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What happens when an Alpha Particle is Emitted from a Nucleus?

After a radioactive nucleus has emitted an alpha particle, the
mass number goes down by 4 and the atomic number goes down by 2.
This change in the atomic number produces a different element.

What is a Nuclear Equation?

A nuclear equation shows the change in the mass number
and the atomic number during a radioactive decay.

The nuclear equation is balanced because the mass number
on the left of the arrow is equal to the sum of the mass numbers
on the right of the arrow,  224 = 220 + 4.
Similarly for the atomic numbers88 = 86 + 2.

How to find the New Element in a Nuclear Equation.

You can work out what the new element is by balancing the equation.
220 = 4 + 216,   86 = 2 + 84.
The element with atomic number 84 is polonium (Po)
(see the periodic table on the GCSE Chemistry site).

The balanced nuclear equation is

If you are given the two elements, you can work out
which particle is emitted by balancing the equation.

What is a Decay Series?

Polonium (see the 2 equations above) is also radioactive
and decays to form other radioactive nuclei.
One element turning into another which turns into another
which turns into another etc. is called a decay series.
Eventually a stable nucleus is formed and the decay series stops.
The stable nucleus is often often lead
(atomic number 82, mass number 206, 207 or 208).

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