Before life appeared, the earth became cool enough for water vapor to condense into small drops in the atmosphere. Clouds formed.  The atmosphere was probably made up of carbon dioxide, water vapor and nitrogen.  The carbon dioxide interacted with the water vapor, a weak acid.  Acid rain fell eroding the rocks and dissolving some of the chemicals on their surfaces.  The runoff from the rain created lakes and oceans, and washed chemicals into them. There the chemicals were mixed together, and many organic molecules were formed. Organic molecules contain carbon (its symbol is C), which is why living things on earth are called carbon-based life forms.

The materials that the earth are made of can be thought of as being of two kinds: elements and compounds.

• Elements, such as oxygen (O), gold (Au), iron(Fe), nitrogen (N), and carbon (C), cannot be broken down into anything else.
  
• Compounds, such as carbon dioxide, salt, and water, are made of combinations of the elements. Their molecules can be separated back into the elements that they are made of, or other, but simpler, compounds.

     Living organisms take chemicals  into their bodies by eating, drinking, and breathing. They can break down, use, and change the elements and compounds that they take in. They are able to move, grow, sustain their bodies and reproduce because of chemical changes that take place within them.

If atoms could not be joined together, life would not be possible. Atoms, however, can be connected to each other. When atoms are linked together, they are called molecules. Many of the elements will form molecules: for example, oxygen will form molecules consisting of two oxygen atoms. However, oxygen can also be linked to the molecules of other elements: for instance, water is made of one oxygen molecule and two hydrogen molecules. How is this possible?

Every atom is made up of a nucleus (which contains protons and neutrons). The nucleus is the center, and it contains most of the mass of the atom. Outside the nucleus there are electrons, which are extremely small, and which move around the nucleus very quickly. The electrons are segregated into "shells" which are like invisible spheres that go around the atoms. Only two electrons can be in the innermost shell. The next track will take eight atoms. Atoms are often drawn as if they were small solar systems, but the electrons do not move in a flat plane.

There are five possible shells available to atoms.  This diagram shows only three shells, and some atoms, such as the little hydrogen atom, use only the one inner shell.      Each of the elements has a different number of the tiny electrons, and a corresponding number of protons. Hydrogen is the smallest, with only one electron. Carbon, nitrogen, and oxygen, are also small, light atoms.

    These electron shells are filled one by one from the inner shell going outward.  When a shell has a few atoms in it but is not filled, the atom will connect to another atom  that also has shells that are not filled.  This is sort of "Plug and Play" system that allows elements to combine in numberless ways.  Several atoms can connect to one or more other atoms at once.

How can you tell how many electrons can fill each shell?  I found this formula here.

http://www.southwest.com.au/~jfuller/chemistry/chem2.htm 

2 * (the shell number *the shell number)= the maximum number of electrons in this shell. 

Look!

     Now, what if an atom has space in its outermost track for more electrons than it actually has? This is where we suddenly see that we have a 'Construct a Universe" kit!   Atoms combine to fill up the spaces in their shells so that the outer shell is complete. Let's see how that works.

This is a diagram of an atom of neon gas. I have colored the nucleus orange and the electrons blue. The inner shell is filled by two electrons. The next shell is filled too -- with eight electrons. Neon doesn't need any more electrons to fill its outermost shell, so it doesn't participate in chemical reactions. It is what we call an inert gas, one that doesn't combine with other elements.

     Now we see a very different situation. Here we have a carbon atom near two hydrogen atoms. Note that the carbon atom has two electrons in the inner ring: that is filled. But in the next ring, it has four empty spaces! The hydrogen atoms, too, could each use another electron.

     Now lets look at some more atoms. Here we see that four hydrogen atoms are sharing their electrons with the carbon atom! They have filled the empty ring spaces in the carbon atom, and the carbon atom is sharing its electrons as well. We now have a molecule of methane gas! This is real chemistry!

     Actually, carbon is an amazing element. It can make long chains with its own atoms, and with other atoms that it can add to the chain. The complex molecules that life forms need are built around carbon atoms and chains.

     So, what do you need to remember from this page? Think of the atoms with their empty spaces for sharing electrons. Got it? Good! Now think of the early seas of our planet, and probably some other planets, too. For millions of years the rains have been bringing all sorts of chemicals together, and stirring the atoms around in the water. Think of the atoms combining with each other, making all sorts of molecules, including amino acids, which are the basis for the construction of the DNA in our chromosomes. This was going on for millions and millions of years! And now you are ready for --