Year 9 Science · Unit 2 · Lesson 07
Challenge Worksheet
Learning Goals
1. Define valency. State the valency of oxygen and of aluminium.
Find the error
A student wrote these six formulas. Mark each one ✓ or ✗, and correct the wrong ones. Use a periodic table to find each ion’s charge.
| Compound | Student’s formula | ✓ or ✗ | Correct formula |
|---|---|---|---|
| Sodium oxide | NaO | ||
| Lithium oxide | Li₂O | ||
| Potassium nitride | K₃N | ||
| Aluminium sulfide | Al₃S₂ | ||
| Calcium sulfide | Ca₂S₂ | ||
| Calcium fluoride | CaF₂ |
2. The student made three different kinds of mistake. Describe each one, and give a tip that would stop it happening again.
Data: the ions in seawater
Marine scientists have measured the ions dissolved in ocean water very precisely. Table 1 shows the six main ions, as the number of each ion in every 1000 dissolved ions.
| Ion | Number per 1000 ions | Charge on each ion | Total charge |
|---|---|---|---|
| Chloride, Cl⁻ | 489 | −1 | −489 |
| Sodium, Na⁺ | 420 | +1 | +420 |
| Magnesium, Mg²⁺ | 47 | +2 | |
| Sulfate, SO₄²⁻ | 25 | −2 | |
| Calcium, Ca²⁺ | 9 | +2 | |
| Potassium, K⁺ | 9 | +1 |
Source: the reference composition of standard seawater (salinity 35 g/kg), converted from grams to numbers of ions and rounded. Small amounts of other ions, such as hydrogencarbonate (HCO₃⁻) and bromide (Br⁻), are left out.
3. Complete the Total charge column. Then add up all the positive charges and all the negative charges.
Total positive Total negative
4. Explain why the two totals should be almost equal. Suggest two reasons why they are not exactly equal in Table 1.
5. A student claims: “Every ion in seawater is a metal ion that formed when a metal atom gave away electrons.” Use Table 1 to evaluate this claim.
6. A classmate says: “Table salt must be dangerous to eat. It is made of sodium, which explodes in water, and chlorine, a poisonous gas.” Evaluate this claim.