I Hate Isotopes…
Atoms of the same element that have different numbers of neutrons.
difference between isotopes of an element is the number of neutrons that it
has and therefore the mass.
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.
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.
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
How does this relate to our penny lab???
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.
way you will get the same answer.
How does this relate to the penny lab???
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.