Atoms and Light Energy
The study of atoms and their characteristics overlap several different
sciences. Chemists, Physicists, and Astronomers all must understand the
microscopic scale at which much of the Universe functions in order to see
the "bigger picture".
Inside the Atom
Just like bricks are the building blocks of a home, atoms are the building blocks of matter. Matter is anything that has mass
and takes up space (volume). All matter is made up of atoms. The
atom has a nucleus, which contains particles of positive charge
(protons) and particles of neutral charge (neutrons). Surrounding the
nucleus of an atom are shells of electrons - small negatively charged
particles. These shells are actually different energy levels and
within the energy levels, the electrons orbit the nucleus of the atom.
The ground state of an electron, the energy level it
normally occupies, is the state of lowest energy for that electron.|
|There is also a maximum energy that each electron can
have and still be part of its atom. Beyond that energy, the electron
is no longer bound to the nucleus of the atom and it is considered to
||When an electron temporarily occupies an energy state greater than its
ground state, it is in an excited state.
An electron can become excited if it is given extra energy, such as if it
absorbs a photon, or packet
of light, or collides with a nearby atom or particle.
Each orbital has a specific energy associated with it. For an electron
to be boosted to an orbital with a higher energy, it must overcome the
difference in energy between the orbital it is in, and
the orbital to which is is going. This means that it must absorb a photon
that contains precisely that amount of energy, or take exactly that amount
of energy from another particle in a collision.
The illustrations on this page are simplified versions of real atoms,
of course. Real atoms, even a relatively simple ones like hydrogen,
have many different orbitals, and so there are many possible energies
with different initial and final states. When an atom is in an excited
state, the electron can drop all the way to the ground state in one go,
or stop on the way in an intermediate level.
Electrons do not stay in excited states for very long - they soon
return to their ground states, emitting a photon with the same
energy as the one that was absorbed. |
Identifying Individual Types of Atoms
Transitions among the various orbitals are unique for each element because the
energy levels are uniquely determined by the protons and neutrons in the
nucleus. We know that different elements have different numbers of
protons and neutrons in their nuclei. When
the electrons of a certain atom return to lower orbitals
from excited states, the photons they emit have energies that are
characteristic of that kind of atom. This gives each
element a unique fingerprint, making it possible to identify the elements
present in a container of gas, or even a star.
We can use tools like the periodic table of elements to figure out exactly
how many protons, and thus electrons, an atom has. First of all, we know
that for an atom to have a neutral charge, it must have the same number of
protons and electrons. If an atom loses or gains electrons, it becomes
ionized, or charged. The periodic table will give us the atomic number of
an element. The atomic number tells us how many protons an atom
has. For example, hydrogen has an atomic number of one - which means it has
one proton, and thus one electron - and actually has no neutrons.
For the Student
Based on the previous description of the atom, draw a
model of the hydrogen atom. The "standard" model of an atom is known
as the Bohr model.
Different forms of the same chemical element that differ only by the number
of neutrons in their nucleus are called isotopes. Most elements have
more than one naturally occurring isotope. Many more isotopes have been
produced in nuclear reactors and scientific laboratories. Isotopes usually
aren't very stable, and they tend to undergo radioactive decay until
something that is more stable is formed. You may be familiar with the element
uranium - it has several unstable isotopes, U-235 being one of the most
commonly known. The 235 means that this form of uranium has 235 neutrons
and protons combined. If we looked up uranium's atomic number, and substracted
that from 235, we could calculate the number of neutrons that isotope has.
Here's another example - carbon usually occurs in the form of C-12 (carbon-12)
, that is, 6 protons and 6 neutrons, though one isotope is C-13, with 6
protons and 7 neutrons.
For the Student
Use the periodic table and the names of the elements given below
to figure out how many protons, neutrons and electrons they have.
Draw a model of an atom of the following element: silicon-28,
magnesium-24, sulphur-32, oxygen-16, and helium-4.
For the Student
Using the text, define the following terms:
energy levels, absorption, emission, excited state, ground state, ionization,
atom, element, atomic mass, atomic number, isotope.
A Optional Note on the Quantum Mechanical Nature of Atoms
While the Bohr atom described above is a nice way
to learn about the structure of atoms, it is not the most accurate way to
Although each orbital does have a precise energy, the
electron is now envisioned as being smeared out in an "electron cloud"
surrounding the nucleus. It is common to speak of the mean distance to the
cloud as the radius of the electron's orbit.
So just remember, we'll keep the words "orbit" and "orbital", though we are
now using them to describe not a flat orbital plane, but a
region where an electron has a probability of being.
Electrons are kept near the
nucleus by the electric attraction between the nucleus and the electrons.
Kept there in the same way that the nine planets stay near the Sun instead of
roaming the galaxy. Unlike the solar system, where all the planets' orbits
are on the same plane, electrons orbits are more three-dimensional. Each
energy level on an atom has a different shape. There are mathematical
equations which will tell you the probability of the electron's location
within that orbit.
Let's consider the
hydrogen atom, which we already drew a Bohr model of.
Probable locations of the electron in the
of the Hydrogen atom.
What you're looking at in these pictures are graphs
of the probability of the electron's location. The nucleus is at the
center of each of these graphs, and where the graph is lightest is
where the electron is most likely to lie. What you see here is sort of a
cross section. That is, you have to imagine the picture rotated around
the vertical axis. So the region inhabited by
this electron looks like a disk, but it should actually be
a sphere. This graph is for an electron in its lowest possible
energy state, or "ground state."
To the right is an excited state of hydrogen.
Notice that at the center, where the nucleus is, the picture
is dark, indicating that the electron is unlikely to be there. The two light
regions, where the electron is most likely to
be found, are really just one region. Remember, you have to
mentally rotate this around a vertical axis, so that in three dimensions
the light region is really doughnut shaped.
Probable locations of the electron in an
The text and images in this section were adapted from Dave Slaven's page on
The Atom (see References below).
The Periodic Table
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