Alpha Decay

Alpha ($\alpha$) decay is the spontaneous emission of an alpha particle from an unstable nucleus. An alpha particle carries a charge of $+2$ and a mass of $4\ u$ -- it is, in fact, identical to a bare helium nucleus, which is why it is also written as 4-2-He.

In general, when a parent nucleus $^{A}_{Z}X$ undergoes alpha decay, it emits a 4-2-He particle and becomes a daughter nucleus $Y$ with 2 fewer protons and 2 fewer neutrons than the parent, i.e. mass number 4 less and atomic number 2 less: $^{A}_{Z}X \rightarrow\ ^{A-4}_{Z-2}Y + \,^{4}_{2}He$. Two concrete examples from the source: radium-226 decays to radon-222, $226$-$88$-$Ra \rightarrow 222$-$86$-$Rn + 4$-$2$-$He$; and uranium-238 decays to thorium-234, $238$-$92$-$U \rightarrow 234$-$90$-$Th + 4$-$2$-$He$. …