All elements have isotopes. There are two main types of isotopes: stable and unstable (radioactive). There are 254 known stable isotopes. All artificial (lab-made) isotopes are unstable and therefore radioactive; scientists call them radioisotopes. Some elements can only exist in an unstable form (for example, uranium). Theoretically, all five can decay into isotopes of element 72 (hafnium) by alpha emission, but only 180 W has been observed to do so. The other naturally occurring isotopes have not been observed to decay, and lower bounds for their half lives have been established: 182 W, t 1/2 7.7×10 21 years 183 W, t 1/2 4.1×10 21 years 184 W, t 1/2.
Sometimes the Atomic Mass, unlike the Atomic number, is usually given as decimals. For example with Chlorine, the Atomic Mass is given as 35.5, this is because there are two forms of Chlorine as an element. This is because in nature there there are two types of Chlorine atoms and they have different numbers of neutrons but the same number of protons. As they have the same number of protons they will have the same atomic number and the same number of electrons. They have the same reactivity due to the number of electrons and also the same atomic number. The key difference is the difference in mass number, this is caused by different number of protons. They are chemically the same, the only difference is Chlorine 35 contains 18 neutrons and Chlorine 37 contains 20 neutrons, both still have the same number of protons 17 and 17 electrons as you can see in the table.
Looking at another example Lithium has many isotopes. It has a mass number of 6.941 which in most periodic tables is rounded up to 7. This is because Lithium has isotopes that range from having 0 neutrons up to 13 but they all have different abundances. The abundance of an isotopes is the percentage of that isotope in existence. 92% of all lithium in the known universe is Lithium 7 - having 3 protons and 4 neutrons.
The relative atomic mass of an element takes into account all its naturally occurring isotopes when compared to 1/12th the mass of carbon 12. It can be mathematically calculated using the formula below.
If we look at the example of chlorine the equation would consider the abundance as a percent of the isotope mass. For Cl 35 there is 75% and for Cl 37 there is 25% so if we put this into our equation
Relative atomic mass = (35x75) + (37 x 25)
- 100
Isotopes Of Elements And Abundance
This gives a relative atomic mass of 35.5 as found in the periodic table.
Isotopes Of Elements In Periodic Table