# Question #ff8f4

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, $\text{^244"Pu}$.

For starters, it;'s worth mentioning that the PeriodicTable tells you the average atomic mass of plutonium, i.e. $\text{244.064 u}$, 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.

$244.064 \approx 244$

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

$244 - 94 = 150$

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.

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

${\text{_ (color(white)(1)94)^244"Pu" -> ""_ (color(white)(1)92)^240"U" + }}_{2}^{4} \alpha$

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

• $244 = 240 + 4 \to$ conservation of mass
• $94 = 92 + 2 \text{ } \to$ conservation of charge

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

$\textcolor{w h i t e}{a}$

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

$240 - 94 = 146$

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.