Why actually are some elements radioactive?

[Desconcertado]

Can anyone explain to me why actually are some elements radioactive? I have the explanation in my textbook but i want to know the main reason why these elements are not stable...

 

[bcrowell]

Hi, Desconcertado -- welcome to Physics Forums!

Here's an online treatment that you could try as an alternative to your text, to see if you just need to see it from a different perspective to make it "click:" http://www.lightandmatter.com/html_books/4em/ch02/ch02.html

It will be hard for anyone here to answer your question in any detail unless you tell us what it is specifically about your textbook that you don't understand, or that isn't satisfying you.

 

[Desconcertado]

My textbook tells me that, as given by the stabliity curve, certain neutron proton ratios are stable and with the increase in atomic number the neutron proton ration diverges from being the stable one.

I do not understand why certain neutron proton ratios are stable and others are not...
The hypertext that brocwell sent was nice.. but i did not get my answer exactly.. It would be great if somebody could explain or give the site where it is explained!

 

[hamster143]

Every nucleus has a certain value of binding energy. For example, the mass of a helium-4 nucleus is smaller than the mass of 2 free protons and 2 free neutrons. The difference is called binding energy.

The exact value of binding energy can be measured experimentally, by weighing the nucleus. The theory behind this is quite complicated and the truth is that we can't compute the binding energy exactly. There are, however, approximation formulas that work quite well.

Any nucleus is radioactive if and only if it has something to decay into. Helium-4 is stable, because it has a high value of binding energy and all its possible decays are prohibited by the law of conservation of energy. Helium-6 is unstable, because it's energetically allowed for it to decay into Lithium-6 and an electron.

 

[bcrowell]

My textbook tells me that, as given by the stabliity curve, certain neutron proton ratios are stable and with the increase in atomic number the neutron proton ration diverges from being the stable one.

It's a trade-off-between two effects. Because of the Pauli exclusion principle, you get a lower energy by putting in equal numbers of neutrons and protons to fill the available energy levels. However, for heavy nuclei the Coulomb repulsion of the protons makes it less favorable to have as many protons. This is expressed mathematically by two of the terms in the liquid-drop formula for the energy. If you minimize with respect to N/Z, you get the average shape of the line of stability (but not the small wiggles that are caused by quantum-mechanical shell effects).