by Ron Kurtus (revised 10 November 2004)
A molecule or compound consists of at least two atoms that are chemically bonded. The chemical formula of a molecule or compound states how many atoms of each element are in one of its molecules.
This formula is similar to an algebraic formula in its use of symbols. The description of a compound with numbers and symbols is called a chemical formula. Some formulae can be quite complex.
Questions you may have include:
- What are chemical compounds?
- What is the numbering system in a chemical formula?
- How do you designate complex compounds?
This lesson will answer those questions.
A molecule is the chemical combination of two or more atoms. They can be of the same element, such as in the oxygen molecule (O2) or different as in the water molecule (H2O).
A compound is a molecule that is made up of at least two different elements. The water molecule is a compound. When atoms of different elements combine to form a compound, the result is a new substance that has different properties than the original elements. A good example is when the poisonous green chlorine gas is combined with the explosive metal sodium to form the white salt crystals we use in our food.
The study of Chemistry is mainly interested in the formation of chemical compounds, since there are so many possible combinations of elements.
Chemical formulas (or more correctly: formulae) are designations of molecules and compounds in shorthand notation, similar to that used in Algebra.
Shorthand for elements
Elements can be written as abbreviations or in a shorthand notation. For example, He denotes helium, Fe denotes iron, and Cl denotes chlorine. A chemical formula is writing the elements of a compound, using their abbreviations.
Designation of a molecule
The combination of two or more elements to form a molecule is designated by writing their abbreviations next to each other. For example carbon monoxide is written as CO. The order in which the elements are written is typically alphabetical, but there are a number of exceptions for historical reasons and to clarify the geometry of the molecule.
Number of atoms in a molecule
If there is more than one atom of a type in the molecule, the formula shows it by a small number after the symbol. For example, water is H2O, which means there are 2 atoms of hydrogen and one atom of oxygen in the molecule. Carbon dioxide is CO2, which means there is one atom of carbon and two atoms of oxygen in the molecule.
Number of molecules
To show the number of molecules, a full sized number is located in front of the molecule. For example 4 molecules of carbon dioxide is designated as:
This means there are a total of 4 C atoms and 8 O atoms in the combination. A way to remember this--taken from Algebra--is to think of it as 4 x (CO2).
Just as in Algebra, you can use parentheses to separate parts in a complex formula. One example is the formula for nitroglycerin, a highly explosive substance.
This formula shows that nitroglycerin consists of 3 atoms of C, 5 atoms of H and then 3 NO3 nitrate ions. If the parentheses were not used, you might have a formula like:
The number of atoms for each element would be correct, but it wouldn't help to describe the true structure of the nitroglycerin molecule.
Remember that molecules are 3-dimensional collections of atoms. In more complex molecules—especially in organic substances—the configuration becomes important.
The number of atoms of each element in a chemical formula is designated by the small number behind each element symbol. If there is no number, it is assumed there is only one of that element. A large number in front of a compound designates how many units there are of that compound. Parentheses can be used to designate a special structure, where other molecules are attached to the larger, complex molecule.
Look for the formula for success
Resources and references
Chemical Formulas - From the University of Waterloo, Canada
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