Balancing nuclear equations for alpha, beta and gamma decay. 10 questions, getting harder as you go. Write your answer first, then tap Show answer. Combined Separate
π Copy into your book
How to read a nuclear symbol: AZX
A = mass number (top) = protons + neutrons
Z = atomic number (bottom) = protons
Alpha particle = 42He β mass number goes DOWN 4, atomic number goes DOWN 2
Beta particle = 0β1e β mass number STAYS THE SAME, atomic number goes UP 1 (a neutron turns into a proton)
Gamma ray = Ξ³ β NO change to mass number or atomic number
Golden rule: the top numbers must add up to the same on both sides. So must the bottom numbers.
Atomic numbers you may need: C 6 Β· N 7 Β· Co 27 Β· Ni 28 Β· Sr 38 Β· Y 39 Β· Pb 82 Β· Bi 83 Β· Po 84 Β· Rn 86 Β· Ra 88 Β· Th 90 Β· Pa 91 Β· U 92
Q1 (2 marks) F A nucleus emits an alpha particle. What happens to its mass number and its atomic number?
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The mass number decreases by 4 (1). The atomic number decreases by 2 (1).
Why? An alpha particle is 2 protons + 2 neutrons, so the nucleus loses 4 particles in total, 2 of them protons.
Q2 (3 marks) F A nucleus emits a beta particle. What happens to its mass number and its atomic number? Explain why.
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The mass number stays the same (1). The atomic number increases by 1 (1). A neutron in the nucleus turns into a proton and an electron, and the electron is emitted as the beta particle (1).
Q3 (2 marks) F Explain why emitting a gamma ray does not change the mass number or atomic number of a nucleus.
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A gamma ray is an electromagnetic wave β it has no mass and no charge (1). So the nucleus loses no protons or neutrons; it just gets rid of some energy (1).
Q4 (1 mark) F Complete the equation for the alpha decay of radium-226:
22688Ra β 22286Rn + ?
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42He (1)
Check: top 226 = 222 + 4 β bottom 88 = 86 + 2 β
Q5 (1 mark) F Complete the equation for the beta decay of carbon-14:
146C β 147N + ?
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0β1e (1)
Check: top 14 = 14 + 0 β bottom 6 = 7 + (β1) β
Q6 (3 marks) F/H Uranium-238 (23892U) decays by emitting an alpha particle. Write the full decay equation, and name the new element.
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23892U β 23490Th + 42He
Alpha particle written correctly (1). Mass number 234 and atomic number 90 (1). Atomic number 90 is thorium (1).
Q7 (3 marks) F/H Strontium-90 (9038Sr) decays by emitting a beta particle. Write the full decay equation, and name the new element.
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9038Sr β 9039Y + 0β1e
Beta particle written correctly (1). Mass number 90 and atomic number 39 (1). Atomic number 39 is yttrium (1).
Q8 (2 marks) H Polonium-210 decays into lead-206:
21084Po β 20682Pb + X
Identify X and state the type of decay.
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Top: 210 β 206 = 4. Bottom: 84 β 82 = 2. So X is 42He (1). This is alpha decay (1).
Q9 (3 marks) H Cobalt-60 (6027Co) emits a beta particle and a gamma ray at the same time. Write the decay equation and explain how the gamma ray affects it.
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6027Co β 6028Ni + 0β1e + Ξ³
Nickel-60 written correctly (1). Beta particle and Ξ³ both included (1). The gamma ray doesn’t change either number β it only carries away energy (1).
Q10 (4 marks) H Thorium-232 (23290Th) decays in several steps to become radon-220 (22086Rn). Each step is either an alpha decay or a beta decay. Work out how many alpha particles and how many beta particles are emitted. Show your working.
Hint: only alpha decay changes the mass number β so start with the top numbers.
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Mass number falls by 232 β 220 = 12. Each alpha takes away 4, so 12 Γ· 4 = 3 alpha particles (1).
3 alphas would lower the atomic number by 3 Γ 2 = 6, from 90 to 84 (1).
But radon is 86, so the atomic number must go back up by 2 (1). Each beta adds 1, so there are 2 beta particles (1).
This is the real decay chain: Th-232 βΞ± Ra-228 βΞ² Ac-228 βΞ² Th-228 βΞ± Ra-224 βΞ± Rn-220.
β Add up your marks out of 24. 18 or more? Brilliant. Less than 12? Re-read the π box and try Q4βQ8 again.
More practice: Radioactive decay and half-life Β· Lesson: Hospital Radiation Officer
