Compound: A substance that contains atoms of different elements, which is formed when different elements chemically react with one another. The relative proportions of two or more elements in a compound never vary, so that the substance can be identified by the proportions of these elements.

The atoms of certain elements have affinities for each other, so that they tend to bind together in specific ways to form compounds, and these compounds will always have the same composition regardless where they are located in nature. One very important example of a compound is water, which always contains two atoms of hydrogen and one atom of oxygen (H2O). The properties of elemental atoms, or even elemental molecules, such as hydrogen gas, which contains molecules of H-H (H2) are vastly different than the properties of compounds. For instance, though hydrogen gas is extremely flammable, water is not flammable at all, even though both hydrogen gas and water are both made up of molecules which contain two hydrogen atoms. The same can be said of the properties of pure oxygen versus water.

Table salt is also a compound, made of sodium and chlorine atoms. Sodium is a very soft, silver element which is extremely reactive is its pure state. An extremely explosive reaction will occur if elemental sodium comes into contact with water, and even moisture in the air could cause a reaction. Chlorine is a highly toxic yellow-green gas. When these two elements react with each they form crystals of salt or sodium chloride (NaCl) These crystals are white and easily dissolve in water, are non-toxic, and are a principle means by which we get sodium in our diet.

big sodium explosion (reaction) from water added to 3lbs of elemental sodium

This is what happens when water is added to 3lbs of elemental sodium

Some elements can only combine with one another in a single way to make one specific chemical compound. Sodium chloride molecules are an example of this type of compound. Sodium atoms always combine with chlorine atoms in a 1:1 atomic ratio to form sodium chloride. Most elements are not so restricted and combine with one another in more than one way. For instance, whereas water is formed when two atoms of hydrogen join with one oxygen, under special conditions, another compound can form which contains two atoms of hydrogen and two atoms of oxygen. This is called hydrogen peroxide. Water and hydrogen peroxide are both formed from the exact same elements, but the amount of the different elements and their structures given them completely different chemical properties. Water is much more stable in most conditions and is easy to form and hard to destroy. Hydrogen peroxide, however, is extremely reactive, is not made as often (although it forms easily in the right conditions), and is easy to destroy. And although water is essential to life on Earth, hydrogen peroxide is destructive, even in small quantities.

water molecule

Water molecule.

Compounds have chemical formulas that give the number of atoms for each element in the substance. As above, the formula for salt, or sodium chloride is NaCl, meaning one atom of sodium and one atom of chloride. Sucrose, or common table sugar, contains 12 carbon, 22 hydrogen, and 11 oxygen atoms, so the formula is written as C12H22011.

Whereas the chemical formula gives only the number of atoms of each element in one molecule of a compound, the structural formula gives the number of atoms and also shows how the atoms are connected to one another. The atoms are held together by attractive forces called bonds which are the result of an attraction between a positively charged atomic nuclei and and negatively charged electrons. A bond is generally a pair of electrons shared between two atoms. A structural formula represents these bonds with lines drawn between elemental symbols to represent the type of bond. A structural formula is more informative than a chemical formula, as the structure of a compound is what determines its behavior. The structural formula of sucrose is shown below:

structural formula of sucrose

Structural Formula of Sucrose

This can also be shown as a more compact line drawing, with types of bonds between elements represented by one, two, or three lines to denote single, double, or triple bonds:

line drawing of structural formula of sucrose

Line Drawing of Structural Formula of Sucrose

The ball-and-stick model is often used to represent, in a simple fashion, the 3 dimensional shape of a compound's molecule. The three dimensional shape of a molecule is important because it is the shape of a molecule that helps determine its chemical reactivity, and even its odor. A sucrose ball-and-stick model is show below:

3D ball-and-stick model of a sucrose molecule

Ball-and-stick model of a Sucrose Molecule

Organic compounds have a backbone of carbon atoms arranged as a chain or ring structure, which inorganic compounds do not share.

For more information see Chemistry: The Molecular Science by John W. Moore, et al. (available used).

This page created 01 Mar 2013 03:46
Last updated 27 Feb 2016 23:04

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