# Questions and Answers

How many atoms are in the human head?

We can calculate the number of atoms in your head if we know the density and a constant called Avogadro's number. This is really just an estimate, but it's going to be a good one. The equation is fairly simple. The number of atoms of ANY substance in a volume is:

# of atoms = N * (density) * volume / (Molecular Weight).

N is a constant called Avogadro's number and its equal to 6.022*1023 atoms/mole. It can also be molecules per mole. In the above formula density times volume is just the mass. If you know how heavy something is or what its volume and density are you can easily do this.

Lets start with a simple problem. A liter of water is 1000 cubic centimeters. Water is easy because each cubic centimeter has 1 gram of mass. Water is made up of 2 hydrogen atoms and one oxygen atom. Hydrogen has an atomic weight of 1 and oxygen has an atomic weight of 16. The water has a molecular weight of 18. The liter of water has 1000 grams. The number of moles is 1000/18 = 55.556 moles. The number of molecules is therefore 6.022*1023 * 55.556 = 3.346*1025 molecules. The number of atoms is 3 times larger because each molecule has three atoms, so there are 1.0038*1026 atoms in a liter of water.