An alpha particle ($\alpha$) is equivalent to a Helium nucleus, consisting of 2 protons and 2 neutrons (${}_2^4\text{He}$). When a nucleus undergoes alpha decay, it ejects this particle, thereby reducing its atomic number ($Z$) by 2 and its mass number ($A$) by 4. The general reaction is: ${}_Z^A\text{X}{\to }_{Z-2}^{A-4}\text{Y}{+}_2^4\text{He}$.