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How long is the life span of an atom?

Atoms are forever! But let me explain. Atoms are made of a central core containing a collection of protons and neutrons. Almost all of the mass (the proper word for "weight") of the atom is contained in the nucleus. Surrounding the nucleus is a cloud of electrons whose number equals that of the number of protons. (This is necessary to keep the electrical charge of the atom zero.) The number of protons determines the identity of the atom. So, hydrogen has 1 proton, oxygen has 8 protons, iron has 26 protons, and so on. The number of neutrons is usually fixed for a particular atom (for example, the most common form of carbon has 6 neutrons), but this does not have to be so. Sometimes, when an atom doesn't have the 'right' number of neutrons, it becomes unstable and shoots out sub-atomic particles, a common one being the electron. What is happening is that one of the neutrons in this neutron-enhanced version of the atom, is changing into a proton plus an electron (plus a zero mass particle called the anti-neutrino, but we can chat about that some other time). This electron is spat out of the nucleus. This is an example of a "radioactive" atom. Essentially they are special versions of a particular atom that are trying to change to a stable type of atom, that is, one that will NOT change with time.

Let's take carbon as an example. The most common (and stable) form has 6 neutrons. This is called "carbon-12" from the fact that there are 6 protons plus 6 neutrons. There is a variety called "carbon-14" which has 8 neutrons. Every once in a while, a carbon 14 atom emits an electron (and yes, that pesky anti-neutrino too). Since the neutron that did this is now a proton, you have 7 protons and 7 neutrons. Remember that the type of atom is determined by the number of protons. So now the carbon-14 atom has transformed into a very stable nitrogen atom (which is identified by its 7 protons). The rate at which this happens is a "stochastic" process - it happens when it happens and you can't predict exactly when THAT atom over in the corner there will transform. However one usually works with many, many atoms, and, in that case, one can use a very reliable average time called the"half-life." This is the time that it takes for half (50%) of a bunch of unstable atoms to decay. For carbon-14, this number is 5,730 years. For different radioactive atoms, this number can be anywhere from a tiny fraction of a second to minutes, hours, days, or even millions of years. But, in all these cases, the point of the decay is to reach a type of atom that is stable.

Ultimately, even these stable atoms have a limit imposed by the lifetime of proton (>1025 years). Remember, though, that the best estimate of the present age of the universe is the much smaller number of 1010 years, so for all practical purposes, atoms are forever.

Now, here's a question for all you hotshots out there. Carbon-14 is a popular atom to look for when figuring out the age of old organic matter, for example, some bits of wood found in an old Egyptian tomb. Now, knowing that the half-life is only about 6,000 years, and the age of the Earth is about 4 billion (4*109) years, it doesn't take an Einstein to see that ALL of the carbon-14 should have converted to nitrogen long ago. Something must be PRODUCING the stuff even as we sit here staring at a computer monitor. Any ideas?


Carl Zorn, Detector Scientist (Other answers by Carl Zorn)

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