1.4 MOLES and AVOGADRO’S NUMBER

French physicist Jean Perrin won the 1926 Nobel Prize in Physics for discovering the number 6.022 x 1023. This is a huge number that gives us the chemical concept of the mole. Here is what it looks like written out:avogadro number

You are probably wondering what this number means. It is called Avogadro’s Number andit is the number of particles it takes to make a single mole. Remember that 12 inches is a foot, 12 eggs is a dozen and that a thousand meters is a kilometer. In that same way, 6.022 x 1023 particles is a mole. Now, why use the word “particles” instead of “atoms?” Because a mole is 6.022 x 1023 atoms or molecules. Remember that in a previous tutorial, we explained that when 2 or more atoms snap together they form a molecule. Just like two or more bits of lego are snapped together to make a single unit of two or more legos. So whether we are dealing with individual atoms or many atoms snapped together into a molecule, we count each unit (a.k.a. particles) when counting up to a mole. And a mole is 602 sextillion units of either atoms or molecules. One of the reasons for using such a huge number is so we can get enough atoms or molecules to register a weight — in grams — on a scale. One mole is one group of 6.022 x 1023 particles. That’s it, that’s all.

How much a mole weighs depends on what element or molecule is being counted. A mole of a single element like hydrogen will obviously weigh less than a mole of a multi-atom cholesterol molecule. This would be like comparing the weight of a dozen thumb tacks to the weight of a dozen pool balls.

Here is a fun fact. Remember the atomic weight reading for each element on the periodic table? It tells you how much one atom of that element weighs in atomic mass units. Yet, that is not all. It also tells you the weight, in grams, of one mole of that element.carbon amu gram mole

For example, if you look at the atomic weight of carbon above, you now know that if you have 12 grams of carbon, you have  exactly 6.022 x 1023 carbon atoms (which is one mole of carbon). It just so happens that a single carbon atom weighs 12 amu, and if you multiply it 6.022 x 1023 times, it forms a pile of carbon atoms that weighs exactly 12 grams. This is the reason Avogadro’s number is precisely 6.022 x 1023. It simplifies our ability to understand the element on a micro and macro level. This rule applies to every element on the periodic table.

To clarify how this duel function is possible, picture a dozen pool balls in a box. Each pool ball represents either a neutron or a proton from a carbon atom. Let’s say each ball weighs one pound in the same way each proton or neutron weighs one atomic mass unit (amu). The 12 balls will therefore weigh 12 pounds the way a carbon atom weighs 12 amu. And let us pretend we have more than one box filled with 12 pool balls. Each separate box of 12 pool balls is like a separate carbon atom. And instead of Avogadro’s number let’s invent a fake number. Let’s say the number is 2000 and is called Pakoyoo’s number or a pole for short. Now, if I gather 2000 boxes of pool balls I will officially have a “pole” of boxes. That is, a Pakoyoo’s number of boxes. If we multiply each 12 pound box by Pakoyoo’s number, the total weight of a “pole” of boxes will be 24,000 pounds of pool balls. This just happens to be exactly 12  tons* of pool balls.

mole example Now if I created a periodic table of pool ball boxes, my 12 ball box would have a symbol that would look like this:

mole example 2

As you can see, Pakoyoo’s number was selected to allow the pool ball boxes to be easily referenced as single boxes, or in large groups. Because atoms are so small, they cannot be weighed on a scale and we need to group them in large groups (such as moles) that can be weighed accurately.

Before moving onto the next tutorial, we suggest you try the sample questions for this tutorial. Have fun!

*non-metric tons (a.k.a. “short tons”)

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