Alpha particles are made of 2 protons and 2 neutrons.
This means that they have a charge of +2, and a mass of 4
(the mass is measured in "atomic mass units", where each proton & neutron=1)
We can write them as [pic], or, because they're the same as a helium nucleus, [pic].
Alpha particles are relatively slow and heavy.
Americium is a metal. It’s radioactive and gives off alpha radiation so we call it an alpha source.
Alpha radiation gets stopped by a few centimetres of air or a thin sheet of paper. You may think that this means alpha radiation is quite weak but in fact the opposite is the case. Alpha radiation is like a big heavy ball rolled across a lawn. It doesn't go very far because it loses a lot of energy flattening out the bumps in the ground. In other words the heavy ball interacts strongly with the ground.
This is what alpha radiation does to air. Each alpha particle loses its energy by ripping the air atoms to pieces as it flies past. Eventually it loses all its energy and just stops harmlessly.
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.
Alpha particles are deflected slightly in a magnetic field.
One use of americium-241 is in smoke detectors. The alpha particles tear up the neutral air molecules.