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Compounds

A compound is a substance formed from the chemical reaction of two or more elements.  Compounds differ from each other in by the ratio or atoms joined together.  Remember a ratio is a fixed relationship between two things.

For example the compound water is the combination of two Hydrogen atoms with one Oxygen atom.  This forms a molecule of water.  If you combined two Hydrogen atoms with two Oxygen atoms you'd get hydrogen peroxide which is quite different than water.  It's ration is 2:2 while with water it was 2:1.  This is very important and why we must write the chemical formula correctly. 

Atoms have things we need to know about them in order to write the chemical equations correctly.  Atoms have subatomic particles called Protons, Neutrons and Electrons.  The Protons and Neutrons reside in the center or Nucleus of the atom while the electrons travel in shells around the nucleus of the atom. 

Protons have a positive electrical charge and electrons a negative charge.  (Remember the Proton Positive PP)  Atoms differ from one another by the number of protons in their nucleus.  Protons, though have about the same mass as protons bear no electrical charge. 

An element with an equal number of protons and electrons is said to be a stable elements.  You'll notice when we move to balancing equations that this isn't always the case.  Once we combine the element with another element we change this by balancing out the negative and positive charges.

A defining characteristic of a compound is that it has a chemical formula. Formulas describe the ratio of numbers of atoms in a substance. For example, in H₂O (water) there are two hydrogen atoms for every one oxygen atom. The formula does not tell you that water is made of molecules. Indeed water ice has the same formula, but it is in the form of a crystal - there are no molecules in ice.

Compounds may have a number of possible phases. For a compound to be a liquid or a gas and still be called a compound, atoms from the various elements must be stuck together in the form of molecules. The formation of molecules is why compounds such as C₂H₄ exist (rather than just CH₂) - the formula is telling you not just the ratios but also how many atoms there are in each molecule.

All compounds will break up into smaller compounds or individual atoms if you heat them to a high enough temperature. This temperature is called the decomposition temperature.

Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number.

Types of compounds:

See list of compounds for a list of all compounds currently in Wikipedia.

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