Most elements can occur as more than one isotope.
The following is an example of the notation that is used to
indicate a particular isotope of an element:
Where 92 is the atomic number of Uranium and 235 is the atomic weight of the isotope in question.
Some elements only have a few isotopes (Helium has 5), and others have many ( Xenon has 16).
Over all, of the 90 naturally occurring elements, there are about 300 stable isotopes, and over 2500 radioactive isotopes.
A stable isotope is one that is not radioactive, and is therefore more or less permanent.
A radioactive isotope is unstable for one of three reasons: it has too many neutrons, too few neutrons, or too many protons.
The graph below shows the “Band of Stability”, all the dots shown in purple represent the stable isotopes. Radioactive isotopes are not shown.
As the number of protons in the nucleus increases (higher atomic number) a higher ratio of neutrons to protons is needed in order to hold the nucleus together. This is because the repelling electric force between the protons in the nucleus acts over a greater distance than the attractive nuclear force between the neutrons and the protons. This explains why there are no stable isotopes of elements 84 and up.
If an isotope has too many neutrons or too few neutrons it will be unstable and transform (decay) into a more stable isotope by emitting a particle from the nucleus. This process is called “radioactive decay”, and the emissions are called “particle radiation”.
An unstable nucleus, one with too many or too few neutrons, can gain stability through one of the following radioactive decay processes:
alpha particle emission, electron emission, positron emission, or electron capture.
When an unstable isotope undergoes nuclear decay, the original isotope is called the parent isotope and the newly formed isotope is called the daughter isotope.
An alpha particle
contains two protons and two neutrons.
It is identical to a helium nucleus, so it is represented with the
symbol
In fact, every helium atom present on earth started out as an alpha particle that later attracted two electrons.
Parent isotope daughter isotope alpha particle
Note that alpha emission causes the atomic number to go down by two and the atomic weight to go down by four.
Click here for animations of alpha decay: link
Alpha decay occurs most often in large isotopes that have both too many neutrons and too many protons (more than 83 protons)
Examples of isotopes that emit alpha particles:
A beta particle is an electron or positron (anti-electron) emitted from an unstable nucleus.
Electron emission (also called negatron
emission) is the most common type of
beta-decay. It occurs when a neutron in
an isotope with too many neutrons emits a negative beta particle
(electron) and in so doing transforms into a proton. Negative beta particles are represented with
the symbol b- or
A neutron that is not in contact with a proton becomes
unstable and decays into a proton by emitting an electron. The following equation shows the
transformation of an unstable neutron into a proton by the emission of a negative beta particle (electron)
After a negative beta particle slows down it is simply an ordinary everyday electron.
Electron
emission causes the atomic number to go up by one (because what was a neutron
is now a proton) and the atomic weight to stay the same (an electron has an
atomic weight of zero).
Click
here for animations: link
Example of electron emission:
Examples of electron emitting isotopes:
Positron emission is a less common form of beta
decay. In positron emission, a proton in an isotope with too few neutrons
transforms into a neutron by emitting a positive beta particle (positron) which
is given the symbol b+ or
A positron is a particle with the same mass as an electron, but with a positive charge instead of a negative charge. Positrons are not ordinary matter, but are instead a form of antimatter. When matter and antimatter come into contact with each other they both annihilate, converting all of their mass into energy via Einstein’s famous e=mc2 formula.
Positron emission causes the atomic number to go down by one (because what was once a proton is now a neutron), and the atomic weight to stay the same (because a positron, like an electron, has an atomic weight of zero).
Click here for animations: link
Example of positron emission:
Examples of positron emitting
isotopes:
Electron capture is another way that isotopes with too few neutrons
can gain stability. In electron
capture, one of the electrons closest to the nucleus is pulled in and unites
with a proton to form a neutron. The
atomic number goes down by one, since what was a proton is now a neutron, and
the atomic weight stays the same, since the only thing that is emitted is
energy in the form of x-rays.
Click here for
animations: link
Example of electron
capture:
Examples of electron capturing isotopes:
Click here for great animated review of concept: Link
These particles are dangerous when they are emitted because they are ejected from the nucleus at extreme velocities. When they pass through a cell they can damage its DNA. This often kills the cell. If it doesn’t, it sometimes causes an error in the DNA that causes it to turn the cell cancerous. Mutations (so popular in movies) can only occur if the particle passes through a sperm cell, and egg cell, or an embryo in the early stages of development. An adult organism subjected to radiation can not become a mutant.