However there are 2 problems with this chemical reaction.  First, because atoms like to have full valence shells, single H or O atoms are rare (and unhappy) creatures.  As we saw in the previous lesson, both hydrogen and oxygen react with themselves to form the molecules H₂ and O₂, respectively.  These hydrogen and oxygen molecules are much more common.  Given this correction, one might guess that the reaction looks like this:

But we still have one problem.  As written, this equation tells us that 1 hydrogen molecule (with 2 H atoms) reacts with 1 oxygen molecule (with 2 O atoms) to form 1 water molecule (with 2 H atoms and 1 O atom).  In other words, we seem to have lost 1 O atom along the way!  To write a chemical reaction correctly, the number of atoms on the left side of a chemical equation has to be precisely balanced with the atoms on the right side of the equation.  How does this happen in our example?  In actuality, the O atom that we 'lost' reacts with a 2nd molecule of hydrogen to form a second molecule of water.  The reaction is therefore written:

In the chemical reaction above, the number in front of the molecule (called a coefficient) indicates how many molecules participate in the reaction.  A simulation of the reaction can be viewed by clicking below (the atoms are represented as spheres in the animation: red = hydrogen, blue = oxygen):

        In order to write a correct chemical reaction, we must balance all of the atoms on the left side of the reaction with the atoms on the right side.  Let's look at another example.  Natural gas is primarily methane.  Methane (CH₄) is a molecule in which 4 hydrogen atoms are bonded to one carbon atom.  If you have a gas stove, lighting the stove causes the methane to react with oxygen in the atmosphere to release heat and the atoms recombine to form carbon dioxide and water vapor.  The unbalanced chemical reaction would be:

Look at the reaction atom by atom.  On the left side we find 1 carbon atom, and 1 on the right.  There are 4 hydrogen atoms on the left, but only 2 on the right.  Therefore, you know 2 water molecules must be formed.  Adding this coeffiecient we get:

 Now we have to balance the oxygen atoms.  On the left you find 2 atoms, on the right 4 (2 in the CO₂ molecule and 1 in each of 2 H₂O molecules).  Therefore we need to start with 4 oxygen atoms, or 2 molecules.  The balanced equation would then be:

Try balancing the following equations on your own (note: while you don't need to write the coefficient 1 in a chemical reaction, these examples will not work unless you input a 1 where needed):

1) Na + Cl₂NaCl

2) N₂ + H₂NH₃

3) Fe + O₂Fe₂O₃

4) Cu + AgNO₃Ag + Cu(NO₃)₂ (a Cu atom bonded to 2 NO₃ groups)

5) H₂SO₄ + NaCN HCN+ Na₂SO₄

        Up until this point we have been talking about atoms and molecules.  The problem with this approach is that atoms and molecules are very small things.  In a single drop of water for example, there are trillions and trillions of water molecules.  A reaction between a single molecule of hydrogen and a single molecule of oxygen, as we discussed above, would be undetectable.  Instead of talking about single molecules in science, we talk about groups of molecules.  You can think of it like buying eggs.  You don't go to the store and buy an egg - you buy a dozen.  Contained within that dozen are the individual eggs.  Its the same thing when we talk about molecules.  We don't talk about single units, we talk about groups.
        But even a dozen molecules is a tiny amount.  What we need is a big number - a huge number!  That number is the mole.  The mole is the scientific community's baker's dozen.  One mole equals 6.02 x 10²³ (also known as Avogadro's number).  A 6 followed by 23 zeros.  Now that's a pretty big number.  But that's all it is, a number.  You can't just have a mole, you have to have a mole of something.  A mole of atoms.  A mole of water molecules.  A mole of pennies (which would make you richer than you can imagine).  Why the mole?  As it turns out, the mole has some interesting properties.  One mole of hydrogen atoms (6.02 x 10²³ H atoms) weighs 1 g.  From the periodic table we know that an He atoms weighs 4 times as much as an H atom, so go figure, 1 mole of He atoms weighs 4 g.  In fact, one mole of any element is equal to the atomic mass of that element (in grams).
        Let's think about that for a second.  If we know the molar mass of an element, and we know how many elements make up a specific molecule, then you can calculate the molar mass of a compound by adding up the atomic weights.  Huh?  Take water for example.  How much does a mole of water weigh?  Well, one mole of water contains one mole of oxygen atoms and two moles of hydrogen atoms.  A mole of hydrogen weighs 1 g and a mole of oxygen weighs 16 g (look at the atomic mass in the periodic table).  So to calculate the weight of one mole of water:

One mole of water weighs 18 grams!

        The mole is also useful in chemical reactions.  Though you can't measure out an atom of hydrogen, you can measure out a mole.  Since the mole is just a constant number, the coefficients in a balanced chemical reaction give you the molar proportions of reactants and products.  In other words:

It also tells us that:

For additional information and practice with balancing chemical reactions and the concept of the mole, you might want to try:

• More info on Reactions and problem sets (a whole bunch)
• Some tips on Determining what is reacting
• Even more Reaction problem sets
• And finally, some really bad mole jokes.

Copyright © 1998-1999, All Rights Reserved, Anthony Carpi
The 'Lab Mole' gif courtesy of the Diving into Math and Science site