National 5ChemistrySQARevision NotesChemistry In SocietyNuclear ChemistryNuclear Equations

Nuclear Equations (SQA National 5 Chemistry): Revision Note

Exam code: X813 75

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Balancing nuclear equations

  • Nuclear equations use nuclide notation to show the changes in an atomic nucleus during radioactive decay

Particle

Composition

Nuclide Notation

alpha (α)

2 protons, 2 neutrons

He presubscript 2 presuperscript 4

beta (β)

electron from nucleus

straight e presubscript negative 1 end presubscript presuperscript 0

proton

proton

straight p presubscript 1 presuperscript 1

neutron

neutron

straight n presubscript 0 presuperscript 1

The rules of balancing nuclear equations

  • A nuclear equation is balanced when the totals on both sides of the arrow are equal

  • There are two rules:

    1. Conservation of mass number:

      • The sum of the mass numbers (top numbers) on the left side must equal the sum of the mass numbers on the right side

    2. Conservation of atomic number:

      • The sum of the atomic numbers (bottom numbers) on the left side must equal the sum of the atomic numbers on the right side

Alpha (α) decay nuclear equations

  • During alpha decay, an alpha particle is emitted from the unstable nucleus

Diagram of nuclear decay showing a parent nucleus emitting an alpha particle, resulting in a daughter nucleus. Neutrons are blue, protons are red.
In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2.

  • An alpha particle has a mass number of 4

    • This means that the mass number of the decaying nucleus decreases by 4

  • An alpha particle has an atomic number of 2

    • This means that the atomic number of the decaying nucleus decreases by 2

    • Since the atomic number decreases by 2, a new element is formed

  • For example, the nuclear equation for polonium-212 undergoing alpha decay to form lead-208 is:

Po presubscript 84 presuperscript 212 → Pb presubscript 82 presuperscript 208 + He presubscript 2 presuperscript 4

Worked Example

Radium-226 (Ra presubscript 88 presuperscript 226) undergoes alpha decay. Write the balanced nuclear equation for this process.

[1]

Answer:

  1. Write the starting radioisotope on the left and the emitted alpha particle on the right

Ra presubscript 88 presuperscript 226 → ? +He presubscript 2 presuperscript 4

  1. Balance the mass numbers (top):

    • Left-hand side = 226

    • So, the right-hand side must equal 226

    • Mass number of unknown = 226 - 4 = 222

  2. Balance the atomic numbers (bottom):

    • Left-hand side = 88

    • So, the right-hand side must equal 88

    • Atomic number of unknown = 88 - 2 = 86

  3. Identify the new element:

    • Use the Data Booklet to find the element with atomic number 86

    • This is radon (Rn)

  4. Write the final balanced equation:

Ra presubscript 88 presuperscript 226 → Rn presubscript 86 presuperscript 222 +He presubscript 2 presuperscript 4 [1 mark]

Beta (β) decay nuclear equations

  • During beta decay, a neutron changes into a proton and an electron inside the nucleus

    • The electron is emitted from the nucleus

    • The proton remains in the nucleus

Diagram of beta decay showing a neutron in a nucleus transforming into a proton with an electron ejected.
In beta decay, a neutron becomes a proton and an electron is emitted. The mass number is unchanged, and the atomic number increases by 1.

  • A beta particle has a mass number of 0

    • This means that the mass number of the decaying nucleus stays the same

  • A beta particle has an atomic number of -1

    • This means that the atomic number of the decaying nucleus increases by 1

    • Since the atomic number increases by 1, a new element is formed

  • For example, the nuclear equation for carbon-14 undergoing beta decay is:

straight C presubscript 6 presuperscript 14 → straight N presubscript 7 presuperscript 14 + straight e presubscript negative 1 end presubscript presuperscript 0

Worked Example

Iodine-131 (straight I presubscript 53 presuperscript 131) undergoes beta decay. Write the balanced nuclear equation for this process.

[1]

Answer:

  1. Write the starting radioisotope on the left and the emitted beta particle on the right

straight I presubscript 53 presuperscript 131 → ? + straight e presubscript negative 1 end presubscript presuperscript 0

  1. Balance the mass numbers (top):

    • Left-hand side = 131

    • So, the right-hand side must equal 131

    • Mass number of unknown = 131 - 0 = 131

  2. Balance the atomic numbers (bottom):

    • Left-hand side = 53

    • So, the right-hand side must equal 53

    • Atomic number of unknown = 53 - (-1) = 54

  3. Identify the new element:

    • Use the Data Booklet to find the element with atomic number 54

    • This is xenon (Xe)

  4. Write the final balanced equation:

straight I presubscript 53 presuperscript 131 → Xe presubscript 54 presuperscript 131 + straight e presubscript negative 1 end presubscript presuperscript 0 [1 mark]

Gamma (γ) decay nuclear equations

  • During gamma decay, a gamma ray is emitted from an unstable nucleus

Illustration of gamma radiation depicted as waves emitted from a nucleus.
Gamma decay releases energy but does not change the mass number or atomic number of the nucleus.

  • The gamma ray has no mass or charge

    • This means that there is no change to the atomic number or mass number

  • For example, the nuclear equation for uranium-238 undergoing gamma decay is:

straight U presubscript 92 presuperscript 238 → straight U presubscript 92 presuperscript 238 + γ

Examiner Tips and Tricks

Questions on balancing nuclear equations in the exam will almost always focus on alpha and beta decay

This is because they are the nuclear equations where the numbers change

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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry Content Creator

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener