What is the defining characteristic of alpha decay?

The defining characteristic of alpha decay is the emission of a helium nucleus, which consists of two protons and two neutrons, commonly referred to as an alpha particle. This process occurs when an unstable nucleus releases energy by expelling this helium nucleus in order to reach a more stable state.

In alpha decay, the loss of the helium nucleus results in a reduction of the original nucleus's atomic number by two and its mass number by four, transforming it into a different element. This process is a key mechanism through which heavy elements can stabilize themselves by shedding excess mass and energy.

Additionally, although other types of decay and particle emissions exist, the distinct formation of a helium nucleus is specifically what characterizes alpha decay, differentiating it from processes involving neutrons or electrons. Options involving the emission of neutrons or the loss of an electron pertain to different decay mechanisms, such as beta decay or neutron emission, and do not accurately describe alpha decay.