I think I understand how radioactivity works. Large unstable atoms break apart and the pieces flying off are the radioactivity. This results in the atom becoming a different, smaller element. This new element may still be too big and continue to decay, so it is radioactive too. Eventually, the atom shoots off enough pieces that it is stable and therefore no longer radioactive.
So how then do stable materials become radioactive? Exposure to radioactivity seems to do it, but how? It seems like a block of pure iron atoms would still be composed of iron atoms even if it was exposed to long term radioactivity. What changes to make the iron radioactive? Is it no longer iron? And if it is now radioactive, is it breaking down into a new, lower element?
One of the particles that is emitted from an unstable atom is a neutron. If neutrons are captured by a previously stable isotope, it can be converted into a radioactive element. BTW, this is how Plutonium is created - U238 + a neutron -> U239 which decays into Neptunium-239 which then decays into Plutonium-239
Generally, if something is radioactive after being exposed to radioactive materials, it’s because it’s covered in uranium dust or whatever. Give your piece of iron a good washing, and it won’t be much more radioactive than it ever was.
This is why in the nuclear power field, they distinguish between “radioactive contamination” (which can cleaned off and is what you are describing), and exposure to “radioactivity,” which I described above.
Most of the naturally occurring long-lived radioactive isotopes were formed (and strewn about) during supernovas. Several of your garden variety short-loved radioactive isotopes are formed from cosmic ray bombardment. E.g.,Carbon-14 is formed in the upper atmosphere from Nitrogen.
Most of the OP’s questions have already been answered, but just to finish them up:
The nucleus is still iron until it decays; however, it can actually decay to higher elements as well as lower through beta decay. In more detail:
As has been said above, when you bombard a stable element with neutrons, it can become heavier than it “wants to be”. There are two ways that it can correct this imbalance: alpha decay, in which the nuclues “expels” an alpha particle, composed of two neutrons and two protons; and beta decay, in which the nucleus either converts a proton into a neutron or a neutron into a proton (along with a positron or an electron to conserve charge.) Thus, the number of protons can either decrease by two, decrease by one, or increase by one. In fact, if you took natural iron and bombarded it with neutrons, you’d end up with a fair amount of the isotope Fe[sup]59[/sup], which decays by converting a neutron to a proton via beta decay; the new nucleus is then Co[sup]59[/sup], which is one element higher on the periodic table.
So the “overstuffed” cobalt-60 decays in the next heavier element, nickel? But just now, I saw that nickel has a lower atomic mass than cobalt, 58.693 vs 58.933. I always thought that atomic mass increased along with the atomic number. Learned something new on top of my original question.
You’re looking at a periodic table. The atomic masses listed in periodic tables, for all of the naturally-occurring elements, are averages over the isotopes seen in nature. For example, the atomic mass listed for carbon is 12.011, even though [sup]12[/sup]C is defined as having an atomic mass of exactly 12. This is because of the presence of small quantities of [sup]13[/sup]C and [sup]14[/sup]C.
For questions about individual isotopes, what you need is a table of the nuclides, like this one. On this chart, atomic number Z grows upward and neutron number N grows rightward. (So neutron capture moves you one step rightward in the chart.) If you examine the individual nuclei in this chart, you find that their masses do grow by about one unit each time you step upward or rightward. The slight discrepancies are because some nuclei are more stable than others. For example, moving northwest through the chart moves you along a line of constant N+Z; all of these nuclides ought to have very similar masses, since the proton and neutron masses are nearly the same. What you will find is a low-mass valley along the blue region of stable elements, with the mass increasing above and below this region. You can think of this extra mass as potential energy; high-mass nuclei would like to decay into lower-energy states if they can.
Alpha decay moves you southwest two squares (down two and left two); and beta decay moves you up or down one. Looking at this chart, you see that [sup]60[/sup]Co, with a mass of 59.9338222, beta-decays to [sup]60[/sup]Ni, with a slightly lower mass of 59.9307906. (The beta particle, an electron or positron, has a very small mass, so beta decay usually occurs between nuclei which are very close in mass.)
Those atomic mass numbers you are using are incorrect when you apply them to isotopes. The numbers you are using are actually weighted averages of the different naturally occurring isotopes for each element.
You have to use the actual atomic mass for each isotope to properly compare cobalt-60 and nickel-60. (FWIW, I could instantly tell you were using the wrong numbers, because atomic masses of isotopes are going to very close to the mass number of the isotope. In other words, both cobalt-60 and nickel-60 are going to have atomic masses that are very close to 60 amu.)
In any event, here are the exact figures:
Cobalt-60 has an atomic mass of 59.933817 amu.
Nickel-60 has an atomic mass of 59.930786 amu.
You didn’t ask, but the mass of an electron is 0.000549 amu. In the process of beta decay, an electron is emitted. This leaves a mass difference of 0.002482 amu that is unaccounted for. That mass is converted to energy during the decay process, which is reflected in the emission of gammas. The emission of the electron and gammas is what makes cobalt-60 “radioactive” as it decays.
The definition of “cobalt” is the element with 27 protons in it. Cobalt-60 has 27 protons in it. It is “strictly” cobalt.
The definition of “nickel” is the element with 28 protons in it. Nickel-60 has 28 protons in it. It is “strictly” nickel.
Cobalt-60 (27 protons, 33 neutrons) is not a stable isotope of the element cobalt. The stable element is cobalt-59 (27 protons, 32 neutrons).
Nickel-60 happens to be one of several stable isotopes for nickel. Natural nickel is composed of a mixture of nickel-58 (68.1% naturally occurring by mass), nickel-60 (26.2%), nickel-61 (1.1%), nickel-62 (3.6%), and nickel-64 (0.9%). If you do a weighted average using the isotopic masses for nickel, and the relative percentages for which they occur, you get the weighted average for nickel shown on a periodic table.