An atom is the basic structure from which all
matter is composed, in the same manner as a brick is the basic
structure from which a wall is built. Although atoms are too
small to be seen with our eyes, scientists have long had indirect
evidence for the existence of atoms. We can now use the world's
most powerful scanning tunneling microscopes to "see" the
magnified images of atoms and to study surface reaction sites
on an atom-by-atom basis.
Figure 1: Just as a brick is basic to the structure
of a wall,
an atom is basic to the structure of matter.
Atoms are made of small particles called protons,
neutrons, and electrons. Each of these particles is described
in terms of measurable properties, including mass and charge.
Mass is the amount of matter that an object contains. The
proton and neutron have roughly the same mass and have approximately
one thousand times the mass of the electron. The proton and
electron have equal, but opposite, electrical charges. A neutron
does not have an electrical charge.
Figure 2: Model of Proton, Neutron, and Electron
the proton and neutron were enlarged, and each had the
approximate mass of a hippopotamus, the electron, enlarged
to the same scale, would have less mass than an owl.
Figure 3: Comparison of Masses
In an atom, the protons and neutrons clump together
in the center and are called the nucleus. Because the protons
are positively charged, the nucleus has a positive electric
charge. The electrons of the atom move rapidly around the
nucleus. If we attempt to detect an electron in an atom, we
might find evidence of it located almost anywhere around the
nucleus. However, if we repeat this experiment many times,
it will be found that the electron is much more likely to
be located in certain regions of space surrounding the nucleus
than in other regions of space. We might think that the electron
is rapidly moving around the nucleus and our experiment "catches"
the electron as an instantaneous "snapshot" of it in motion.
The probability of finding the electron in any region of space
can then be described by a cloud that rapidly thins out as
one goes farther from the nucleus. The density of the cloud
at any point is the probability of finding the electron at
of an atom is empty space. The nucleus of the atom contains
almost all of the mass of the atom. A greatly enlarged
atom might look like a marble (the nucleus) inside an
empty football stadium (the electron probability cloud).
Figure 4: Electron Probability Cloud
Around a Nucleus
The attractive electric force between the positively-charged
protons in the nucleus and the negatively-charged electrons
around the nucleus holds the atom together. Atoms containing
the same number of protons and electrons have no net charge.
Atoms that have extra electrons or are missing electrons have
a net electrical charge and are called ions. Ions can interact
with other ions due to the electrical attraction between opposite
Figure 5: Diagram Comparing a Beryllium Atom and a
Di-positively-Charged Beryllium Ion
Atoms interact with other atoms by sharing or
transferring electrons that are farthest from the nucleus.
These electrons are sometimes called valence electrons. These
outer electrons determine the chemical properties of the element,
such as how readily it interacts with other elements and the
allowable ratios for its combinations with other substances.
Figure 6: Model of Covalent and Ionic Bonds
An element is a substance made up of a single type of atom.
It can't be broken into simpler components by chemical processes.
There are 92 naturally occurring elements. They may be solids,
liquids, or gases.
Figure 7: Modern Periodic Table
The elements are distributed unevenly, with
some much more common than others. The ten most abundant elements
of the earth's crust make up more than 99% of our planet.
% of Earth's Mass
Figure 8: The Ten Most Abundant Elements
A bar of gold can be shaved into gold
dust, and still be recognizable as gold.
is one example of an element.
How fine can the dust become
and still be considered gold?
particle that would still have the properties associated
with gold is an atom. To get an idea of how small an atom
is, consider that a small gold coin may contain over 20,000,000,000,000,000,000,000
The mass of an atom or particle is expressed
in atomic mass units or amus. One atomic mass unit is a very
small amount of mass. An amu is 1/12 the mass of one atom
of 12C, or about 1.66 x 10-27 kg.
Atomic mass values for elements are almost
never an integer. The only exception is carbon, whose mass
was used as a standard reference. The mass of a 12C
carbon atom is specified as exactly 12. The mass of an atom
of another element is the ratio of its mass to the mass of
a carbon atom. Even the masses of protons, neutrons, and electrons
are ratios of their mass to carbon. These ratios are not integers.
Since atoms are made of various numbers of these particles,
it is unlikely that the mass of an atom other than carbon
would add up to exactly a whole number.
Figure 12: Diagram Comparing Hydrogen,
Deuterium, and Tritium Atoms
When an element has atoms that differ in the
number of neutrons in the nuclei, these atoms are called different
isotopes of the element. All isotopes of one element have
identical chemical properties. This means it is difficult
to separate isotopes from each other by chemical processes.
However, the physical properties of the isotopes, such as
their masses, boiling points, and freezing points, are different.
Isotopes can be most easily separated from each other using
Most atoms of the element hydrogen contain
only one proton in their nuclei. Each of these atoms has a
mass of 1.008 amu. There exist atoms of hydrogen that have
either one or two neutrons in the nucleus in addition to the
single proton. These are called deuterium or tritium, having
masses of 2.014 amu and 3.016 amu respectively. Deuterium
and tritium are isotopes of hydrogen. An atom of deuterium
has two particles in its nucleus, and tritium has three. Since
atoms of both deuterium and tritium have only one proton in
their nuclei, they only have one electron. They behave, chemically,
like other hydrogen atoms.
Figure 13: Mass Number and Atomic Mass of Hydrogen,
Deuterium, and Tritium Atoms
The sum of the number of protons and neutrons
in the nucleus of an atom is called that element's mass number.
This is not the same as the element's mass. Since different
isotopes of an element contain different numbers of neutrons
in the nuclei of their atoms, isotopes of the same element
will have different atomic masses. This was shown above for
the three isotopes of hydrogen. The symbol for an isotope
is the symbol for the element followed by the mass number.
Hydrogen is symbolized as H1, while deuterium is symbolized
What would we call an atom that had three particles
in its nucleus, like tritium, but two were protons and one
was a neutron? This would be an uncommon isotope of a different
element, helium (He3). Because there were two protons in this
nucleus, there would also be two electrons in the probability
cloud around it. Since it is the outer electrons that determine
the chemical properties of an atom, this would be a different
kind of atom than hydrogen. The presence of two rather than
one electron would cause it to have distinctive chemical properties.
Thus, this must be a different element, and it is named helium.
The most common isotope of helium (He4) has two protons and
two neutrons in the nucleus of each atom.
Figure 14: Diagram of Tritium, 3He,
and 4He Atoms
To distinguish between elements, we often refer
to their atomic numbers. The atomic number is the number of
protons in the nucleus of an atom of that element (which is
equal to the number of electrons around that atom's nucleus).
Hydrogen's atomic number is 1, while helium's atomic number
is 2. Gold has an atomic number of 79, which means it has
79 protons in its nucleus. The modern periodic table of the
elements shows the different elements arranged in increasing
order of atomic number.
There are 92 elements found in nature and several
more exotic, manmade elements. Based on their chemical and
physical properties, scientists have invented a tool to show
relationships among these elements. It is known as the periodic
table of the elements.
Figure 15: Periodic Table of the Elements, Los Alamos
Observations show that the same elements exist
throughout the known universe. We organize information about
the elements in the form of a periodic table. The elements
and their interactions are studied in all disciplines of science,
as chemicals form the basis of life science, physical science,
and earth and space science. As the American Association for
the Advancement of Science wrote in their Project 2061, "Science
for All Americans" publication, "All humans should
participate in the pleasure of coming to know their universe