Year 8 Science · Unit 2 · Lesson 08
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Learning Goals
Match each isotope to a fact about it
Write the matching letter in the middle column. Each letter is used once.
| Isotope | Letter | Fact |
|---|---|---|
| Carbon-14 | A. Stable; a tracer for tracking water in the body. | |
| Tritium (hydrogen-3) | B. Radioactive; used for medical imaging. | |
| Technetium-99m | C. Stable; about 99% of all carbon. | |
| Deuterium (hydrogen-2) | D. Radioactive; makes some exit signs glow. | |
| Carbon-12 | E. Stable; 1 proton and 1 neutron. | |
| Oxygen-18 | F. Radioactive; used to date once-living things. |
Odd one out
In each row, three atoms are isotopes of one element. Circle the odd one out and name its element.
| Row | Atoms | Odd one out and its element |
|---|---|---|
| 1 | ¹²₆C · ¹³₆C · ¹⁴₇N · ¹⁴₆C | |
| 2 | ¹⁹₉F · ¹⁶₈O · ¹⁷₈O · ¹⁸₈O | |
| 3 | ¹₁H · ²₁H · ³₁H · ³₂He |
1. Tritium and deuterium are both hydrogen. State what they have in common and how they differ, using particle numbers.
Classify with a reason
Write “isotopes”, “identical” (same isotope) or “different elements”, with a reason.
| Pair | Atom 1 | Atom 2 | Verdict and reason |
|---|---|---|---|
| a | 6 protons, 6 neutrons | 6 protons, 7 neutrons | |
| b | 17 protons, 18 neutrons | 18 protons, 18 neutrons | |
| c | 1 proton, 1 neutron | 1 proton, 1 neutron |
Scenario: a water sample
A laboratory checks lake water (made of hydrogen and oxygen atoms) and records the particles in four of its atoms.
| Atom | Protons | Neutrons | Name (for example oxygen-16) |
|---|---|---|---|
| J | 8 | 8 | |
| L | 8 | 10 | |
| M | 1 | 1 | |
| N | 8 | 9 |
Source: sample data made up for this worksheet, using real isotopes.
2. Complete the Name column of Table 1. Then list the atoms that are isotopes of oxygen.
3. A water molecule made with atom L is heavier than one made with atom J. Explain why it still behaves as water in the body, and why this makes oxygen-18 useful as a tracer.
4. Carbon-14 (6 protons, 8 neutrons) and nitrogen-14 (7 protons, 7 neutrons) share a mass number but are different elements. Carbon-12 and carbon-14 have different mass numbers but are the same element. Use the isotope rule to explain both facts, and say which number to check first, and why.
Match each isotope to a fact about it
Carbon-14 → F; Tritium (hydrogen-3) → D; Technetium-99m → B; Deuterium (hydrogen-2) → E; Carbon-12 → C; Oxygen-18 → A.
Odd one out
Row 1: ¹⁴₇N, nitrogen (7 protons; the others have 6). Row 2: ¹⁹₉F, fluorine (9 protons; the others have 8). Row 3: ³₂He, helium (2 protons; the others have 1).
1. Same: both have 1 proton (so both are hydrogen, atomic number 1). Different: tritium has 2 neutrons (mass number 3) and deuterium has 1 neutron (mass number 2). [2 marks]
Classify with a reason
a: isotopes (both 6 protons, so carbon; different neutrons). b: different elements (17 and 18 protons: chlorine and argon). c: identical (same protons and same neutrons, so the same isotope, hydrogen-2).
Go Deeper
2. J oxygen-16; L oxygen-18; M hydrogen-2 (deuterium); N oxygen-17. Isotopes of oxygen: J, L and N. [3 marks: 2 all four names (1 if two or three), 1 list]
3. Atom L is still oxygen (8 protons). Isotopes of an element have the same chemical properties (same electron arrangement), so the molecule behaves as water. Only its mass differs (2 extra neutrons), so scientists can detect the heavier water and follow where it goes. [3 marks: 1 same element; 1 same chemical behaviour; 1 extra mass lets it be tracked]
Show What You Know
4. Marking guide (points expected): 1 rule: isotopes have the same proton number but different neutron numbers, and proton number decides the element; 1 carbon-14 has 6 protons and nitrogen-14 has 7, so different elements; 1 the mass numbers match only because 6 + 8 and 7 + 7 both equal 14; 1 carbon-12 and carbon-14 both have 6 protons (both carbon) but 6 and 8 neutrons, so they are isotopes; 1 check the proton (atomic) number first: it alone identifies the element, and mass numbers can match by coincidence. [5 marks]
Common error to watch for: in pair c, writing “isotopes”. Two atoms with the same protons and the same neutrons are the same isotope, not isotopes of each other.