Each element has a unique number of protons, denoted by its atomic number and its position in the periodic table. Besides protons, the nuclei of all elements, with the exception of hydrogen, also contain neutrons, which are electrically neutral particles with the same mass as protons. The number of protons in the nucleus of a particular element never changes, or it would become a different element. The number of neutrons can change, however. Each variation in the number of neutrons in the nucleus of a particular element is a different isotope of that element.
How to Denote Isotopes
The word "isotope" comes from the Greek words isos (equal) and topos (place), which signify that isotopes of an element occupy the same place in the periodic table, even though they have different atomic masses. Unlike atomic number, which is equal to the number of protons in the nucleus, atomic mass is the mass of all the protons and the neutrons.
One way to to denote an isotope is to write the symbol of the element followed by a number that denotes the total number of nucleons in its nucleus. For example, one isotope of carbon has 6 protons and 6 neutrons in its nucleus, so you can denote it as C-12. Another isotope, C-14, has two extra neutrons.
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Another way to denote isotopes is with subscripts and superscripts before the symbol of the element. Using this method, you would denote carbon-12 as 126C and carbon-14 as 146C. The subscript is the atomic number and the superscript is the atomic mass.
Average Atomic Mass
Every element that occurs in nature has multiple isotope forms, and scientists have managed to synthesize many more in the laboratory. Im all, there are 275 isotopes of the stable elements and around 800 radioactive isotopes. Because each isotope has a different atomic mass, the atomic mass listed for each element in the periodic table is an average of the masses of all the isotopes weighted by the total percentage of each isotope that occurs in nature.
For example, in its most basic form, the hydrogen nucleus consists of a single proton, but there are two naturally occurring isotopes, deuterium (21H), which has one proton, and tritium (31H), which has two. Because the form containing no protons is by far the most abundant, the average atomic mass of hydrogen is not much different from 1. It is 1.008.
Isotopes and Radioactivity
Atoms are most stable when the number of protons and neutrons in the nucleus are equal. Adding an extra neutron often does not upset this stability, but when you add two or more, the binding energy that keeps nucleons together may not be strong enough to hold them. The atoms throw off the extra neutrons and with them, a certain amount of energy. This process is radioactivity.
All the elements with atomic numbers higher than 83 are radioactive because of the large number of nucleons in their nuclei. When an atom loses a neutron to revert to a more stable configuration, its chemical properties do not change. However, some of the heavier elements may shed a proton to achieve a more stable configuration. This process is transmutation because the atom changes into a different element when it loses a proton. When this happens, the atom undergoing the change is the parent isotope, and the one left after the radioactive decay is the daughter isotope. An example of transmutation is the decay of uranium-238 into thorium-234.