Atoms and Subatomic Particles
An element is a substance that contains only one type of atom.
An atom is the smallest particle of an element that can exist alone or in combination with other atoms.
Atoms are neutral (have no electric charge), but they are composed of still smaller subatomic particles,
which carry positive, negative, and neutral charges.
There are three subatomic particles out of which all atoms are made: protons, electrons, and neutrons
Protons: have a positive electric charge, are found in the nucleus (center) of the atom,
it is the number of protons that differentiates one atom from another and determines its properties
Neutrons: are neutral (no electric charge), are found in the nucleus, and glue protons together with nuclear
force. Neutrons have almost the same mass as protons.
Electrons: have a negative electric charge, are just as negative as a proton is positive, and orbit the nucleus.
Electrons have very little mass when compared to protons and neutrons, it takes about
2,000 electrons to equal the mass of one proton.
Protons and neutrons are made of still smaller pieces called quarks. We will not concern ourselves with
sub-nuclear particles in this class. If, however, you are curious about these particles, click on the following
link
http://library.thinkquest.org/18188/english/universe/theory/element.htm
Under certain condition (as in nuclear reactions and particle collision experiments) antimatter is formed.
Antimatter has the same mass as normal matter, but it has an opposite charge.
An anti-proton has the same mass as a proton, but it has a negative electric charge. Anti-electrons have the
same mass as electrons, but have a positive electric charge (for this reason they are also called positrons).
If anti-particles are shattered, they break into anti-quarks.
When matter and antimatter come into contact the particles annihilate, converting their mass completely
into energy by the famous e=mc2 equation.
For a discussion of anti-matter:
http://www.lbl.gov/abc/Antimatter.html
http://www.crystalinks.com/antimatter.html
The center of the atom is called the nucleus. Since it contains only protons and neutrons, the nucleus is
positive. Virtually all of the mass of the atom is found in the nucleus.
Because both protons and neutrons are found in the center, or nucleus, of the atom, they are collectively
referred to as nucleons (isn’t that a way cool word?)
The electrons orbit the nucleus at various energy levels. Although electrons have almost no mass, they are
just as negative as protons are positive. Since the number of electrons in an atom equals the number of
protons, atoms are neutral.
The electron cloud, or region of the atom which contains the electrons in their various energy levels, makes
up the vast majority of the atom’s volume. If an atom was as tall as tall as a 60 story building, the nucleus
would only be as big as a nickel, yet it would contain 99.99999% of the atom’s mass. This means that
atoms are mostly empty space, and that the density of the nucleus is 2.3  1014 g/ml
or about a mountain per ml!