Radioactivity is the spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation.

#### Background information:

• Radioactivity was discovered by Henri Becquerel with photographic paper.
• Pierre and Marie Curie discovered polonium and radium.

alpha $( \alpha )$, beta $( \beta )$, gamma $( \gamma )$.

## Alpha particles

• Groups of two protons and neutrons struck together $( _{2}^{4}\textrm{He} )$.
• They have a positive charge (+).
• Have low penetrating power and are stopped by a sheet of paper.
• General equation:$_{A}^{Z}\textrm{x} \to _{Z-2}^{A-4}\textrm{(x-2)} + _{2}^{4}\textrm{He}$-2 from atomic number, -4 from mass number, move two places back in the periodic table and add the alpha particle $(_{2}^{4}\textrm{He})$.

## Beta particles

• Formed when neutron in unstable nucleus is changed into proton.
• They have a negative charge (-).
• More penetrative and are stopped by 5mm of aluminium.
• General equation:$_{A}^{Z}\textrm{x} \to _{Z+1}^{A}\textrm{(x+1)} + _{-1}^{0}\textrm{e}$

## Gamma rays

• Has no mass or charge.
• Only energy is emitted in waves of energy.
• It has high penetrating power.
• Can be stopped by thick lead.

### Distinction between nuclear and chemical reactions

#### Nuclear Reactions

• New element may be formed.
• Change occur in nucleus.
• No chemical bonds are broken or formed.

#### Chemical Reactions

• No new element formed.
• No change in nucleus.
• Chemical bond-breaking and forming occurs.

The half-life of an element is the time taken for half of the nuclei in any given sample to decay.