It's really difficult to count the number of atoms in any measurement of material because you can't see them! You also can't simply pick up five atoms and put them on a scale to find the weight. For this reason, scientists use Avogadro's number in order to talk about atoms.
Counting Particles: Avogadro's Number
Avogadro's number is 6.022x1023 particles, which is also known as one mole (abbreviated mol). These particles can be molecules, atoms, ions, etc. Avogadro's number tells you that there are 6.022x1023 particles in one mole of a substance. If there is one mole of ping pong balls this means there are 6.022x1023 ping pong balls. It also means that if there is one mole of nitrogen that means there are 6.022x1023 atoms of nitrogen.
In other words:
Avogadro's number tells you the number of particles there are in one mole of a substance.
How Many Grams are There in One Mole?
How many grams of a substance there are in one mole depends on the substance in question.
The molar mass of an element or compound is the mass per mole of that substance. This value is reported in gram/mol or g/mol. You can find the molar mass by looking at the periodic table at the number under the element name. For example, the molar mass of chlorine is 35.45 g/mol.
If you need to find the molar mass of a compound you will need to add the molar mass of the individual elements together. For example, the molar mass of water requires you to take twice the molar mass of hydrogen and add the molar mass of oxygen to get 18.02 g/mol.
How Do You Convert From Atoms to Grams?
Now that you know the number of particles in one mole, and how to calculate the number of grams in one mol of that substance, you can convert from atoms to grams.
Say you have 3x1024 atoms of chlorine. How many grams of chlorine is that?
First, you need to begin by converting from atoms to mols.
Now that you know how many mol of chlorine 3x1024 atoms is equal to, you can use the molar mass to convert from mol to grams.
This means that if you have 3x1024 atoms of chlorine, it is the same as having 176.6 grams of chlorine.
What about if you know how many atoms you have of an element, but the actual compound you're talking about isn't just that element? For example, let's say you know you have 5.55x1023 atoms of oxygen, but the compound you want to find the mass of is CO2. How would you do that?
You know that there are two oxygen atoms per CO2 molecules. This means that for each oxygen atom you have, there is an entire CO2 molecule. You can now use this conversion factor to get to the mass of CO2.
Take a look:
The only difference between what was done before and this problem is the first step. You will need to figure out how many molecules of CO2 there are:
Now you can do exactly what you did before!
Knowing only the number of atoms of oxygen, you can find the grams of CO2.
When converting from atoms to grams always make sure to keep track of your units!
About the Author
Riti Gupta holds a Honors Bachelors degree in Biochemistry from the University of Oregon and a PhD in biology from Johns Hopkins University. She has an interest in astrobiology and manned spaceflight. She has over 10 years of biology research experience in academia. She currently teaches classes in biochemistry, biology, biophysics, astrobiology, as well as high school AP Biology and Chemistry test prep.