Atoms are particles of elements,
substances that could not be broken down further. In examining
atomic structure though, we have to clarify this statement. An atom
cannot be broken down further without changing the chemical nature
of the substance. For example, if you have 1 ton, 1 gram or 1 atom
of oxygen, all of these units have the same properties. We can
break down the atom of oxygen into smaller particles, however, when
we do the atom looses its chemical properties. For example, if you
have 100 watches, or one watch, they all behave like watches and
tell time. You can dismantle one of the watches: take the back off,
take the batteries out, peer inside and pull things out. However,
now the watch no longer behaves like a watch. So what does an atom
look like inside?
Atoms are made up of 3 types of particles electrons  ,
protons 
and neutrons  .
These particles have different properties. Electrons are
tiny, very light particles that have a negative electrical charge
(-). Protons
are much larger and heavier than electrons and have the opposite
charge, protons have a positive charge.
Neutrons are
large and heavy like protons, however neutrons have no electrical
charge. Each atom is made up of a combination of these particles.
Let's look at one type of atom:
The atom above, made up of one proton and one electron, is called
hydrogen (the abbreviation for hydrogen is H). The proton and
electron stay together because just like two magnets, the opposite
electrical charges attract each other. What keeps the two from
crashing into each other? The particles in an atom are not still.
The electron is constantly spinning around the center of the atom
(called the nucleus). The centrigugal force of the spinning
electron keeps the two particles from coming into contact with each
other much as the earth's rotation keeps it from plunging into the
sun. Taking this into consideration, an atom of hydrogen would look
like this:
A Hydrogen Atom
Keep in mind that atoms are extremely small. One hydrogen atom,
for example, is approximately 5 x 10-8 mm in diameter.
To put that in perspective, this dash - is approximately 1 mm in
length, therefore it would take almost 20 million hydrogen atoms to
make a line as long as the dash. In the sub-atomic world, things
often behave a bit strangely. First of all, the electron actually
spins very far from the nucleus. If we were to draw the hydrogen
atom above to scale, so that the proton were the size depicted
above, the electron would actually be spinning approximately 0.5 km
(or about a quarter of a mile) away from the nucleus. In other
words, if the proton was the size depicted above, the whole atom
would be about the size of Giants Stadium. Another peculiarity of
this tiny world is the particles themselves. Protons and neutrons
behave like small particles, sort of like tiny billiard balls. The
electron however, has some of the properties of a wave. In other
words, the electron is more similar to a beam of light than it is to
a billiard ball. Thus to represent it as a small particle spinning
around a nucleus is slightly misleading. In actuality, the electron
is a wave that surrounds the nucleus of an atom like a cloud. While
this is difficult to imagine, the figure below may help you picture
what this might look like:
Hydrogen: a proton surrounded by an electron cloud
While you should keep in mind that electrons actually form clouds
around their nucleii, we will continue to represent the electron as
a spinning particle to keep things simple.
In an electrically neutral atom, the positively charged
protons are always balanced by an equal number of negatively charged
electrons. As we have seen, hydrogen is the simplest atom with only
one proton and one electron. Helium is the 2nd simplest atom. It
has two protons in its nucleus and two electrons spinning around the
nucleus. With helium though, we have to introduce another
particle. Because the 2 protons in the nucleus have the same charge
on them, they would tend to repel each other, and the nucleus would
fall apart. To keep the nucleus from pushing apart, helium has two
neutrons in its nucleus. Neutrons have no electrical charge on them
and act as a sort of nuclear glue, holding the protons, and thus the
nucleus, together.
A Helium Atom
As you can see, helium is larger than hydrogen. As you
add electrons, protons and neutrons, the size of the atom
increases. We can measure an atom's size in two ways: using the
atomic number (Z) or using the atomic mass (A, also known as the
mass number). The atomic number describes the number of protons in
an atom. For hydrogen the atomic number, Z, is equal to 1. For
helium Z = 2. Since the number of protons equals the number of
electrons in the neutral atom, Z also tells you the number of
electrons in the atom. The atomic mass tells you the number of
protons plus neutrons in an atom. Therefore, the atomic
mass, A, of hydrogen is 1. For helium A = 4.
Ions and Isotopes
So far we have only talked about electrically neutral atoms,
atoms with no positive or negative charge on them. Atoms, however,
can have electrical charges. Some atoms can either gain or lose
electrons (the number of protons never changes in an atom). If an
atom gains electrons, the atom becomes negatively charged. If the
atom loses electrons, the atom becomes positively charged (because
the number of positively charged protons will exceed the number of
electrons). An atom that carries an electrical charge is called an
ion. Listed below are three forms of hydrogen; 2 ions and
the electrically neutral form.
|
|
 |
| H+ : a positively
charged hydrogen ion |
H : the hydrogen atom |
H- : a negatively
charged hydrogen ion |
Neither the number of protons nor neutrons changes in any
of these ions, therefore both the atomic number and the atomic mass
remain the same. While the number of protons for a given atom never
changes, the number of neutrons can change. Two atoms with
different numbers of neutrons are called isotopes. For
example, an isotope of hydrogen exists in which the atom contains 1
neutron (commonly called deuterium). Since the atomic mass is the
number of protons plus neutrons, two isotopes of an element will
have different atomic masses (however the atomic number, Z, will
remain the same).
Two isotopes of hydrogen
|
 |
|
Hydrogen
Atomic Mass = 1
Atomic Number = 1
|
Deuterium
Atomic Mass = 2
Atomic Number = 1
|
Copyright © 1998-1999, All Rights
Reserved,
Anthony Carpi
Copied From the web page below
|
