# What particle is needed to complete this nuclear reaction? #"_86^222Rn##->##"_84^218 Po + underline# ?

##### 1 Answer

An alpha particle.

#### Explanation:

The thing to remember about nuclear equations is that **mass** and **charge** must always be **conserved**.

In other words, in any nuclear equation

theoverall mass numberremains unchangedtheoverall atomic numberremains unchanged

Your unbalanced nuclear equation looks like this

#""_ (color(white)(1)color(blue)(86))^color(darkgreen)(222)"Rn" -> ""_ (color(white)(1)color(blue)(84))^color(darkgreen)(218)"Po" + ""_ (color(blue)(Z))^color(darkgreen)(A)"?"#

The goal here is to find the **atomic number**, **mass number**,

Since the overall mass number must be conserved, you can say that

#color(darkgreen)(222) = color(darkgreen)(218) + color(darkgreen)(A) -># conservation of mass

This will get you

#color(darkgreen)(A) = 222 - 218 = 4#

The overall atomic number is also conserved, so you can say that

#color(blue)(86) = color(blue)(84) + color(blue)(Z)#

This will get you

#color(blue)(Z) = 86 - 84 = 2#

The unknown particle has a mass number equal to **alpha particle**.

In essence, an alpha particle is simply the nucleus of a *helium-4 atom*, i.e. it contains **protons** and **neutrons**.

You can now complete the nuclear equation that describes the **alpha decay** of radon-222 to polonium-218

#""_ (color(white)(1)86)^222"Rn" -> ""_ (color(white)(1)84)^218"Po" + ""_ 2^4alpha#