Iodine Half-Life Experiment

Iodine is element number 53. That is, Iodine has 53 protrons and 53 electrons. That is what determines that it is Iodine. We could also say that for Iodine, Z = 53.

How many neutrons does Iodine have?

Most naturally-occuring Iodine is Iodine-127. We could say A = 127. Remember, A is the "atomic mass number" of an isotope -- the total number of "nucleons" inside the nucleus. A is the total number of the protons and neutrons inside the nucleus,

A = Z + N

N = A - Z

N = 127 - 53

N = 74

Most naturally-occuring Iodine has 74 neutrons. We can write this as


What happens if this naturally-occuring Iodine-127 absorbs a neutron?

127I53 + 1n0 --> 128I53

Notice how this equation is "balanced". A neutron has a mass of 1 unit so

127 + 1 = 128

and the neutron has zero charge so

53 + 0 = 53

There are no more electric charges in the new nucleus so there are no more electrons in the new atom. The new isotope is still an isotope of Iodine since Z = 53. Naturally-occuring Iodine-127 becomes a new isotope of Iodine, Iodine-128. This new isotope is not stable. It is radioactive,

128I53 --> 128Xe54 + 0e-1 + 0 0

or, we can write this as

128I53 --> 128Xe54 + 0-1 + 00

The Iodine-128 nucleus sends out an electron. We might also call this electron a beta particle. We can detect this beta particle with a Geiger counter. The nucleus also sends out a neutrino which is very difficult to detect. And, with one less negative electronic charge -- or one more positive electronic charge -- inside the nucleus, it is now Xenon-54.

This nuclear reaction is as if

1n0 --> 1p1 + 0e-1 + 0 0

That is, this nuclear reaction is as if a neutron had decayed into a proton plus an electron (plus a neutrion). We will detect this electron which the nucleus sends out with a Geiger counter.

How long does all this take?

We can not say when one particular Iodine nucleus will decide to send out an electron and transform itself into a Xenon nucleus. But if we have a large number of Iodine nuclei, we know that "on the average" -- or statistically -- if we wait 25 minutes only half of them will remain Iodine-128. If we wait another 25 minutes, only half of those will still be Iodine-128.

That is, the half-life for this nuclear reaction is 25 minutes.


Geiger Counter
Return to Ch 30, Nuclear Physics
(c) Doug Davis, 2002; all rights reserved