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Alpha particles are made of 2 protons with 2 neutrons. This means that when a nucleus emits an alpha particle, it loses 2 protons and so its atomic number decreases by 2. Also, when a nucleus emits an alpha particle, its atomic mass decreases by 4 (that's 2 protons plus 2 neutrons) So Americium-241 ( an a-source used in smoke detectors), which has an atomic number of 95 and an atomic mass of 241 will decay to Neptunium-237 (which has an atomic number of 93 and an atomic mass of 237).

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The equation would look like this:-  

Note that an alpha particle is the same as the nucleus of a Helium atom (2 protons and 2 neutrons). Thus we can write   or   in the equation.

Alpha-decay occurs in very heavy elements, for example, Uranium and Radium.

These heavy elements have too many protons to be stable. They can become more stable by emitting an alpha particle.

Alpha particles have a large charge(+2), so they easily ionise other atoms that they pass. Ionising atoms requires energy, so alpha particles lose energy rapidly as they travel. Thus they have a range of only a few centimetres in air.

Remember, in Alpha decay:- atomic number decreases by two atomic mass decreases by four.

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Created by Andy Darvill, www.darvill.clara.net,Science teacher at Broadoak Community School, Weston-super-Mare, England