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: The [[Periodic Table]] orders the [[element]]s due to their [[Chemical Property|chemical properties]] but since [[isotope]]s of the same [[element]] have the same [[Chemical Property|chemical properties]] then different [[isotope]]s are not included.
: On the [[Periodic Table]] an [[average]] [[Relative Atomic Mass|atomic mass]] is given based on the [[Relative Atomic Mass|mass]] of each [[isotope]] and how common it is.
====Calculating the Average Relative Atomic Mass====
[[Chlorine]] has two common [[Stable Isotope|stable]] [[Isotope]]s; [[Chlorine-35]] and [[Chlorine-37]]. [[Chlorine-35]] is three times more common than [[Chlorine-37]] so the ratio is 3:1.
Ratio: 3:1
3xChlorine-35 : 1xChlorine -37
To find the [[Mean Average|average]]:
: <math>Average=\tfrac{35+35+35+37}{4}</math>: <math>Average=\tfrac{35 \times 3 + 37\times 1}{4}</math>: <math>Average=35.5</math> So the [[Relative Atomic Mass]] of [[Chlorine]] is quoted as 35.5 on the [[Periodic Table]]. [[Lithium]] has two common [[Stable Isotope|stable]] [[Isotope]]s; [[Lithium-7]] and [[Lithium-6]].92.5% of all [[Lithium]] is the [[isotope]] [[Lithium-7]] while [[Lithium-6]] makes up only 7.5%. Ratio: 92.5:7.5 92.5xLithium-7 : 7.5xLithium-6 To find the [[Mean Average|average]]:: <math>Average=\tfrac{7 \times 92.5 + 6 \times 7.5}{100}</math>: <math>Average=6.925</math> So the [[Relative Atomic Mass]] of [[Lithium]] is quoted as 6.9, 6.93 or 6.925 depending on the [[precision]] of that particular [[Periodic Table]].