As we described in the article about the Bohr atomic model, the amount of neutrons in an atom can vary. When you have two atoms which both contain the same amount of protons, but different amount of neutrons, you say that they are isotopes of the same element.
Describing an isotope
When you want to specify that you are talking about a certain isotope of an element, you write the mass number as a superscript on the left side of the element symbol. The mass number is the sum of protons and neutrons within the atom. The amount of protons can be written as a subscript in the left side of the element symbol. Let's look at an isotope of chlorine containing 17 protons and 18 neutrons, which are summarized to the mass number 35:
The information on how many protons that an element contains is usually considered superfluous, since all chlorine atoms contain 17 protons. If there would have been any other amount of protons, the element symbol would have changed. In other words, you most often describe an isotope like this:
If we wish to describe the isotope in words, we call it chlorine-35. There is also a version of chlorine containing 17 protons and 20 neutrons. That one is called chlorine-37. In the same way, you may have heard about carbon-14, when determining the age of really old objects. We place the name of the element first, and the mass number last during the verbal description of the isotope.
Many isotopes of the same element
The same element may have several different isotopes. Most of the isotopes we come into contact with in our daily lives are stable isotopes which do not change over time. If we were to put the atoms inside a box (also preventing them from interacting with each other or their environment), we could pick out the same atoms after 1 million years without them having changed.
An important thing to be aware of is that when you look at the mass of an element (for example in the periodic table), an average of all the commonly existing isotopes is listed. We discuss this further in the article about atomic mass.
An unstable isotope of an element is called a radioactive isotope. Many of the heavy elements contain no stable isotopes at all, and they all spontaneously decay at a certain pace after being created. During the decay, they emit radiation, and decay into other isotopes of other elements or elementary particles.
A term that is commonly used for radioactive isotopes is half-life. Half-life is a measure of the amount of time it takes until half of the original atoms are left. If the half-time is 100 years, only half of the radioative isotope remains after 100 years, and the other half decayed into more stable elements. Note that a half-time can be shorter than a second, so there is a enormous variation in the half-times for different radioative isotopes.
An example of a radioactive isotope
A weakly radioactive isotope is carbon-14, which we mentioned earlier in the article. Carbon-14 is not stable, but decays with a half-time of approximately 5700 years. Carbon-14 is created in the atmosphere due to cosmic particles, and is incorporated into all life through the fixation of carbon by photosynthesis. After something died, it will be desynchronized with the amount of carbon-14 that is present in all other presently living things, since the carbon-14 in the fossil will decay without being replaced. By measuring the content of carbon-14 in something old, we can calculate how long it was since it stopped exchanging carbon with its environment.