Section 1 of 7Unstable nuclei and what they emit
Why nuclei decay
- Why nuclei decay Some nuclei have an unstable combination of protons and neutrons, so they release energy and particles to become more stable.
Types of emission
- Types of emission An unstable nucleus may emit an alpha particle, a beta particle, a neutron, or a gamma ray.
Alpha emitters
- Alpha emitters Alpha decay mainly happens in large nuclei, which have many protons and neutrons.
Section 2 of 7The alpha particle
Alpha particle
Structure
- Structure It is the same as a helium nucleus: 2 protons and 2 neutrons.
Symbol
- Symbol Written as 42He or as the Greek letter alpha with mass number 4 and atomic number 2.
Charge and mass
- Charge and mass It has a relative charge of +2 and a relative mass of 4.
Section 3 of 7What alpha decay does to the nucleus
| Property of the nucleus | Change | Reason |
|---|---|---|
| Mass number (A) | Decreases by 4 | 2 protons and 2 neutrons leave |
| Atomic number (Z) | Decreases by 2 | 2 protons leave |
| Nuclear charge | Decreases by 2 | Each proton carries +1 |
| Element | A new element forms | Atomic number defines the element |
Section 4 of 7Writing a decay equation
1
Write the parent nucleus on the left of the arrow, with its mass number on top and atomic number below.
2
Write the alpha particle (42He) on the right of the arrow, together with the daughter nucleus.
3
Subtract 4 from the parent mass number to get the daughter mass number.
4
Subtract 2 from the parent atomic number to get the daughter atomic number, then use that number to identify the element.
5
Check the equation balances: the mass numbers on both sides add up equally and so do the atomic numbers.
Section 5 of 7Worked example
Polonium-212
- Polonium-212 21284Po → 20882Pb + 42He
- Mass numbers
- 212 = 208 + 4.
- Atomic numbers: 84 = 82 + 2.
From protons and neutrons
- From protons and neutrons A nucleus with 84 protons and 126 neutrons has mass number 84 + 126 = 210.
- After alpha decay the atomic number is 82 and the mass number is 206, giving lead-206.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Alpha particle | A helium nucleus (2 protons, 2 neutrons) emitted from an unstable nucleus. |
| Mass number | The total number of protons and neutrons in a nucleus. |
| Atomic number | The number of protons in a nucleus; it decides which element the atom is. |
| Decay equation | A nuclear equation showing the nucleus before decay and the particles produced after it. |
| Unstable nucleus | A nucleus that can emit radiation to become more stable. |
Section 7 of 7Exam technique
1
Reducing the mass number by 2 instead of 4; the alpha particle contains four nucleons.
2
Forgetting that the atomic number falls by 2, so the element changes.
3
Treating 'alpha particle' and 'helium nucleus' as different things - they are the same and either may appear in exam questions.
4
Not checking that the totals balance on each side of the arrow; always add the top numbers and the bottom numbers separately.