Thursday, 9 February 2012

Relative Atomic Mass (Ar)

The actual mass of an atom is approximately 10-23 g (now that is small!) and working with it when doing calculations can be a bit difficult and to be honest a waste of time! Instead we use what is known as its relative atomic mass.
The 12C isotope is used as a standard to which all other atoms are compared. One atom of 12C has a mass of 12 so 1/12 of an atom of 12C has a mass of 1….so now it is a lot more easier to work out how many times heavier an atom is compared to 1/12 of the mass of an atom of 12C.
If you are still confused, it can be put into even simpler terms…

The average mass of an atom of the element
1/12 of the mass of one atom of 12C

We also use the same idea for the masses of individual isotopes – Relative isotopic mass:

The average mass of an atom of the isotopes
1/12 of the mass of one atom of 12C

As you may or may not know, all elements contain isotopes (isotopes are atoms of the same element with the same number of protons but different number of neutrons), so when calculating the relative atomic mass of an element we need to take this into account. There are two things you need to know when doing these calculations:
1.     the relative isotopic mass of each isotope
2.   the percentage of each isotope in the element
Below are a couple worked examples to calculate the relative atomic mass (There are a few ways to set your working out but I find this way really simple):

1.    Chlorine consists of two isotopes – 75.5% of 35Cl and 24.5% of 37Cl.

35Cl  x  75.5%  à  (75.5 ÷ 100)  x  35  =  26.425
37Cl  x  24.5%  à (24.5 ÷ 100)  x  37  =  9.065

26.425  +  9.065  =  35.49 (35.5 to 3sf)

2.    Magnesium consists of three isotopes – 78.6% of 24Mg, 10.1% of 25Mg and 11.3% of 26Mg.

24Mg  x  78.6%  à  (78.6 ÷ 100)  x  24  =  18.864
25Mg  x  10.1%  à  (10.1 ÷ 100)  x  25  =  2.525
26Mg  x  11.3%  à  (11.3 ÷ 100)  x  26  =  2.938

18.864  +  2.525  +  2.938  =  24.3 to 3sf

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