A large atom decays and emits a particle. After the reaction is complete, the atom’s mass has changed substantially. What kind of particle has most likely been emitted?
alpha particle
beta particle
positron
photon

alpha particle

Explanation:

Alpha decay occurs when a nucleus is unstable because it has too many protons. The nucleus emits an alpha particle and energy. An alpha particle consists of two protons and two neutrons, which is actually a helium nucleus. Losing the protons and neutrons makes the nucleus more stable.

Hence, alpha emission means that the nucleus looses four mass units. Loosing four mass units leads to a substantial change in the mass of the nucleus. Hence the daughter nucleus is four mass units less than than the parent.

thats how its supposed to be