History of the Universe

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HONC Atoms

About one hundred different types of atoms emerged from exploding stars, ranging from the simplest (hydrogen) to large heavy atoms such as uranium. Fortunately for us, life depends mainly (although not exclusively) on only four different types of atom: hydrogen, oxygen, nitrogen and carbon. In this story we will focus on these very important atoms, which I will call the HONC atoms.

Valence 1234

It turns out that not only were the HONC atoms abundant, but they can each form a different number of covalent bonds: hydrogen can make only one, oxygen can make two, nitrogen three and carbon can make four covalent bonds. This can be summarized as:
















Here I show some of the HONC atoms joined together into an invented molecule. Can you work out which atom is which? Count the bonds for each one.

To see the solution click here.

Second Electron Shell

You might wonder WHY do the HONC atoms have valence of 1234?

The answer is given by the subject of quantum mechanics when applied to atoms.

We have already seen that two electrons formed a hard shell around the helium atom. Atoms larger then helium have more than one shell of electrons. The first two electrons form the first shell, as we have seen for helium, and further electrons go into a second, third and further shells outside the first.

Diagram of atom showing first two shells of electrons

The first shell can hold two electrons, the second can hold up to eight. So let's examine the second shell of each main atom.


A carbon atom has six electrons. Two are held in its first shell and the next four go into its second shell, so there are four "holes" in the second shell, waiting to be filled by sharing four electrons with other atoms.

Carbon can share 4 electrons with other atoms, so it can form 4 covalent bonds.

Ball and stick model of methane CH4

In this diagram we see a carbon atom sharing electrons with 4 hydrogen atoms to form a methane molecule.

Carbon atoms form part of almost all the molecules of life.

In models, carbon atoms are usually colored black.

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History of the Universe eBook
History of the Universe eBook
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Written by Wyken Seagrave
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