Could someone clarify and define these for me? Critical mass is when you have enough of a substance to start a chain reaction right? How does an element become radioactive? Can every element become radioactive? Is uranium-238 radioactive? How much uranium is used in chemotherapy? If plutonium was found in the ground, even a minute amount, would you receive radiation poisoning?

Thanks.

Not every element can be radioactive. See, radioactive ones are so large that the bonds holding them together are weak, and so they throw off a particle every once and a while. U-238 is radioactive.

If there was ENOUGH plutonium, yes, you would get sick.

Some elements that are not noramlly considered highly radioactive can become so through induced radiation were they are made to become unstable eg fireing a nuetron into a nucleus

 

Hmm well most of the bigger atoms such as thorium,plutonium and uranium are radioactive, this is due the the nuclear binding energies being to weak to hold the larger nucleuses together,

 

Critical mass is when there is a large enough amount of a subsatnce to self sustain fission eg enough neutrons flying around to continue the fission process

 

as to how they got to be radioactive well the awnsers are numerous, they can be the products of a radioactive decay themselves or they were fromed like they are

Cap'n Refsmmat said in post # :

Not every element can be radioactive. See, radioactive ones are so large that the bonds holding them together are weak, and so they throw off a particle every once and a while.

When they throw off particles until their mass is half of the original, it's called a half-life? Will they keep on throwing off a particle? Or will they stop at a certain stage?

They will stop, eventually. They will run out of particles to throw out.

ores rich in Pu and U underground can indeed make you quite ill with prolonged exposure, Radon gas seeps out of the ground at a high rate in these areas and can cause all manor of illnesses.