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A chemical formula is a concise way of expressing information about the
atoms that constitute a particular chemical compound. It identifies each
type of element by its chemical symbol and identifies the number of atoms of
such element to be found in each discrete molecule of that compound. The
number of atoms (if greater than one) is indicated as a subscript.
For example methane, a simple molecule consisting of one carbon atom bonded
to four hydrogen atoms has the chemical formula:
and glucose with six carbon atoms, twelve hydrogen atoms and six oxygen
atoms has the chemical formula:
A chemical formula may also supply information about the types and spatial
arrangement of bonds in the chemical, though it does not necessarily specify
the exact isomer. For example ethane consists of two carbon atoms
single-bonded to each other, each having three hydrogen atoms bonded to it.
Its chemical formula can be rendered as CH3CH3. If there were a double bond
between the carbon atoms (and thus each carbon only had two hydrogens), the
chemical formula may be written: CH2CH2, and the fact that there is a double
bond between the carbons is assumed. However, a more explicit and correct
method is to write H2C:CH2 or H2C=CH2. The two dots or lines indicate that a
double bond connects the atoms on either side of them.
A triple bond may be expressed with three dots or lines, and if there may be
ambiguity, a single dot or line may be used to indicate a single bond.
Molecules with multiple functional groups that are the same may be expressed
in the following way: (CH3)3CH. However. this implies a different structure
from other molecules that can be formed using the same atoms (isomers). The
formula (CH3)3CH implies a chain of three carbon atoms, with the middle
carbon atom bonded to another carbon:
and the remaining bonds on the carbons all leading to hydrogen atoms.
However, the same number of atoms (10 hydrogens and 4 carbons, or C4H10) may
be used to make a straight chain: CH3CH2CH2CH3.
The alkene 2-butene has two isomers which the chemical formula CH3CH=CHCH3
does not identify. The relative position of the two methyl groups must be
indicated by additional notation denoting whether the methyl groups are on
the same side of the double bond (cis or Z) or on the opposite size from
each other (trans or E).
For polymers, parentheses are placed around the repeating unit. For example,
a hydrocarbon molecule that is described as: CH3(CH2)50CH3, is a molecule
with 50 repeating units. If the number of repeating units is unknown or
variable, the letter n may be used to indicate this: CH3(CH2)nCH3.
For ions, the charge on a particular atom may be denoted with a right-hand
superscript. For example Na+, or Cu2+. The total charge on a molecule may
also be shown in this way. For example H3O+
Although isotopes are more relevant to nuclear chemistry than to
conventional chemistry, different isotopes may also be indicated as a
left-hand superscript in a chemical formula. For example, the radioactive
phosphate ion is 32PO4-.