The Nature of Matter
matter is made of atoms composed of protons, neutrons, and electrons.
The center, or nucleus, of the atom is composed of positively charge
protons and neutral neutrons. The outside of the atom has negatively
charged electrons in various orbits. This is shown schematically in the
picture to the right where the electrons are shown orbiting the center,
or nucleus, of the atom in much the same way that the planets orbit the
sun in our solar system.
All atoms have
the same number of protons (positively charged) and electrons
(negatively charged). Therefore all atoms have a neutral charge (the
positive and negative charges cancel each other). Most atoms have
approximately the same number of neutrons as they do protons or
electrons, although this is not necessary, and the number of neutrons
does not affect the identity of the element.
The number of
protons (atomic number) in an atom determines which kind of atom we
have, and the atomic mass (weight) of the atom is determined by the
number of protons and neutrons in the nucleus (the electrons are so
small as to be almost weightless).
There are over
100 different elements that have been discovered. These are shown in the
Periodic Table of the Elements below. The letter symbols for the
elements come from their Latin names, so for example, H stands for
hydrogen, C for Carbon, O for oxygen, while Fe stands for iron and Cu
stands for copper.
Only a few of the
elements are common, and most corrosion occurs due to only a dozen or so
metallic elements (iron, aluminum, copper, zinc, etc.) reacting with
common nonmetallic elements (oxygen, chlorine, sulfur, etc.).
Ions are formed
when atoms, or groups of atoms, lose or gain electrons.
Metals lose some
of their electrons to form positively charged ions, e.g.
Al+3, Cu+2, etc.
electrons and form negatively charged ions, e.g.
groups of metals and nonmetals that form distinct chemicals. Most of us
are familiar with the formula H2O, which indicates that each
water molecule is made of two hydrogen atoms and one oxygen atom. Many
molecules are formed by sharing electrons between adjacent atoms. A
water molecule has adjacent hydrogen and oxygen atoms sharing some of
Water is the most
common chemical on the face of the earth. It is made of three different
constituents, hydrogen ions, hydroxide ions, and covalently bonded
(shared electron) water molecules. Most of water is composed of water
molecules, but it also has low concentrations of H+ ions and
Neutral water has
an equal number of H+ ions and OH- ions. When
water has an excess of H+ ions, we call the resultant liquid
an acid. If water has more OH- ions, then we call it a base.
We measure the
strength of an acid or a base on the pH scale. pH is defined by the
pH = -log [H+]
explanation of pH and acids and bases is beyond the scope of this web
site. It is sufficient to note that some metals (e.g. zinc and aluminum)
will corrode at faster rates in acids or bases than in neutral
environments. Other metals, e.g. steel, will corrode at relatively high
rates in acids but have lower corrosion rates in most neutral and basic
Even a strong
acid, with a pH of 0, will be less than 1/1000th by weight
hydrogen ions. Neutral water, at a pH of 7, is less than 1 part H+ in 10 million parts covalently bonded water molecules.
device: Many people have a hard time remembering whether an acid or
a base has a high pH number. Just remember that acid comes before base
in the alphabet and that low numbers come before high numbers. Acids
have low numbers (less than 7), bases have high numbers (greater than
7). Neutral waters have pH's near 7 and tend to be relatively
noncorrosive to many materials.
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