Isotopes

An ISOTOPE of an element is a different form of the element. The atoms of all forms of the element have the same number of Protons (and electrons), but the number of Neutrons differs.

Hydrogen, for example, exists in three different forms. The most common has one Proton and no Neutrons. A second type has one proton and one Neutron and the third type has one Proton and two Neutrons.

All of these forms of Hydrogen have the same number of electrons (one). They are therefore chemically identical. The only difference between them is their Mass Number.

NOTE: You may have heard of "heavy water" used in Nuclear research. Heavy water is H20 composed of the deuterium isotope. Water made up of Deuterium is heavier than 'normal' water.

Many elements have isotopes. In most cases the ratio of isotopes to 'normal' atoms is quite low.  Since the Mass Number is the number of Protons plus Neutrons you would expect it to be a whole number. Most Periodic Tables  indicate Mass Number with a small decimal component. This decimal component is a result of the occurence of small numbers of isotopes in any sample. In Chlorine, however, almost half the atoms have 18 Neutrons and half have 19 Neutrons. The Mass Number of Chlorine is therefore recorded as 35.5 (Both isotopes of Chlorine have 17 Protons.)

Some isotopes are radioactive. These isotopes are called "Radio Isotopes". Radio Isotopes are used in medicine and in a number of industrial applications.