(C) - Copyright, 2001 F.W. Boyle, Jr, PhD
One more in Dr Boyle's Chemistry Series
One of my favorite phrases is "because we said so!" I use it to
impress upon the minds of those learning chemistry that much of
what the science is based upon is/was decided by those of us who
profess and work in the chemistry. In order to facilitate
communications, there must be agreement, either implicitly or
explicitly, as to how those communications will be
Communication is of the utmost importance. If we all just
grunted or made stick drawings without a common standard,
communication would breakdown and frustration or even combat
might result. Each language of the world has agreed upon a
standard alphabet and spellings of phonemes (speech sounds).
Why? Because to communicate agreement is essential. And so it is
in chemistry that a standard format has evolved.
One of the most important yet basic areas is the use of the
chemical symbol and the various numerical values which are placed
in specific positions around the symbol. Below I have created a
basic symbol box with colored, numbered boxes for this
discussion. The student should spend time committing this
information to memory so that when the student comes upon one of
these written symbols, the student will be able to decipher the
writing in the accepted manner. Boxes and colors are used for
clarity and are not used when writing the formal symbol and its
associated number values.
For this discussion, we will begin with box 1, the large box in
the center of the graphic. Through out this discussion, Box 1 is
the standard position for the alphabetic symbol of the element.
These alphabetic symbols are agreed upon by the Chemistry Union.
As the student hopefully has noticed, the symbols come from the
proper name of the element. For some elements, the substance
name is old and in the early tradition carries a Latin name.
Box 2, placed in the upper righthand area following of the
symbol, is the standard position for writing the charge. The
number with it's sign is always written as a superscript. An
atom which carries a charge is known in Chemistry as an
ION. As a rule, charges range from +1 to +7 or from -1 to
-7. The sign of the charge indicates which subatomic particle is
in excess. A positive sign shows that the ION has more
protons than electrons. A negative sign indicates that the
ION has more electrons than protons. No other information
is authorized to be written in this position.
Box 3, placed in the upper lefthand area in front of the symbol,
is used strictly for indicating he mass of isotopes. Technically
ALL atoms are isotopes even though usage is generally
limited to a discussion of radioactive atoms. The mass of the
atom is written as a whole number in this position. Some
elements have many mass numbers while others have only one or
two. The mass of all elements is dependent primarily upon the
number of protons and the number of neutrons in the nucleus of
the atom. Differences in mass for a specific element are due to
differences in the number of neutrons in the nucleus of that
element. Thus the same element may have different mass
numbers. The letter, A, is sometimes used to represent the mass
number in examples.
Box 4, placed at the lower lefthand area in front of the symbol,
is another isotope position. The atomic number, which is the
number of protons in the nucleus, is written in this position
when formally writing the symbol for an isotope. When dealing
with a given element, this number never changes. In the generic
format, the letter, Z, is used to represent the atomic number.
Box 5, placed at the subscript position following the symbol, is
used only when dealing with molecules to indicate the number of
atoms of the element in the molecule. No other use is allowed.
For example, the number 2 in this position, as in H2O,
tells the reader that the substance contains 2 atoms of hydrogen
in each molecule.
Box 6, placed directly in front of the symbol and at the same
level, indicates area reserved for numerical coefficients.
Values are placed here for balancing reaction equations just as
one would place numbers in front of any mathematical term to
indicate the number of times the term appears in the equation.
Numbers do not appear in front of the symbol when a reaction is
It is my hope that this explanation of the standard use positions
will aid students in deciphering Chemical symbols. Good luck.