Why atoms are radioactive

All atoms are not radioactive. It is also good for living beings because the radiation emitted by radioactive substance is very harmful to living beings. We will discuss the hazards of this in our later posts.
Those atoms which have unstable nucleus are radioactive. The stability of nucleus depends upon two factors. First is size of the nucleus and second is the neutron to proton ratio simply known as n/p ratio. All nucleus containing more than 209 proton in it are generally radioactive. The largest stable nucleus is bismuth-209. Nucleus containing less than 209 proton have at least one stable isotopes. Note technetium and promethium are exceptions.
I know after reading this you have come across a question in your mind that why nucleus become unstable when the number of proton increases. Keep reading for answer to your quires.
Due to large number of proton in a nucleus the size of nucleus grew up. As you know the force that binds proton together in a small area of nucleus (Nuclear force) acts only over a small area of 10^-15 meter and due to large size of nucleus this force is unable to hold the proton at the extreme to break away from the repulsion of other protons in a nucleus so the nucleus become unstable. Do you now got why the nucleus get unstable when number of proton increases.
Now to give more stronger nuclear force for binding large number of proton it is necessary to get some more neutron in the nucleus. So as the number of proton goes increases in the nucleus to keep nucleus stable their goes more neutron than proton in the nucleus. You can see that stable atoms of atomic number from 1 to 20 has equal number of proton and neutron. but as the atomic number goes increasing this is not the case. See the graph below.



The red line shows where the number of protons is equal to the number of neutrons. For small atoms, this is the most stable ratio (1:1). For example, oxygen-16, magnesium-24, silicon-28, and calcium-40 are all stable atoms. As the atoms get larger, they diverge from this line, and are stable with more neutrons than protons (the maximum ratio is 1.3:1). Larger stable atoms include gold-197, tungsten-184, and indium-115, all of which have more neutrons than protons. The most logical explanation for this is that since neutrons are neutral, they don't repel each other, but are attracted by the strong force, so they held offset the repulsion of the protons.