Re: Is there a limited number of neutrons an atom can have?

Ryan:

    That is a good question.  In theory, there is no reason that a 
hydrogen atom should be limited only to 3 isotopes (those with zero, one 
and two neutrons).  Why, for instance couldn't the isotope H-100 exist?  
That isotope would be one with a single proton and 99 neutrons in the 
nucleus.  Or, at the other extreme, an isotope of U-93, a uranium atom 
with 92 protons and a single neutron?

    Conceptually, either of those isotopes (or any other combination of 
protons and neutrons) could exist.  But, when we look at isotopes that we 
can actually detect, we only see certain isotopes for each element, and 
they seem to be grouped near what is called the "line of stability."

    If you plot the number of neutrons along the x-axis and the number of 
protons along the y-axis of a rectangular coordinate system, for 
approximately the first 20 elements, the line of stability would be 
approximately x = y.  Or, the stable isotopes would be those that had 
approximately the same number of protons and neutrons.  As the number of 
protons (usually called the Z number)increases above 20, the number of 
neutrons to define stable isotopes increases above the 1 : 1 ratio found 
below Z = 20.  The n/p ratio gradually increases from 1 to about 1.5 for 
the heavier atoms.  And above the element bismuth, none of the elements 
have ANY stable isotopes.

    For any given element, there could be anywhere from 2 or 3 to as many 
as 30 known isotopes.  (On the average, for the first 104 elements, there 
are between 2 and 3 stable isotopes and around 13 unstable, or 
radioactive, isotopes.)  In general, as the isotopes move away from 
the "line of stability," they become more and more unstable.  That is, 
they have shorter and shorter half-lives.  (The "half-life" is the time it 
takes for 50% of a group of atoms to decay, or transform to a different 
isotope.)  We can measure half-lives of less than a millionth of a second, 
but there is a limit below which we cannot measure half-life.  When an 
isotope has too many or too few neutrons, we can think of it as being so 
unstable that its half-life can't be measured.  That is why isotopes with 
n/p ratios too far from the "line of stability" are said to not exist.  
They may even be theoretically possible within the limits of nuclear 
quantum mechanics, but they are SO unstable that they cannot be detected 
even with our best current measurement equipment.

    So, from a practical point of view, there is a limit to the number of 
neutrons that any element can have.  Quantum physics imposes some limits, 
but our ability to detect extremely unstable isotopes also imposes a 
practical limit.  Out of the first 104 elements, there are about 1575 
isotopes that have been detected, and about 275 of those are stable.  This 
is only about 15% of the total number of isotopes that are theoretically 
possible.  You may be able to look up the known isotopes in your library 
by getting a copy of a booklet put out by the General Electric Company 
called, The Chart of the Nuclides.

    If you have any further questions or if I can clarify anything for 
you, please feel free to email me at gelsg@aol.com.  Thanks for the 
question, and good luck in your science class.