Before life appeared, the earth
became cool enough for water
vapor to condense into
small drops in the
probably made up of
carbon dioxide, water
interacted with the
water vapor, a weak
acid. Acid rain
fell eroding the rocks and
dissolving some of the
chemicals on their
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
| 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.
atoms, such as
use only the
Each of the elements has a different
number of the tiny electrons, and a corresponding
Hydrogen is the smallest, with only one electron. Carbon, nitrogen,
and oxygen, are also small, light atoms.
electron shells are filled one
by one from the inner
outward. When a
shell has a few atoms
in it but is not
filled, the atom will
connect to another
also has shells that
are not filled.
This is sort of
Play" system that
allows elements to
combine in numberless
atoms can connect to
one or more other
atoms at once.
can you tell how many
electrons can fill
each shell? I
found this formula
* (the shell number
*the shell number)=
the maximum number of
electrons in this
* (the shell number *
the shell number)
the maximum number of electrons in
of Electrons in the Shell
(1 x 1) = 2
||2 x 1 = 2
(2 x 2) = 8
||2 x 4 = 8
( 3 x 3) = 18
||2 x 9 = 18
x (4 x 4) = 32
||2 x 16 = 32
(5 x 5) = 50
||2 x 25 = 50
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.
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
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
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
Image from a
First Graphic from Demo of Crystal Maker, a software program.
1996,1997, 1998, 1999,
2000, 2002, 2003.
All rights reserved.
This material may be
used by individuals
purposes but not sold.
Please inform the
author if you use it