All life on planet Earth is bound by one common factor: carbon. Why is carbon important to life? The molecular makeup of all living things is determined by the size and shape of carbon molecules. Unlike other elements, carbon has the unique ability to form a variety of bonds with itself and many other elements which make it the perfect foundation of all life forms.

We often describe carbon as the building block of living organisms. That is because the biomolecules like proteins, DNA, and carbohydrates that we talk and hear about are all carbon-based molecules. If you look at the structures of these biomolecules, you will quickly see that the carbon atoms essentially serve as their “backbone” where the carbons are bonded to one another in a chain-like fashion or in the form of a ring.

But, you may be wondering – what exactly makes carbon so special that allows it to be used this way?

If you are a science fiction enthusiast, you may have thought about alternative life forms that are not carbon-based. A common example used is silicon, which is the element found directly below carbon on the periodic table.

There is a reason why silicon is directly compared to carbon, and it is because carbon and silicon are similar in the fact that their atoms have 4 valence electrons in their outer shell.  This shared characteristic means that silicon can also make the same number of bonds that carbon can make to generate similar molecules.

However, despite the fact that silicon can make the same number of bonds as carbon and is also about 135 times more abundant on our planet, we are NOT silicon-based organisms here on earth.

There are of course many different reasons as to why we are not silicon-based organisms, but one reason is that these two seemingly similar atoms actually differ in terms of their size. And although the difference may seem trivial in words, this issue of size greatly influences, for instance, the strength of bonds that carbon and silicon can make, which then affects the types of molecules that we can generate with these atoms.

The differences in size can be easily understood by simply looking at the number of electrons each atom owns, which differs for each element on the periodic table. The electrons reside in areas outside of the atom’s nucleus that we call atomic orbitals, and as the number of electrons increases, this would mean that the number of atomic orbitals would have to increase, too.

Specific to our conversation, carbon has a total of 6 electrons while silicon has a total of 14 electrons, which makes silicon comparatively larger than carbon.

When larger atoms come together to form a bond, this will result in the formation of longer bonds, which can be more easily broken, than shorter bonds, which are much stronger. As a reference, the average bond energy for carbon to carbon single bond is 346 kJ/mol while the bond between silicon atoms is 222 kJ/mol. Higher the bond energy, stronger the bond, so quantitatively we can see that carbon to carbon bonds are stronger than bonds between silicon.

But, let’s visualize this as a molecule.

Take a look at the size difference between a 6-membered ring made up of carbon atoms (on the left) versus silicon atoms (on the right), and see the obvious difference in the bond lengths between the atoms.

Let’s put this into context with a common 6-membered biomolecule found in nature, glucose, which serves as an important energy source for us living organisms. (images 2&3 or simulation?)

Again, if glucose is made up of silicon atoms, it would create a molecule filled with longer, weaker bonds that would break apart more easily than glucose molecules that we take for granted that are carbon-based.

And remember, it wouldn’t be just glucose molecules. Your DNA structure, proteins, and fats would all be affected if carbon atoms did not serve as their foundation, and life as we know it here on earth would certainly not exist the way it does now.