Mole Day

Mole Day

It’s 23rd October again, so it’s mole day again.  A mol is the least understood or used of the 7 SI units by ordinary people.  It is the standard unit for a quantity of atoms or molecules.  A mol used to be called a “gram molecule”, which is probably more descriptive than mol which sounds like a gangsters girlfriend or a furry insectivore.  It just a number like Pi except much larger which it needs to be as its describing such tiny particles.  One mol is equal to 6.022 x 10²³.  Which is basically 6 with 23 zero’s after it.  That’s why mole day is on 23rd day of the 10th month and officially starts at 6:02 in the morning.  This time, I thought I would try to show how much a mol of a substance is; how big does 6.022 x 10²³ molecules look like.

 The above picture shows comparatively what a mol, in the form of a sphere, looks like arranged from less dense to more dense common elements.  The first sphere is water but the rest are common solid elements.  I must admit, I was a little surprised when I made this as I expected a bigger difference between different densities of materials.  The following list, is of each material in order from left to right followed by its atomic number (number of protons) in parentheses;-
Water (10), amorphous Carbon (6), graphite Carbon (6), diamond Carbon (6), Aluminium (13), Silicon (14), Iron (26), Copper (29), Zinc (30), Silver (47), white Tin (50), grey Tin (50).
I have shown Carbon in three of its allotropes (structural forms) and Tin in two.  When Carbon is compressed and heated many km underground it can change its form into the more dense diamond form by changing its atomic structure or way that its atoms link together in the solid.  Remember that each sphere is not by weight but by the number of atoms.  For reference, the first water sphere is about 1.6cm radius.

It is only when you include, the much less dense, air that things change dramatically.
This is the same picture with 1 mol of air included at the front.  This really does show how much more space atoms and molecules in the form of a gas take up.  Of course, a gas expands or contracts, much more than liquids or solids, according to the temperature and pressure they are exposed to.  This picture is at 15 degrees C and at standard atmospheric pressure at sea level.  For reference, 1 mole of air is about 17.5cm radius.

This picture shows, 1 mol of air, 2 mols of air, 3 mols of air and 1 mol of Water(10).  The interesting thing about quantities of a gas, is that whatever the gas, they always have the same number of molecules in the same space, as long as the pressure and temperature is constant.  This is because, the space between the molecules is very much larger than the molecule itself, however big it is.  Even Xenon (54) gas, which has atomic number 54 would occupy the same space.  So in out picture, 1 mol of Hydrogen, would be the same size as 1 mol of Oxygen.  The only difference between the first 3 spheres is the number of atoms inside them, regardless of what gas it contains.  There would be a difference in its mass or weight, but not the number of atoms or molecules within.  Thats why a balloon filled with hydrogen floats and one filled with air sinks, even though they are both the same size and therefore contain the same quantity of molecules.

Look at the last picture again.  The volume of the second sphere (2 mol) is twice the volume of the first sphere (1 mol).  The volume of the third sphere (3 mol) is equal to the first two spheres added together.  One mol of water consists of 2 mol of Hydrogen and 1 mol of Oxygen.  If we filled a balloon with 2 mol of Hydrogen and another with 1 mol of Oxygen, then combined the contents of the two balloons, we would get a balloon filled with 3 mol of the mixture.  In fact, it doesn’t really work like that, because as soon as we collect hydrogen atoms (H) together, they will react and combine with each other to make hydrogen molecules (H₂).  Oxygen atoms (O) do the same thing to make Oxygen molecules (O₂).  Once they form molecules they will take the same space as half the amount that they would in atomic form.  Effectively, we would have 1 mol of Hydrogen molecules added to 1/2 mol of Oxygen molecules producing a balloon filled with 1.5 moles of the combined gas.  This balloon would not react to make water but remain as a gas.  Energy is required to start the reaction.  Once the reaction starts it is self sustaining because when these gasses combine to make water the reaction produces its own energy which will sustain further reactions.  If we have a balloon filled with such a mixture and add energy, in the form of a lit match, the reaction would take place very quickly.  In the time it takes to blink, you would have missed it.  Once the reaction starts the balloon would explode in a puff of light and sound leaving only water vapor and water droplets.  In fact, if all this water is collected together, one exactly the size of the blue 1 mol sphere on the right.