The mole is the SI unit for the quantity of a substance. The mole is defined as the number of atoms in exactly 12 grams of the isotope carbon-12. The number is 6.022 times 10 to the 23rd power. This number is also called Avogadro's number. "Mole" is used similar to the way "dozen" is used. You have a mole of something just like you have a dozen of something. For example, if I have one dozen cookies, I have 12 cookies. Likewise, if I have one mole of cookies, I have 6.022 times 10 to the 23rd power of cookies. That's a huge number of cookies! If I have one mole of water molecules, however, it would amount to about 1 tablespoon of water. When we're talking about atoms or molecules, one mole is a reasonable quantity to bridge the difference in size between an atom and the microscopic world. We'll use Avogadro's number anytime we want to convert between moles of something and the number of that something. The mole is also very conveniently defined with regards to atomic mass. The atomic mass listed on the periodic table for each element is in atomic mass units. Numerically, the atomic mass is also equal to the molar mass for an element. An element's molar mass is defined as the mass in grams of one mole of atoms for an element. So, from the periodic table, aluminum has an atomic mass of 26.98 atomic mass units. This also means that 26.98 grams of aluminum is equal to one mole of aluminum atoms or 6.022 times 10 to the 23rd power of aluminum atoms. Likewise, carbon's atomic mass is 12.01 atomic mass units, so 12.01 grams of carbon is one mole of carbon atoms. We can use an element's molar mass anytime we want to convert between mass and moles.