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In the chemical equation CH4 for methane gas why is there more hydrogen than carbon?

This is a very good question, and the answer is at the heart of modern atomic physics.

The nucleus is at the center of the atom, like the sun is at the center of the solar system. Electrons move around in orbits around the nucleus, like the planets around the sun. But there is an important difference: electrons can only have very special energies, which correspond to specific orbits. The orbits in the atoms are called shells, and each shell can only hold so many electrons. For example, the first shell can only hold two electrons and the second shell can only hold eight electrons. The last thing you need to know is that stable molecules like to form bonds that have full shells and they can do that by sharing electrons with other atoms.

Returning to your question, carbon has six electrons and hydrogen has one. Let us count the number of electrons in each shell. Hydrogen has only one electron in the first shell. Carbon has two electrons in the first shell, so the first shell is full. The second shell has the four remaining electrons, but it needs four more to complete the shell. To make up the CH4 molecule, carbon "borrows" four electrons from four different hydrogen atoms to make a stable molecule. And this is why there are more hydrogen atoms than carbon.

Now that we have gone through one example, can you explain why water molecules have two hydrogen atoms and only one atom of oxygen? What about carbon dioxide CO2? Hint: Oxygen has eight electrons. Place them in atomic shells and figure out how two atoms can share electrons to create complete shells.


Elton Smith, Staff Scientist

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