An examination of the structure of the atoms of the elements shows that the number of protons in the nucleus determines the position of the element on the Periodic Table; this used to be called the Atomic Number of that element (now called the Proton Number) and is unique to it. The number will be between 1 and 104. The number of orbital electrons in the atom also follows this pattern and is therefore the same as the number of protons.
However, the number of neutrons in the atoms of the elements does not follow a regular pattern.
For example, an atom of element number 1, hydrogen, has:
1 proton
1 electron
no neutrons.
An atom of element number 8, oxygen, has:
8 protons
8 electrons
8 neutrons.
An atom of element number 13, aluminium, has:
13 protons
13 electrons
14 neutrons.
So the number of protons follows a pattern, as does the number of electrons, but the number of neutrons does not follow a regular pattern.
It is worth remembering also that the Nucleon Number (or Mass Number) is the total number of the particles in the nucleus, that is, the number of protons plus the number of neutrons.
So for hydrogen the nucleon number is 1 + 0 = 1
For oxygen the nucleon number is 8 + 8 = 16
For aluminium the nucleon number is 13 + 14 = 27.
Another pattern exists in relation to the chemical properties of an element, which depends largely upon the number of outermost orbital electrons. It is this factor that determines to which vertical group on the Periodic Table the element belongs.
Irregularities:
There is a further complication with neutrons: in many elements different numbers of neutrons occur naturally in atoms of the same element. Observation of the Periodic Table shows that chlorine has a Proton Number of 17 but a Nucleon Number of 35.5. This would seem to be impossible, as the Nucleon Number is the sum of the protons and neutrons in the nucleus. It must have 17 protons, as determined by its Proton Number, which implies that it must have 18.5 neutrons, which is not possible, as 'half a neutron' cannot exist. This irregularity is explained by the fact that all chlorine is made up of two types of atoms.
One type of chlorine atom has:
17 protons
17 electrons
18 neutrons giving a Nucleon Number of 17 + 18 = 35
The second type of chlorine atom has:
17 protons
17 electrons
20 neutrons giving a Nucleon Number of 17 + 20 = 37
Any sample of chlorine has 3 times as many of the first type of atom as of the second type. For example, if four atoms were studied, three of them would have a Nucleon Number of 35 while the fourth would have a Nucleon Number of 37. There is, therefore, an average Nucleon Number, calculated as follows:
(35 + 35 + 35 + 37) / 4 = 142/4 = 35.5 which is why chlorine has 35.5 as its Nucleon Number.
Many other elements have Nucleon Numbers that are not a whole number; these elements, like chlorine, have different atoms.
If more than one atom of an element exists then the different forms are called ISOTOPES. From the above example we can see that there are two naturally occuring isotopes of chlorine.
Isotopes:
ISO means 'the same' and TOPE means 'place', so ISOTOPE means 'the same place'. The word is used because isotopes are substances with the same number of protons in their atoms and therefore appear at the same place on the Periodic Table. They do, however, have different numbers of neutrons.
Isotopes of an element have identical chemical properties; that is, they behave the same way in chemical reactions; however, they usually have different physical properties. Chemical reactions are the same because these are largely determined by the outer orbital electrons, while physical properties largely depend upon the whole atomic structure. As different isotopes have different numbers of neutrons the Nucleon Numbers are different and it is this mass difference that shows itself in the physical behaviour of the isotopes.
As described above, isotopes of the same element exist naturally; chlorine, for example, with 17 protons and 18 neutrons; and chlorine with 17 protons and 20 neutrons. These two isotopes are usually written as {U35}{S17} Cl and {U37}{S17}Cl, the top number being the Proton Number (the total number of protons), while the lower number is the Nucleon Number (the total number of protons and neutrons).
Another element with naturally occuring isotopes is the gas, neon, which has two forms, {U20}{S10}Ne and {U22}{S10}Ne. There are many other examples.
It is possible to artificially make many isotopes of all elements; these are called RADIOISOTOPES.
Radioisotopes:
These are isotopes of elements produced by bombarding stable atoms of an element with either alpha particles, protons or neutrons. Many such isotopes of this type can be artificially produced, although a few are produced naturally as a result of bombardment by radiation from outer space (these radiations are often called cosmic rays).
In addition to the fact that radioisotopes are man-made they also differ from natural isotopes in that they tend to be unstable. This means that they are radioactive and the nucleus of such isotopes usually decays until it becomes stable. Over twenty different isotopes have been made for some elements.
Hydrogen can be made to exist in three different isotopic forms. Normal hydrogen has the following atomic structure:
1 proton
1 electron
no neutrons.
However, a neutron can be introduced into the nucleus to give an isotope known as heavy hydrogen or deuterium. Its structure is therefore:
1 proton
1 electron
1 neutron
A third isotope can be produced by the addition of a second neutron. This isotope is called tritium and has the following structure:
1 proton
1 electron
2 neutrons.
The full range of all known isotopes is available for you to explore in this section.
Uses of isotopes:
Radioisotopes have a wide range of uses, largely because they are radioactive and their radioactive emissions can be detected.
(1) Tracing leaks:
Leaks in underground pipes can be traced if an isotope is introduced into the pipe. The isotope will spread out where the leak is and, using a suitable measuring device, the radiation can be detected from above the ground. Once the location of the leak has been found then repair work can be carried out with the minimum of disruption.
(2) Monitoring pollution:
Certain radioisotopes can be safely released into rivers, where they attach themselves to polluting material. By detecting the isotope the progress and movement of pollutants can be monitored. The source of pollutants can also be located by this method.
(3) Cancer treatment:
Some isotopes emit high-energy gamma radiation and these are suitable for the treatment of cancer. Cobalt-60 is such a source. The high energy radiation can be accurately directed towards cancerous cells in the body, destroying the cells.
(4) Thickness testing:
It is important that materials produced in sheet form, such as paper, have a consistent thickness. The thickness can be continuously checked using a suitable source of beta radiation. The emitter is placed on one side of the paper while a detector is placed on the other. Beta radiation is passed through the paper as it comes off the rollers in the paper-pressing factory and the detector measures how much radiation has passed through. If the paper is of the correct thickness then a certain reading on the meter will be shown, but if the paper becomes thinner then the reading will increase, as less of the radiation will have been absorbed. Similarly, if the paper becomes thicker then the reading will decrease, as more of the radiation will have been absorbed. Adjustments can be made to the machinery to ensure a constant thickness.
(5) Carbon dating:
One of the naturally occuring radioisotopes is carbon-14. Most carbon on the earth is carbon-12, having a structure of 6 protons, 6 electrons and 6 neutrons. Carbon-14 has an extra two neutrons.
The quantity of carbon-14 on the earth is constant; as it decays by the emission of beta particles it becomes nitrogen, but more is formed by the bombardment of nitogen by neutrons from space. All living material - animals, plants etc. - take in carbon, including carbon-14, while they are alive, but stop doing so when they die. Carbon-14 has a long half life of about 5,600 years and by measuring the amount of radioactive carbon-14 remaining in a once-living material it is possible to accurately determine the age of the material. Carbon dating is therefore important to historians, archaeologists and anthropologists.
It should be remembered that carbon dating can only be used for materials that were once part of a living organism, for example wood, bone, wool, leather and so on. It cannot be used for stone or metal objects.
