Which emission causes the atomic number of a nuclide to decrease by 2 and its mass to decrease by 4

Alpha

Explanation:

Alpha decay occurs when a nucleus is unstable and loses an alpha particle. An alpha particle has a mass of 4 amu and an atomic number of 2; this also means that an alpha particle is the same as Helium-4. The law of conservation of mass tells us that if an alpha particle is released, then the nuclide must lose 2 protons and 4 mass units.

For example, Uranium-238, which has 92 protons, undergoes alpha decay and becomes Thorium-234, which has 90 protons. As you can see by the mass number, the original atom's mass decreased by 4. Additionally, the atomic number decreased by 2, meaning 2 protons were lost to alpha decay.

water is used to store them because it would keep the radiation from getting out in the open and causing harm to the workers