Year 8 Science · Unit 2 · Lesson 02
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Learning Goals
1. State two reasons chemists write symbols instead of full element names.
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. The symbol K for potassium comes from an English word.
Correct it:
2. The formula NaCl shows two sodium atoms and one chlorine atom.
Correct it:
3. Co is an element, but CO is a compound.
Correct it:
4. Every chemical symbol has exactly two letters.
Correct it:
5. CO₂ contains one carbon atom and two oxygen atoms.
Correct it:
Name, symbol, sample or model?
Each row is one way of showing helium. Write whether it is the element’s name, its symbol, a real sample or a model, and give a reason.
| What you see | Name, symbol, sample or model? | Reason |
|---|---|---|
| A drawing of a circle labelled He | ||
| The gas inside a sealed lab cylinder of pure helium | ||
| He | ||
| helium |
Put the substances in order by number of atoms
Count the atoms shown in each formula. Then number the rows from 1 (fewest atoms) to 5 (most atoms).
| Order (1–5) | Substance | Formula | Atoms shown |
|---|---|---|---|
| Water | H₂O | ||
| Glucose | C₆H₁₂O₆ | ||
| Iron | Fe | ||
| Aspirin | C₉H₈O₄ | ||
| Table salt | NaCl |
Sam copied the formula of one substance from each of four kitchen products.
| Product | Substance | Formula |
|---|---|---|
| Soft drink | carbon dioxide (the bubbles) | CO₂ |
| Baking soda | sodium hydrogen carbonate | NaHCO₃ |
| Table sugar | sucrose | C₁₂H₂₂O₁₁ |
| Vinegar | acetic acid | C₂H₄O₂ |
Source: formulas as given on typical ingredient information.
2. Sam says baking soda contains nitrogen, because its formula starts with N. Explain Sam’s mistake. Then list each element in NaHCO₃ and how many atoms of it the formula shows.
3. Sugar and vinegar are both made of carbon, hydrogen and oxygen. Use their formulas to explain how they can be different substances, and why a chemist must copy every small number exactly.
Scenario: two research teams
A team in Australia and a team in Germany are working on the same new medicine. In German, iron is “Eisen” and sodium is “Natrium”.
4. Explain how both teams can share the ingredients of their medicine precisely without translating anything.
Warm Up
1. Any two: understood the same way in every language; short, so they save space; precise, so there is no confusion about which element is meant. [2 marks]
True or false
1. False: K comes from the Latin name kalium. 2. False: NaCl shows one sodium atom and one chlorine atom; with no subscript, there is one of each. 3. True. 4. False: a symbol has one or two letters, for example H and He. 5. True.
Name, symbol, sample or model?
Circle labelled He: model (a picture that represents one atom, not what it looks like). Gas in the cylinder: real sample (actual matter). He: symbol (standard label, capital first). helium: name (the word for the element).
Put the substances in order by number of atoms
Reading down the table as printed: 3, 5, 1, 4, 2.
Atoms shown: water 3; glucose 24; iron 1; aspirin 21; table salt 2.
Go Deeper
2. Na is one symbol, sodium; a capital letter starts a new symbol, and the lowercase a belongs to the N before it. NaHCO₃: sodium 1, hydrogen 1, carbon 1, oxygen 3. [3 marks: 1 mistake explained; 2 for all four counts, 1 if three are right]
3. Sucrose has 12 C, 22 H, 11 O; acetic acid has 2 C, 4 H, 2 O. Different numbers of each atom make different substances with different properties. Changing one subscript describes a different substance, so a copying error names the wrong chemical. [3 marks: 1 compare counts; 1 different numbers means different substance; 1 why exact copying matters]
Show What You Know
4. Marking guide (points expected): 1 symbols are international (Fe and Na mean iron and sodium everywhere, although the German names are Eisen and Natrium); 1 the system does not depend on one language, so nothing needs translating; 1 formulas show which elements are in each substance; 1 subscripts show exactly how many atoms of each, so both teams make the same substance; 1 the case rule means each symbol reads only one way (Co is cobalt, CO is carbon monoxide). [5 marks]
Common error to watch for: reading a formula one letter at a time. A capital letter always starts a new symbol, so Na, Cl and Co are single symbols.