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ATOMIC STRUCTURE · CHAPTER 5 OF 12
How does decay change a nucleus?
An alpha particle leaves. A neutron becomes a proton. A nucleus releases energy. Each change leaves a different pattern in the numbers. Can you work out what remains?
By the end, you’ll be able to…
- describe changes in proton and neutron numbers during alpha, beta minus, beta plus, gamma and neutron emission
- predict whether the element changes
- balance nuclear equations using mass number and charge
- distinguish nuclear decay from ionisation and electron-shell changes.
Keep track of two numbers
The mass number, A, counts protons and neutrons. The atomic number, Z, counts protons. The proton number identifies the element.
After a nuclear change, ask: how many nucleons remain, and how many of those are protons? Neutron number is A − Z.
Read carbon-14
Click each tile to remind yourself what it tells you.
14
6
C
Alpha decay: four nucleons leave together
An alpha particle contains two protons and two neutrons. When a nucleus emits one, its mass number decreases by 4 and its proton number decreases by 2. Its neutron number also decreases by 2.
What is left after an alpha particle leaves?
Start with polonium-210: A = 210 and Z = 84. Predict the two new numbers, then reveal the change.
Two protons leave, so the remaining nucleus belongs to a different element: lead.
* The large pink disc represents the whole nucleus; the numbers give its contents. It is not one proton. The moving alpha particle is shown enlarged. Not to scale.
Beta minus: a neutron changes into a proton
In beta-minus decay, a neutron changes into a proton and a high-speed electron is emitted. The new proton remains in the nucleus.
The mass number stays the same because one nucleon has changed type. The proton number increases by 1 and the neutron number decreases by 1.
Fourteen nucleons. A different element.
The dashed ring marks the neutron that will change. Predict the new proton and neutron counts, then show the change.
One neutron becomes a proton. Carbon becomes nitrogen. The emitted electron is created in the nuclear change; it does not come from an electron shell.
* All 14 nucleons are shown. Electron size is enlarged. A beta-minus decay also emits an antineutrino, omitted from this GCSE equation and diagram because its mass-number and charge entries are both zero.
Beta plus: a proton changes into a neutron
In beta-plus decay, a proton changes into a neutron and a positron is emitted. A positron has the same mass as an electron but a relative charge of +1.
The mass number stays the same. The proton number decreases by 1 and the neutron number increases by 1.
Can the proton count decrease without a nucleon leaving?
Watch the ringed proton become a neutron as a positron leaves. Compare this with beta-minus decay.
A nucleon changes type, so the total stays at 11. The positron carries positive charge away; the remaining nucleus is boron.
* Pink + within the nucleus means a proton; the separate small pink + particle is a positron, as labelled. A neutrino is also emitted, omitted from this GCSE equation and diagram.
Gamma: energy leaves, nucleon numbers stay
After another decay, a nucleus may be left in an excited state. It can rearrange into a lower-energy state and emit a gamma photon.
No proton or neutron is lost or changed into the other. Both A and Z stay the same, so the element and isotope stay the same.
What changes if no nucleons leave?
Reveal gamma emission. Check whether the proton and neutron symbols change.
The nucleus loses energy. Gamma emission can follow alpha or beta decay, but it does not cause an extra change in A or Z.
* The nucleus is schematic, not a named isotope. The dashed pink ring marks an excited state, not extra positive charge or a physical shell. The wave symbol represents electromagnetic radiation.
Neutron emission: one uncharged nucleon leaves
When a nucleus emits one neutron, A decreases by 1 while Z stays the same. The element stays the same, but the isotope changes because there is one fewer neutron.
Does every nuclear emission change the element?
Reveal one neutron leaving. Compare what happens to A and Z.
Only the neutron count changes. The proton number still identifies the same element.
* Schematic particles, not a particular isotope. The neutron is represented in equations by .
The pattern at a glance
| Emission | Change in A | Change in Z | Change in neutrons |
|---|---|---|---|
| Alpha | −4 | −2 | −2 |
| Beta minus | 0 | +1 | −1 |
| Beta plus | 0 | −1 | +1 |
| Gamma | 0 | 0 | 0 |
| One neutron | −1 | 0 | −1 |
A change in Z means a change of element. An unchanged Z means the same element, even if A changes.
Balance a nuclear equation
A nuclear equation shows the original nucleus on the left, then the remaining nucleus and emitted radiation on the right.
Balance the upper and lower numbers separately:
Total lower numbers on the left = total lower numbers on the right
The lower number counts protons for a nucleus. For an emitted particle, it represents charge in units of the proton charge. That is why a beta-minus electron has −1 below it, rather than a negative number of protons.
Can both rows balance?
The daughter nucleus is lead, Pb. Work out its missing upper and lower numbers before revealing the checks.
Step 1 — Balance the mass numbers (top row)
210 = ? + 4
? = 210 − 4
Mass number = 206
Step 2 — Balance the proton numbers (bottom row)
84 = ? + 2
? = 84 − 2
Atomic number = 82
The completed nuclear equation
Three worked examples
Calculate daughter numbers:
Radium-226 emits an alpha particle. The daughter is radon, Rn.
Adaughter = Aparent − 4; Zdaughter = Zparent − 2
A = 226 − 4 = 222; Z = 88 − 2 = 86
Calculate daughter numbers:
Carbon-14 emits a beta-minus particle. The daughter is nitrogen, N.
Adaughter = Aparent; Zdaughter = Zparent + 1
A = 14; Z = 6 + 1 = 7
Calculate daughter numbers:
Carbon-11 emits a positron. The daughter is boron, B.
Adaughter = Aparent; Zdaughter = Zparent − 1
A = 11; Z = 6 − 1 = 5
* Daughter element symbols are provided so you can focus on balancing. Neutrinos are omitted from the beta equations.
A beta-minus equation that looks nearly right
Writing gamma emission
An asterisk can mark an excited nucleus. Here X is a general element symbol; the numbers A and Z do not change.
Gamma emission removes energy from the nucleus. It does not turn one element into another and is different from an electron moving between shells.
Check your understanding
Choose one answer per question. Feedback appears underneath.
A nucleus emits an alpha particle, then a beta-minus particle, then a gamma ray. It started with A = 238 and Z = 92. What are its final numbers?
Track each change separately. Explain whether it is still the same element. You do not need to name the daughter.
Reveal the worked answer
After alpha emission: A = 238 − 4 = 234 and Z = 92 − 2 = 90.
After beta-minus emission: A stays at 234 and Z increases to 91.
Gamma emission removes energy without changing either number.
Final result: A = 234, Z = 91. The neutron count is 234 − 91 = 143. It is a different element from the original because its proton number changed.
CHAPTER 5
Chapter summary
- Alpha emission: A decreases by 4; Z decreases by 2.
- Beta minus: a neutron becomes a proton; A stays the same and Z increases by 1.
- Beta plus: a proton becomes a neutron; A stays the same and Z decreases by 1.
- Gamma emission removes nuclear energy without changing A or Z.
- One-neutron emission decreases A by 1 but leaves Z unchanged.
- A change in proton number means a different element.
- Balance the upper numbers and lower numbers separately in nuclear equations.