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The nuclei of unstable isotopes emit radiation. In the case of α- and β-radiation, it means the nucleus has changed, the number of protons and neutrons is no longer the same. To analyse this, we need to write a nuclear equation, just like a chemical equation.
Remember, the nucleus of an atom can be represented as:
Where:
A = mass number (number of protons + neutrons)
Z = atomic number (number of protons)
X = chemical symbol (as shown on the Periodic Table)
Alpha decay (α)
When an alpha particle is emitted from a nucleus, the nucleus loses two protons and two neutrons. This means the mass number decreases by 4 and the atomic number decreases by 2. A new element is formed that is two places lower on the Periodic Table than the original element.
Example:
Radon decays into polonium when it emits an alpha particle. Here is the equation for that radioactive decay:
Beta decay (β)
In Beta decay, a neutron changes into a proton and an electron. The proton stays in the nucleus and the electron leaves the atom with high energy. This electron is what we call a beta particle.
This means that after the decay the nucleus has one more proton and one less neutron. This means the mass number remains unchanged but the atomic number increases by 1.
Example:
Carbon-14 is a radioactive isotope of carbon. (It's a carbon atom with 8 neutrons instead of the usual 6.) Here is the equation for the beta decay of carbon-14 into nitrogen-14
Why does the β-particle have an atomic number of -1? The atomic number is also the charge of the nucleus. A β-particle is of course an electron but in this notation, it is the nucleus of its own “ion” giving it a negative charge in its nucleus. Also, it balances out the equation nicely.
Gamma decay (γ)
Gamma-rays are photons, just like visible light but with a shorter wavelength. This means gamma radiation is just energy. No particles are changed or moved. Because the contents of the nucleus don’t change, gamma decay is not a nuclear reaction. What does happen; After alpha or beta decay, nuclei often rearrange themselves. This process causes a surplus of energy which is emitted in the form of gamma rays. There is no effect on either the mass number (A) or atomic number (Z) by emitting this energy.
Summary
When writing nuclear equations, remember the following:
The rules:
· The sum of all mass numbers before and after must remain the same.
· The sum of all atomic numbers before and after must remain the same.
Charge
The sum of all charges before and after must remain the same too.
The next decay starts with an atom (no charge) and the beta particle has a charge of -1 (since it is an electron)
This equation shows that the beryllium must have a charge of +1 since the total charge after the decay must be zero again. Because we’re dealing with small numbers, it’s easy to see what happens.
Lithium starts with 3 electrons in orbit.
In the nucleus, a neutron changes in a proton and electron.
The electron shoots out and become the beta particle.
The electrons in orbit remain where they are.
The amount of protons in the nucleus is now 4 and the amount of electrons in orbit is 3, this gives the ion a total charge of +1.
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