*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 ^{35}Cl and ^{37}Cl 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 ^{79}Br and ^{81}Br. That means
that if you were obtain a sample (pick up a chunk) of Br; it would contain both
isotopes of ^{79}Br
and the other 50% of the sample is ^{81}Br. 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 ^{35}Cl and 25% for ^{37}Cl. Put
another way… you obtained a sample of Cl that had 100 atoms in it; 75 of the
atoms would be ^{35}Cl and 25 of the atoms would be ^{37}Cl. 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.

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

- 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.