Atomic mass Video Lecture - Class 12

If you could look at just one atom of an element
you would see the nucleus with protons and neutrons
and the electrons on the outside
Most of the mass of the atom comes from the protons and the neutrons
This version of an atom or isotope happens to have six protons and six neutrons
The protons and neutrons in the nucleus of an isotope is the mass number
so I would expect the mass to be exactly twelve
but if you look at the Periodic Table for the same element
the atomic mass is not exactly twelve
This is because there isn't just one isotope of Carbon
There are multiple isotopes of Carbon
They have different numbers of neutrons
and therefore different mass numbers
we don't use mass number on the Periodic Table
because there are multiple isotopes from many of the elements
instead we use atomic mass
Atomic mass is the weighted average of the atomic masses of the different isotopes of an element
To understand this definition we need to know the difference between weighted average
and a regular average
Let's look at this sample of Boron atoms
there are two different isotopes here
Boron-10 and Boron-11
In a regular average we would add up ten and eleven and then we would divide by two
and our answer would be ten point five
The problem with this is that it doesn't take into account which isotopes are more common
Boron-11 is more common than Boron-10
so this really isn't good enough
In this sample we have twenty four Boron-11 atoms
and six Boron-10 atoms
with a grand total of thirty atoms
That means that there are naturally eighty percent Boron-11 isotopes
and twenty percent Boron-10 isotopes in nature
we can use that information, the natural abundance in our weighted average
The average atomic mass is equal to the mass of isotope one, times its abundance
which is a percentage in decimal form
plus the mass of isotope two
times the abundance of isotope two
and this goes on, so-on and so-forth
until all the isotopes are represented
so, let's find the atomic mass for Boron
The mass of Boron-11 times 0.8
which is eighty percent in decimal form
plus the mass of Boron-10 times 0.2
gives us 10.8 atomic mass units
10.8 is closer to eleven than it is to ten
and that tells us that there are probably more Boron-11 atoms in nature
than Boron-10
and this is the number that we see on the Periodic Table
Let's try another example
Calculate the average atomic mass of Magnesium
which has three isotopes
Magnesium-24 at 78.99%
Magnesium 25 at 10%
and Magnesium-26 at 11.01%
What we need to do is multiply the mass with the percent abundance
and remember to write those as a decimal
Magnesium-24 times its percentage
all I have to do is move the decimal two places to the left to get 0.7899
plus Magnesium-25 times 0.1
plus Magnesium-26 times 0.1101
When we add that all up we get 24.31
and that's the number on the Periodic Table
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