Question

In my text book it states: The relative atomic mass of an element is the average mass of an atom of the element, taking into account all its isotopes and their relative abundance, compared to one atom of carbon 12. At the same time it states that generally, the relative atomic mass is the mass of one mole of atoms of an element. So is it the mass of one moles of atoms of an element or the average mass of an atom of the element?

Answer 1

Relative atomic mass(RAM) is actually a dimensionless quantity. Consider an element A. Let's assume that one atom of A, when put on a weighing machine, weighs 'x' units.

(Since an atom can have different nuclei, we always take those factors into consideration, and assume an averaged out atom when weighing ).

Also, let's assume that one atom of carbon ( the C-12 isotope ) weighs 'y' units.

Then , RAM = x/(y/12)

This is the definition of RAM. We take (1/12)th of the weight of the carbon atom. Since it is a ratio of similar things, it is dimensionless !

This RAM is also known as atomic weight. (Atomic weight, or atomic mass is not dimensionless, although RAM is. We will solve this confusion in the next section !)

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Now, 1 mole is a notation used for representing the quantity 6.023*10²³ , in the same fashion as 'dozen' is used to represent 12.

When we put these many atoms together and weigh them, it is termed as their molar weight (because all these atoms count to 1 mole).

Mostly, molar mass is represented in units of grams.

For instance, He atom has a molar mass of 2 grams.

You will also notice that RAM of He is also 2.( no units, because RAM is dimensionless, remember !).

As a matter of coincidence, when you put together 1 mole atoms of any element, and express their weight in grams, it is equal to the RAM of that particular element.

This is the reason why you will see that RAM of O-atom is written as 16 , and its molar mass is also written equal to 16 grams. ( It's all a matter of coincidence, but remember that the molar mass has to be expressed in grams only and no other unit. In some other unit, RAM will not be equal to molar mass ).

Sometimes, molar mass is also simply called as atomic mass ( not relative atomic mass !! ).

Thus, RAM is dimensionless, and atomic mass ( = molar mass ) is equal to RAM, but in units of grams. :)