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Unlock This CourseYou have met the numbers that describe a nucleus and the radiations an unstable nucleus can give out. Now put them together. When a nucleus gives out radiation, it is called nuclear decay. The nucleus left behind can have different numbers of protons and neutrons from before. So we ask two questions each time: what happens to the atomic number (the number of protons), and what happens to the mass number (protons plus neutrons)?
The atomic number decides which element an atom is. So if the atomic number changes, the atom becomes a new element.
Key terms:
An alpha particle is 2 protons and 2 neutrons. When the nucleus throws one out, it loses 2 protons and 2 neutrons.
Uranium-238 has 92 protons. It emits an alpha particle.
Mass number: 238 − 4 = 234.
Atomic number: 92 − 2 = 90.
The new nucleus has 90 protons and mass number 234. An element with 90 protons is thorium, so uranium-238 becomes thorium-234.
A beta particle is a fast electron, but there are no electrons inside the nucleus to start with. Instead, a neutron changes into a proton and an electron. The electron is thrown out as the beta particle and the new proton stays in the nucleus.
Carbon-14 has 6 protons and 8 neutrons. It emits a beta particle.
Mass number: stays at 14.
Atomic number: 6 + 1 = 7.
The new nucleus has 7 protons and 7 neutrons. An element with 7 protons is nitrogen, so carbon-14 becomes nitrogen-14.
A gamma ray is a burst of energy, not a particle of matter. The nucleus does not lose any protons or neutrons. The mass number and the atomic number both stay the same. It is still the same element. Gamma rays are often given out after an alpha or a beta particle, when the nucleus still has spare energy to get rid of.
Some nuclei throw out a single neutron. The mass number goes down by 1 because one particle has gone. The atomic number stays the same, because the number of protons has not changed. So it is still the same element, but it is a different isotope of it.
Imagine a nucleus with 7 protons and 10 neutrons, mass number 17, that emits a neutron.
Mass number: 17 − 1 = 16.
Atomic number: stays at 7.
It now has 7 protons and 9 neutrons: the same element with one fewer neutron, so an isotope of the same element.
Learn this table. It is one of the most useful things to be able to write from memory.
| Radiation | Mass number | Atomic number | New element? |
|---|---|---|---|
| Alpha | Down by 4 | Down by 2 | Yes |
| Beta | No change | Up by 1 | Yes |
| Gamma | No change | No change | No |
| Neutron | Down by 1 | No change | No (an isotope of the same element) |
This lesson uses words only. Writing the decay as a full balanced equation comes in the next lesson, Balancing Nuclear Equations. What each radiation is made of and how far it travels was covered in Alpha, Beta and Gamma Radiation.
(a) A nucleus of radium-226 has an atomic number of 88. It emits an alpha particle. State the mass number and the atomic number of the nucleus that is left. [2 marks]
(b) A nucleus of sodium-24 has an atomic number of 11. It emits a beta particle. Describe what happens to its mass number and its atomic number. [2 marks]
(c) Explain why the emission of a gamma ray does not turn an atom into a new element. [2 marks]
(a) Mass number 222 (1) and atomic number 86 (1).
(b) The mass number stays the same, at 24 (1). The atomic number goes up by 1, to 12 (1).
(c) A gamma ray is energy only, so no protons or neutrons are lost (1). The atomic number stays the same, so it is still the same element (1).
For part (a), write the two sums down: 226 − 4 and 88 − 2. Do not do them in your head, because mixing up the 4 and the 2 is the easiest way to lose both marks.