Question #ff8f4

1 Answer
Dec 18, 2017

Here's my take on this.

Explanation:

This is a bit difficult to decipher without more information, but my guess would be that you're looking at the alpha decay of plutonium-244, 244Pu.

For starters, it;'s worth mentioning that the PeriodicTable tells you the average atomic mass of plutonium, i.e. 244.064 u, not the mass number of plutonium-244 per se.

If you round the atomic mass of plutonium to the nearest whole number, you will get the mass number of the most abundant isotope of plutonium, which in this case, is plutonium-244.

244.064244

Since plutonium has an atomic number equal to 94, you can say that plutonium-244 contains

24494=150

neutrons inside its nucleus. This is the case because the mass number of a nuclide is calculating by adding the number of protons, which is given by the atomic number, and the number of neutrons.

Now, when plutonium-244 undergoes alpha decay, it emits an alpha particle, which is essentially the nucleus of a helium-4 isotope.

http://www.ck12.org/book/CK-12-Physical-Science-Concepts-For-Middle-School/section/3.60/http://www.ck12.org/book/CK-12-Physical-Science-Concepts-For-Middle-School/section/3.60/

This means that the nuclear equation that describes the alpha decay of plutonium-244 looks like this

244194Pu240192U+42α

This is the case because an alpha particle contains 2 protons and 2 neutrons, so its mass number is equal to 4.

In order for charge and mass to be conserved, you need to have

  • 244=240+4 conservation of mass
  • 94=92+2 conservation of charge

This is why the resulting nuclide, uranium-240, has a mass number equal to 240.

a

ALTERNATIVE INTERPRETATION

Now that I think about it, it's possible that you're referring to plutonium-240, an isotope of plutonium.

This isotope has a mass number equal to 240 because it contains

24094=146

neutrons inside the nucleus, as opposed to 150 neutrons located inside the nucleus of plutonium-244, the most abundant isotope of plutonium.

As you know, isotopes are atoms that have the same number of protons, i.e. the same atomic number, but different numbers of neutrons, i.e. different mass numbers.

In this case, both plutonium-240 and plutonium-244 have 94 protons, which is why they're isotopes of plutonium, but they differ in the number of neutrons, 146 for the former and 150 for the latter.