Chemistry • Year 11 • Module 1 • Lesson 14
Isotopes and Relative Atomic Mass
Build the core vocabulary and structural understanding of isotopes, nuclide notation, and relative atomic mass before tackling calculations.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: isotopes, relative atomic mass (Ar), mass spectrometry, isotopic abundance, mass number (A), atomic number (Z), nuclide notation, weighted average, fractional abundance, stable isotope. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | Atoms of the same element that have the same number of protons but different numbers of neutrons. | |
| 1.2 | The weighted average mass of all naturally occurring isotopes of an element, relative to 1/12 the mass of a ¹²C atom. It is dimensionless. | |
| 1.3 | An analytical technique that separates ions by their mass-to-charge ratio (m/z) to determine the mass and relative abundance of each isotope. | |
| 1.4 | The percentage (or fractional) occurrence of each isotope in a naturally occurring sample of an element. | |
| 1.5 | The total number of protons and neutrons in the nucleus of an atom; always a whole number for a specific nuclide. | |
| 1.6 | The number of protons in the nucleus; determines which element the atom is and equals the number of electrons in a neutral atom. | |
| 1.7 | A symbol such as ³⁵Cl that shows both the mass number (top) and the atomic number (bottom) to the left of the element symbol. | |
| 1.8 | An average in which each value is multiplied by its proportional contribution (its fractional abundance) before summing. | |
| 1.9 | Isotopic abundance expressed as a decimal between 0 and 1; equals percentage ÷ 100. All fractional abundances for one element must sum to 1.000. | |
| 1.10 | An isotope with a nucleus that does not spontaneously undergo radioactive decay under normal conditions. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Isotopes of the same element have different chemical properties because they have different masses. T / F
2.2 The relative atomic mass of chlorine is 35.5 because it is a mixture of two isotopes: ³⁵Cl (~75.77%) and ³⁷Cl (~24.23%). T / F
2.3 The mass number (A) of a specific nuclide is always a decimal, just like relative atomic mass. T / F
2.4 In the nuclide notation for ¹²C, the 12 is the mass number and it equals the total number of protons and neutrons in the nucleus. T / F
2.5 To calculate relative atomic mass correctly you use the percentage abundance directly in the formula: Ar = Σ(isotopic mass × % abundance). T / F
2.6 The relative atomic mass of an element is always pulled toward the mass of its most abundant isotope. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 8 marks (1 per blank)
Word bank:
chemical · electrons · fractional · mass number · neutrons · protons · radioactive · weighted average
Isotopes are atoms of the same element that share the same number of ___________ but differ in the number of ___________ in the nucleus. Because ___________ behaviour is determined by the number and arrangement of electrons, isotopes of the same element have identical chemical properties. The ___________ (A) of a specific isotope is the total count of protons and neutrons; it is always a whole number. Some isotopes are ___________ and will spontaneously decay; others are stable. The relative atomic mass (Ar) is a ___________ of all naturally occurring isotopic masses, where each isotopic mass is multiplied by its ___________ abundance (i.e. the percentage ÷ 100). All fractional abundances for a given element must sum to 1.000.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the function of mass spectrometry in determining the relative atomic mass of an element?
4.2 Why does carbon-14 (¹⁴C) behave identically to carbon-12 (¹²C) when reacting with oxygen to form CO2?
4.3 Why is the relative atomic mass of chlorine (35.45) not a whole number?
4.4 In nuclide notation, what information does the top number and the bottom number each convey?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “is calculated from”, “differs in”, “is measured by”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: isotopes · neutrons · relative atomic mass · isotopic abundance · mass spectrometry · weighted average.
6. Complete the isotope data table
Fill in the missing values for each nuclide. Use the information given and your knowledge of isotopes to complete every blank cell. 10 marks (1 per blank)
| Nuclide | Symbol & notation | Protons (Z) | Neutrons (A−Z) | Mass number (A) | Element |
|---|---|---|---|---|---|
| Protium | ¹H | 0 | 1 | Hydrogen | |
| Deuterium | ²H | 1 | 2 | Hydrogen | |
| Carbon-12 | 6 | 6 | 12 | Carbon | |
| Carbon-14 | ¹⁴C | 6 | 14 | Carbon | |
| Chlorine-35 | ³⁵Cl | 18 | 35 | Chlorine | |
| Chlorine-37 | ³⁷Cl | 17 | 37 | Chlorine | |
| ²⁴Mg | 12 | 12 | 24 | ||
| Magnesium-25 | 12 | 13 | Magnesium | ||
| Copper-63 | ⁶³Cu | 34 | 63 | Copper | |
| Copper-65 | ⁶⁵Cu | 29 | 65 | Copper |
Q1 — Term–definition match
1.1 isotopes • 1.2 relative atomic mass (Ar) • 1.3 mass spectrometry • 1.4 isotopic abundance • 1.5 mass number (A) • 1.6 atomic number (Z) • 1.7 nuclide notation • 1.8 weighted average • 1.9 fractional abundance • 1.10 stable isotope.
Q2 — True / false with correction
2.1 False. Isotopes have identical chemical properties because chemical behaviour is determined by electron configuration, which is the same for all isotopes of an element (same number of protons = same number of electrons in a neutral atom). Isotopes differ in physical properties (mass, density, boiling point) and nuclear stability.
2.2 True.
2.3 False. The mass number (A) of a specific nuclide is always a whole number (it is a simple count of protons + neutrons). Relative atomic mass (Ar) is a decimal (weighted average). The two must not be confused.
2.4 True.
2.5 False. You must use fractional abundance (= % ÷ 100) in the formula: Ar = Σ(isotopic mass × fractional abundance). Using raw percentages gives an answer ~100× too large (e.g. Ar(Cl) = 2646 instead of 35.45).
2.6 True.
Q3 — Cloze paragraph
In order: protons / neutrons / chemical / mass number / radioactive / weighted average / fractional.
Note: only 7 blanks appear despite 8 words in the bank — “electrons” is the unused distractor. Award 1 mark per correct blank (7 blanks × 1 = 7 marks); award the 8th mark for having all 7 correct with the correct word “electrons” identified as the unused distractor, OR adjust total to 7 marks.
Q4.1 — Function of mass spectrometry
A mass spectrometer ionises atoms and separates the resulting ions by their mass-to-charge ratio (m/z) using a magnetic field. Heavier ions deflect less than lighter ions. The detector records the relative number of ions at each mass value, producing a mass spectrum with peaks at each isotope’s mass number; peak height is proportional to relative abundance. This data is then used to calculate Ar.
Q4.2 — Why ¹⁴C and ¹²C react identically with O2
Both isotopes have 6 protons, giving them 6 electrons in an identical electron configuration. Chemical behaviour (bonding, reactivity, valence) is determined solely by electron configuration. Because ¹²C and ¹⁴C have the same electron arrangement, they form the same bonds with oxygen — two C=O bonds — producing CO2 in both cases. The different number of neutrons has no effect on electron configuration.
Q4.3 — Why Ar(Cl) is not a whole number
Chlorine occurs naturally as a mixture of two isotopes: ³⁵Cl (~75.77%) and ³⁷Cl (~24.23%). The relative atomic mass is the weighted average of their masses: Ar = (35 × 0.7577) + (37 × 0.2423) = 26.52 + 8.97 = 35.45. Because the two isotopes are present in unequal proportions, the weighted average falls between the two mass numbers but is not a whole number. For Ar to be a whole number, the isotopes would need to have abundances that produce an exactly integer average.
Q4.4 — Nuclide notation numbers
In nuclide notation (e.g. ³⁵Cl), the top number is the mass number (A) — the total count of protons and neutrons in the nucleus. The bottom number is the atomic number (Z) — the number of protons, which identifies which element the atom is. The number of neutrons = A − Z.
Q5 — Sample concept map
Correct maps should include arrows such as:
- isotopes — differ in number of → neutrons
- mass spectrometry — measures → isotopic abundance
- isotopic abundance — is used to calculate → relative atomic mass
- relative atomic mass — is a → weighted average
- weighted average — uses → isotopic abundance
- isotopes — are detected by → mass spectrometry
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).
Q6 — Isotope data table
Row 1 (¹H): Z = 1. Row 2 (²H): neutrons = 1. Row 3 (¹²C): symbol = ¹²C. Row 4 (¹⁴C): neutrons = 8. Row 5 (³⁵Cl): Z = 17. Row 6 (³⁷Cl): neutrons = 20. Row 7 (²⁴Mg): nuclide name = Magnesium-24; element = Magnesium. Row 8 (²⁵Mg): symbol = ²⁵Mg; A = 25. Row 9 (⁶³Cu): Z = 29. Row 10 (⁶⁵Cu): neutrons = 36.