So just how big is a molecule?

Molecules are made up of atoms and all things are made of atoms. Not quite everything; light isn’t, and sound isn’t either. But the things that we see when light hits them are made of atoms, and we hear sounds because the molecules in the air transmit sound energy from one place to another.

Molecular Weight

Molecules have size and different molecules are different sizes. When talking about the size of a molecule, most scientists will mention its molecular weight. That is the total weight of all of the atoms in the molecule. So water, which is made up of two hydrogen atoms (1 atomic mass unit each) and one oxygen atom (16 atomic mass units) has a molecular weight of 18 (well, 18.01528 to be exact). Glucose has a molecular weight of 180. DNA, which is very long, can have a molecular weight that is around 1010 (about the weight of one human chromosome).

Measuring in Nanometers

In addition to mass, we can also measure how big a molecule is. A water molecule is about 0.27 of a nanometer across. DNA is about 2 nanometers across and can stretch out to meters in length. Amazing, when you think about that length of DNA being stuffed inside a cell. But that is because it is so thin, about 1/1,000,000,000 as thick as it is long. That width-to-length ratio is like a human hair that is the length of a football field.

Shapes and Sizes

Molecules come in many shapes and sizes. Water and carbon dioxide are some of the smallest molecules, proteins are some of the largest.