ELEMENTS, COMPOUNDS, AND CHEMICAL FORMULAS

The Atomic Theory, introduced on a scientific basis by John Dalton in 1806, revolutionized the way that humans thought about matter in the universe.  It created a view of matter that was elegant in its simplicity - all of the changes that take place around us every day are simply due to rearrangements of nature's fundamental building blocks, which we call atoms

ELEMENTS
SUBSTANCES
CHEMICAL FORMULAS


ELEMENTS

There are a limited number of different types of atoms (92 that occur in nature), from which all of the millions of substances around us are "built."  Each different type of atom (called an element) is represented by a unique symbol.  One interesting fact is that John Dalton himself (who was a bit idiosyncratic) wished to use pictorial symbols for the elements, using pictures that he thought represented some aspect of the chemical or physical behavior of that particular element.  He was overruled in favor of using various combinations of letters as the elemental symbols (less creative perhaps, but a lot faster to write and more consistent from one person to the next).  A list of the names and symbols of all of the known elements is provided just inside the front cover of the textbook.


Examples:          H     Hydrogen             O     Oxygen             He    Helium             Na     Sodium


It is important to note that some of the elements are represented by 2-letter symbols (e.g., He).  In these cases, only the first letter of the symbol is capitalized.  This allows one to distinguish between things such as Co (the element cobalt) versus CO (a compound consisting of the two elements carbon and oxygen).  Also, note that some elements have symbols that don't seem to correspond to their names (such as Na for sodium).  This is because these elements have been known for quite a long time, and the symbol usually corresponds to an earlier name in another language, often Latin or Greek (for example, sodium is still known in many countries by its earlier Latin name, natrium).




SUBSTANCES:  ELEMENTS & COMPOUNDS

The term element can be used in two different ways, which can at times be confusing.  The term can be used to describe the type of atom (e.g., water is a compound composed of the elements hydrogen and oxygen).  However, it can also be used to describe a substance that is composed of only one type of atom (e.g., air is a mixture of the elements nitrogen and oxygen).  These two statements imply very different things from an atomic perspective, as illustrated below.


Water molecules with two hydrogen atoms and one oxygen atom.  Mixture of nitrogen and oxygen molecules.  Each molecule has two atoms.


As we can see, water is a single pure substance--it consists of only one type of molecule.  Each of these molecules consists of 2 hydrogen atoms and 1 oxygen atom.  Because water is composed of more than one type of atom, it is called a compound.  Air, on the other hand, is a mixture--it consists of two different types of molecules (i.e., two different substances).  Each substance (each molecule) is an element, because it consists of only one type of atom.  Note that nitrogen and oxygen both exist as what we call diatomic (meaning 2 atoms) molecules.  Other diatomic elements are hydrogen, fluorine, chlorine, bromine, and iodine.

One important implication of this is that if the compound water is split into its two elements hydrogen (H2) and oxygen (O2), it always results in twice as much hydrogen as oxygen.  (Note, by the way, that this is a chemical change.)  If, on the other hand, air is separated into its constituent substances nitrogen (N2) and oxygen (O2), we would usually get about 21% oxygen, but we could get less (for example, if the air is from someone's exhaled breath) or more (for example, if the air is from an 'oxygen bar').  There is no specific ratio of nitrogen to oxygen required for a mixture of these two gases.  The same is true for a mixture of salt and water (or for any mixture).





CHEMICAL FORMULAS

Chemists use a system of symbolic notation that allows them to communicate about the chemical identity and behavior of matter in a very efficient manner.  In addition to incorporating the elemental symbols discussed earlier, the system also makes use of numbers in two different ways, as indicated below.


hydrogen molecule, H2 and water molecule, H2O


Subscripts are used to indicate the number of each type of atom in a molecule.  For example, hydrogen (H2) molecules consist of 2 hydrogen atoms.  Water (H2O) molecules each consist of 2 hydrogen atoms and 1 oxygen atom (note that if no subscript is written, a 1 is implied).


3 water molecules, 3 H2O


Coefficients, which are written in front of the chemical formula of a substance, are used to indicate the number of molecules.  For example, 3 H2O indicates the presence of 3 water molecules.  Note that to determine the number of each type of atom in the collection of molecules, one need only multiply the coefficient and the subscript.  In other words, in
3 H2O there are 3 x 2 = 6 hydrogen atoms, and 3 x 1 = 3 oxygen atoms.