The formula will identify all the atoms making up the chemical, and their number and relative order. The atom are identified using their [chemical symbol]?. For example methane, a simple molecule consisting of one carbon atom bonded to four hydrogen atoms is given the chemical formula of CH4. Note that the number of atoms of hydrogen are in sub-text.
A slightly more complex molecule, ethane?, consists of two carbon atoms bonded singly to each-other, and each having three hydrogen atoms bonded to them. The chemical formula for this molecule is CH3CH3. If there was a double bond between the carbon atoms (and thus each carbon only had two hydrogens), the chemical formula may be written thus: 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. The two dots indicate that there is a double bond between the atoms that are either side of them.
A triple bond may be expressed with three dots, and in the case where there may be ambiguity, a single dot 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. Note however that 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:
C-C-C
|
C
with the remaining bonds on the carbons all bonded to hydrogen molecules. However the same number of atoms (10 hydrogens and 4 carbons) may be used to make a straight chain: CH3CH2CH2CH3.
Polymers
In the case of polymers, brackets are placed around the repeating unit. For example, in the case of polythene?: CH3(CH2)50CH3, would indicate a molecule of polythene 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.
Ions
In the case of ions, the charge on a particular atom may be denoted in superscript. For example Na+, or Cu2+. The total charge of a molecule may also be shown in this way. For example H3O+
See Also:
empirical formula