If you cut a grain of salt in half, you'll end up with two grains of salt. If you keep doing that over and over you'll eventually end up with the smallest possible grain of salt that still acts like salt. That's a molecule. If you cut that final grain of salt in half one more time, you're left with something that isn't salt at all. The final split would leave you with the two atoms that make up salt: one sodium and one chlorine atom.
Each type of molecule is built from its own unique set of atoms. The number of atoms in a molecule, the kinds of atoms in a molecule and the arrangement of atoms in a molecule all combine to determine the properties of the molecule. That's why it's important to be able to describe a group of molecules in terms of the atoms that compose them.
To illustrate the calculations for converting molecules to atoms, we will calculate the number of atoms in four molecules of carbon dioxide.
Determine the number of molecules you're converting to atoms.
Often you'll be given a number, but sometimes you'll be given a mass of a substance. In that case, the number of molecules is equal to 6.02 X 10^23 times the mass of the sample in grams divided by the sum of all the atomic masses in one molecule of the substance.
As an example, assume you have four molecules of carbon dioxide.
Calculate the number of atoms in one molecule of your compound.
Carbon dioxide has two oxygen atoms and one carbon atom in a molecule, for a total of three atoms in one molecule of carbon dioxide.
Take the number of molecules obtained from Step 1 and multiply this number by the number of atoms you calculated in Step 2.
In the example you have four molecules of CO2. Multiply this number by the number of atoms in one molecule. Therefore, four molecules of CO2 = 4 molecules x 3 atoms/molecule equals 12 atoms. You can also calculate the total number of each type of atom. Four molecules of CO2 times two oxygen atoms per molecule means there are a total of eight atoms of oxygen.