Are nitrogen, arsenic, and tantalum radioactive?
The answer is yes and no. Let's see why. When you want to know about elements, you go look in the periodic table. But what you see listed in the periodic table of the elements is only part of the picture. For every element listed, there are different "flavors" called isotopes. All of the elements have at least one isotope that is radioactive. So, we can say that there is such a thing as radioactive nitrogen, arsenic and tantalum. Some elements have isotopes that are all radioactive. So if you had asked "is thorium radioactive" the answer would be yes, because ALL the isotopes of thorium are radioactive.
Another part of the picture is this. Let's say an element has 4 radioactive isotopes and one stable (non-radioactive) one. It is often the case that the radioactive isotopes of that element are very rare and the stable isotope is very common. So, just because there is such a thing as radioactive nitrogen, it does not mean you will be able to find much of it around. If we wanted to have a lot of radioactive nitrogen, we'd have to make it ourselves, because there is so little of it around. Part of the reason for this is that the "half-life" is very short. For example, there are about 7 known radioactive isotopes of nitrogen and two stable ones. The longest half life of the radioactive ones is less than ten minutes. So if you have some of this stuff, it decays away right before your eyes and becomes non-radioactive pretty quickly. So the only way to keep it around is to be making it all the time. There are certain processes in nature that produce many of these radioactive isotopes. For instance, some radioactive nitrogen is produced in the atmosphere when high energy cosmic radiation from the sun hits the earth. Other radioactive isotopes are only available by artificial production. We can get these from nuclear reactors and high energy particle accelerators like Jefferson Lab.
So, the short answer is: Most of the nitrogen, arsenic and tantalum in the world is not radioactive. But, if you wanted some that was, it can be produced.
Keith Welch, Radialogical Controls Group (Other answers by Keith Welch)