; 1 name: calcium chloride; 1 steps: the first element (calcium) keeps its name and the second element (chlorine) changes its ending to -ide. [5 marks]
Common error to watch for: applying a subscript to the whole formula, for example reading Mg(NO₃)₂ as 2 Mg, or forgetting that the 2 outside the bracket doubles both the N and the O (giving 1 + 1 + 3 = 5 instead of 9).