The N-Z curve
The N-Z curve is a plot of the number of neutrons(N) against the number of protons(Z).
lines:
i) the 'stability' line - a gentle curve starting from the origin and of increasing gradient
ii) the line of N = Z - a straight line of gradient '1' through the origin
regions
i) beta minus(electron) particle emitters
ii)beta plus(positron) particle emitters
iii) alpha particle emitters top of curve(not shown)
description:
- for proton numbers(Z) up to 20, N=Z a straight line
- for all nuclei with Z>20 , stable nuclei have more neutrons than protons, the line curves upwards
- unstable nuclei above the stability curve are called neutron-rich
- unstable nuclei below the stability curve are called neutron-poor
the decay process:
Unstable neutron-rich nuclei can become more stable by losing neutrons. They do this by 'beta decay'. The effect of this for a single nucleus is to raise its the proton number (Z) by 1 and decrease its neutron number(N) by 1, bringing the N-Z plot of the nucleus closer to the stability curve. The movement of the point is right one unit and down one unit.
beta decay: Z + 1 N - 1
Unstable neutron-poor nuclei can become more stable by gaining neutrons. They do this by 'positron decay'. The effect of this for a single nucleus is to lower its proton number (Z) by 1 and increase its neutron number(N) by 1 , bringing N-Z plot of the nucleus closer to the stability curve. The movement of the point is left one unit and up one unit.
positron decay: Z - 1 N + 1
Alpha decay has very little effect on the position of a nucleus relative to the stability curve. This is is because the loss of an alpha particle(2 protons + 2 neutrons)does not upset the N-Z ratio too much. The point representing a nucleus has Z - 2 and N - 2 . Only large nuclei participate in alpha decay. So the effect is only confined to the very top section of the curve.
alpha decay: Z - 2 N - 2
Decay Chains
A decay chain (or radioactive series) charts the different types of radioactive decay a nucleus undergoes until a stable isotope is reached.
There are only 3 naturally occuring decay chains called the:
- actinium series
- thorium series
- uranium series
plus one other involving a trans-uranium element
- neptunium series
A decay chain is accurately described using a graph of nucleon number(A) against proton number(Z).
The graph illustrates the complete Thorium-232 decay chain.
click on image to magnify (in pop-out page)
Important observations are:
alpha decay ...........2 units to the left, 4 units down
beta- decay ...........1 unit to the right
Bismuth .................2 possible decay outcomes
Equations describing the Thorium Series:
. . . etc.
As an exercise, it is left to the reader to complete the series using the decay chain graph(above).
