alpha particle – positively charged particle consisting of two protons and two neutrons

To remember the meaning of alpha particle, use the following mnemonic:

The alpha of the group of particles (alpha particle) charged at its rival with two peas and two newts (two protons and two neutrons).

Alpha particles are emitted from the nucleus of some radioactive atoms during radioactive decay. It consists of two protons and two neutrons, giving it a positive charge of +2 and a relatively large mass compared to other types of radiation. Alpha particles are the least penetrating form of radiation and can be stopped just by a few centimetres of air or a sheet of paper, making them relatively harmless outside the body, but dangerous if inhaled or ingested. 

When an atom emits an alpha particle, its atomic number decreases by 2 and its mass number decreases by 4, resulting in the formation of a different element. For example, when uranium-238 undergoes alpha decay, it becomes thorium-234.