# When Plutonium-239 undergoes alpha decay, what does it become?

##### 1 Answer

Aug 21, 2016

#### Answer:

It becomes **uranium-235**.

#### Explanation:

An alpha particle is a helium-4 nucleus, it contains gwo protons and two neutrons. So, alpha decay, giving off an alpha particle, does the following:

1)reduces the atomic number (number of just protons) by two. Plutonium has atomic number of 94, take two protons away and you are left with 92 which corresponds to **uranium**.

2) reduces the mass number (protons plus neutrons) by 4. If you start with 239 and subtract two protons and two neutrons, you are left with **235**.