What is an Isotope

The word isotope is used in chemistry to refer to atoms, which have the same atomic number, but different atomic mass. These isotopes will be chemically identical, but will have slightly different physical weights, and may either be stable or radioactive.

Every atom in the universe is made up of a central nucleus, surrounded by one or more electrons (think of the sun with the planets orbiting around). If we could take a closer look at the nucleus, we would find it consists of 2 types of particles, called protons and neutrons. These 2 types of particles have similar mass, but are different in other ways.

Looking first at the protons, we would find that they have a positive electric charge. This positive charge in the nucleus is equaled by the negative charge from the electrons orbiting around, resulting in an electrically neutral atom. Not only that, but the number of electrons will equal the number of protons. This is important, as it’s the number of protons in an atom that defines its chemical properties; hydrogen has 1 proton, carbon has 6, oxygen has 8, and lead has 82. All 104 elements have a different number of protons, and are therefore chemically different. The number of protons also defines the atomic number of an atom.

It is different if we look at the neutrons in the nucleus. Neutrons are electrically neutral, but their mass is the same as a proton. Theoretically we could add or take away neutrons from an atom without changing its chemical properties – all we would do is change it’s mass. In fact, this is exactly what we see in nature (and in science labs), atoms with the same number of protons, but with different numbers of neutrons. These are called isotopes. The number of protons and neutrons is defined as the atomic mass.

If we look at carbon, we see that most of the atoms occur with 6 neutrons in the nucleus giving an atomic mass of 12 (there are always 6 protons in a carbon atom). About 1% of carbon atoms have an atomic mass of 13, and there is a very small number with atomic mass 14. So, carbon-12, 13 and 14 are isotopes of carbon. Why not more or less? Well, the other effect the number of neutrons has on an atom is to alter its stability. At just the right number the atom will be stable for long periods, perhaps millions of years. Add one or two neutrons, and the atom can become unstable, lasting only thousands of years, or even just a fraction of a second. When it becomes unstable, it will radiate a combination of protons, neutrons and electrons away until it reaches a stable state. This is the process that causes radiation. As mentioned above, if the number of protons changes, then the atom will turn into a different element.

The isotope carbon-14, has a half life of 5,730 years. This just means that half of a sample will have become unstable in that period of time. When it becomes unstable it will decay into nitrogen-14. The importance of this is that if you know how much carbon-14 you had at sometime in the past, and you can measure how much you have now, then you can work out how old the sample is – i.e. carbon-14 dating. You can learn which radioactive isotope is used in geological dating.

Isotopes are used in a wide variety of industries, from medicine to building nuclear weapons, and from smoke alarms to archaeology. They occur in nature or can be created in laboratories or nuclear reactors. Their importance has lead to a great deal of research into the isotopes of all elements, defining their physical properties such as stability.