I Hate Isotopes…

 

Isotopes: Atoms of the same element that have different numbers of neutrons.

 

The only difference between isotopes of an element is the number of neutrons that it has and therefore the mass.

 

ex:   Cl

All Cl atoms have 17 protons. Some Cl atoms have 18 neutrons – some atoms of Cl have 20 neutrons. The only difference between 35Cl and 37Cl is the number of neutrons and therefore the atomic mass.

 

In similar fashion… All pennies have a value of one cent. Some pennies have a lot of copper (old ones) some don’t (new ones). The only difference between old pennies and new pennies is the amount copper and therefore the mass.

 

NOW Then… In nature there can be several isotopes of an element. For example: The element Br has 2 stable isotopes 79Br and 81Br. That means that if you were obtain a sample (pick up a chunk) of Br; it would contain both isotopes of Br. The thing is – that the amount of a specific isotope (relative abundance) in a given sample is not always an even proportion. As it happens – the proportion or relative abundance of a sample of bromine is 50:50; or 50% of the sample is 79Br and the other 50% of the sample is 81Br. If you were to find the average atomic mass of a sample of Br, you simply add the 2 atomic mass together and divide by 2. The answer is 80.

 

Q: How does this relate to our penny lab???

 

Things get weird when the relative abundances of the isotopes in a sample are not equal (50:50). As is in the case for Cl... The relative abundances for the 2 stable isotopes of Cl are: 75% for 35Cl and 25% for 37Cl. Put another way… you obtained a sample of Cl that had 100 atoms in it; 75 of the atoms would be 35Cl and 25 of the atoms would be 37Cl. If you were to find the average atomic mass for a sample of Cl, you need to include the relative abundances. There are 2 ways to do the math here.

 

  1. add up all the parts: (75x35)+(25x37)=3550 then divide by the number of parts: 3550/100=35.5

 

  1. multiply by the percentage: (.75x35)+(.25x37)=35.5

 

Either way you will get the same answer.

 

Q: How does this relate to the penny lab???

 

The reason that average atomic masses, as listed on the periodic table, are not whole numbers (decimals) is because the relative abundances of the stable isotopes is rarely (if ever) 50:50.