Seeing Molecules

Molecules are pretty tiny things, aren't they? Well, the smallest is. H2 is the hydrogen molecule consisting of just two hydrogen atoms - the smallest atoms we know. Its length is about twice the 74 picometres (0.74 Å) distance between the two hydrogen nuclei.

The biggest molecule is theoretically limitless! A diamond is a three dimensional network of bonded carbon atoms. It is because diamond is a huge, single, very rigidly structured molecule that it has its unique optical and physical properties.

As well as being able to be cut to make stunning jewels, diamond is also the hardest known material, the least compressible and the best thermal conductor with an extremely low thermal expansion. It is chemically inert to most acids and alkalis and transparent to light from the deep ultraviolet range, right through visible light to the far infrared. An amazing molecule, and one you can see with no assistance from a microscope.

A molecule is a combination of two or more atoms in a definite arrangement held together by chemical bonds.

A molecule is generally considered the smallest particle of a pure substance that still retains its composition and chemical properties.

However, many scientists consider crystals, such as diamond, to be best understood as being composed of aggregates of atoms or ions rather than as molecular units.

Most molecules are microscopic. Those which make up your body vary from being fairly large, like proteins, to very very small.

One of the largest in your body is your DNA, which is often called a macromolecule. DNA can reach macroscopic sizes, as can molecules of many polymers, such as plastic and amber.


Molecules can consist of hundreds of different atoms bonded together, or just two or three. Atoms are represented by coloured balls.
Getting the little stuff in perspective

Table courtesy The National Center for Research Resources (NCRR), National Institutes of Health (NIH), USA

Scaling this little stuff

To visualise the biological structures in your body we are going to model their relative sizes. How do biological structures, such as cells, organelles, bacteria, and viruses, compare in size with each other and to you?

Let's start with the room you are in. Mark out a 10 metre cube in it. Assume the ceiling is about 10 metres. Check the walls, If they are about 10metres, then that will be fine. We are looking at scale here, not accuracy in measuring with a ruler.

Look at the table above. Your room now represents an average cell in your body. A desk will be about right for a bacterium.

The smaller ones are up to you. Can you find items which are about the right scale for the rest of the biological structures on the list, right down to the essential water molecule?

1. Scale the biological structures as described and then draw your result, indicating the size of the objects you chose to represent each of the structures given in the table.

2. How big are you?

Let's now put you into this scale of things. Measure your own height. Adjust it for the scales we have been using where one class room is a cell in your body and a water molecule is barely visible.

How tall would you need to be for this scaling to be correct. Take a guess first. Then do the calculations.

Take a few minutes to think about the fact that every bit of this massive you is filled with these tiny tiny things, all in the right places and all working together so you are a living, breathing, thinking animal who can actually start to comprehend how astounding you are.

3. How astounding are you?