Keep in mind that ionic compounds are formed by two ions of opposite charge and that they are held together by a relatively weak attraction between the ions.  This does not form a molecule!  The resulting unit is known as a formula unit.  NaCl is the chemical formula for the main ingredient of table salt (A small amount of NaI is mixed with the salt to help supply an essential substance, iodine, to our diet).  NaCl represents one formula unit of this substance.  The smallest unit of any ionic compound is one formula unit, while the smallest particle in one formula unit is represented by one of the smallest ions which make it up.  In writing formulas,  the total mathematical charge(s) on an ion must be the same as the other ion in the formula... (except opposite in charge).
If this is not so, then add more ions on one side or the other to make it so.
Example #1:  Write a formula for aluminum chloride.  First, consider the ionic charge of both of these ions (Al⁺³ and Cl^-1).  Now, think about what the charges mean.  Aluminum has a tendency to lose 3 electrons.  Chlorine has a tendency to gain one electron.  Thus it takes 3 chlorine atoms to gain the 3 electrons that one aluminum atom can donate.  When this transfer occurs, 1 aluminum ion will be formed and three chloride ions will be formed.  These particles will all be held together by the opposite electrostatic charges.  The formula is AlCl₃.
Think of writing a formula as looking for a lowest common denominator for the two charges involved.  Then divide this by the charge of each ion in order to determine the subscript for the ion.
Example #2:  Write a formula for cobalt (III) sulfide.  Again consider the ion charges for cobalt(III) and sulfide (Co⁺³ and S^-2).  The lowest common denominator for these two charge numbers is 6.  By dividing the charge number of 3 for Cobalt into 6 we see that we will need 2 cobalt ions.  For sulfur 6/2 results in 3 ions of sulfide.  The formula is Co₂S₃.
...Or use the "crossing over" method for writing a formula.  You still must first know the ions' formulas and charges, but be sure to reduce the final formula down to lowest terms if possible.  Reducing is dependent upon whether or not a subscript is already present in the ion.
Example #3:  Write a formula for aluminum sulfate.  By crossing over the numbers of the charges
of the Al⁺³ ion and the SO₄^-2 ions, we get the formula  Al₂(SO₄)₃.
Example #4:  Write a formula for magnesium oxide.  The ions are charged +2 and -2 respectively.
Thus by crossing over, we would arrive at Mg₂O₂.  However, we must reduce this to lowest terms (an easy matter since neither ion has a subscript already in it).  The final answer to this problem is simply MgO.
Example #5:  Write a formula for calcium peroxide.  The ions are Ca⁺² and O₂^-2.  When crossing over, we get Ca₂(O₂)₂.  However, this should be reduced to CaO₂.  Notice that we did not change the subscript that was already a part of the peroxide ion.
Example #6:  Write a formula for sodium peroxide.  Since the charge of the sodium ion is +1, the formula we get by crossing over is Na₂O₂.  This is the correct formula for this comound since we cannot reduce a subscript which is already set as a part of the original ion.
So, do you understand why the formula for sodium oxide is Na₂O ?

Writing formulas for molecular compounds

The logical first question here would be... "How do I know if a compound is molecular?"
If it is molecular, the atoms will be bonded together by covalent bonds.  The smalllest particle will be a molecule, not an ion or formula unit.  Even though there may be polar charges involved, these will be on the entire molecule and not on the individual atoms which make it up.  Finally, even though we can determine whether or not the electronegativity differences in two atoms are large enough to constitute an ionic bond (this can be calculated mathematically), we can generally tell that a bond is covalent and will form a molecular compound if we notice that it is two non-metals combined.
Once this has been determined, we are basically going to write the formula based on its name.  This must be done because molecular compounds notoriously may have several different possible ratios in which the elements may combine.  Thus the formula for carbon dioxide is based on the prefix on the oxygen and is CO₂.  Carbon monoxide is CO.  It is necessary to learn the common latin prefixes for the different numbers of atoms in formulas, as follows:

mono = one     hexa= six
di = two          hepta = seven
tri = three        octa = eight
tetra = four      nona = nine
penta = five     deca = ten

Try these practice problems by writing formulas for the following compounds and check your answers HERE.
1. potassium hydroxide
2. aluminum acetate
3. aluminum phosphate
4. dinitrogen trioxide
5. nitrogen dioxide
6. iron (III) chloride
7. magnesium bicarbonate
8. diphosphorous pentoxide
9. sulfur dioxide
10. carbon disulfide

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NAMING COMPOUNDS

IONIC COMPOUNDS:
Most of the time, prefixes are not considered here because the ion charges determine an exact number of ions that will "fit" together,  Thus, ionic compounds are easily named by simply putting the names of the two ions together, but you should be aware that anions of single elements consistently end in "ide".  Thus the name of NaCl is sodium chloride and MgO is magnesium oxide, etc.  If the ion is polyatomic, it has a specific name... MgSO₄ is magnesium sulfate.  Note that in the name and in the formula, the cation always comes first. Another consideration is made in the naming of compounds when a metal has 2 or more oxidation numbers.  For example, sometimes copper loses one electron and forms a +1 ion while other conditions may cause it to form a +2 ion.  Therefore, there is a difference in CuCl and CuCl₂ and thse two compounds must have different names based on which from of copper is present.  The names of these two compounds are copper (I) chloride and copper (II) chloride.  This same concept is used in naming compounds of iron, cobalt, nickel, mercury, and several others.

MOLECULAR COMPOUNDS:
The IUPAC rules for naming molecular compunds  when you already know the formula, are different than for ionic compounds.  The names are based on HOW MANY ATOMS of each element are known to be in each molecule of the compound.  So the rules are the same as those for writing a formula when you already know the name.  Thus, AsCl₃. is
arsenic trichloride.  Now obviously this does not include the rules for naming the many categories and thousands of organic compound.  This will be discussed at another time.  CCl₄ is carbon tetrachloride.  Many times the "ide" ending is used even in these molecular compounds even though no ions are present.

ACIDS:
All acids contain ionizable hydrogen which combines with water and produce the hydronium (H₃O⁺¹) ion.  Naming acids is based on whether they are binary or ternary.  Binary acids contain only two elements and always start with "hydro".  The second part of the name is based on the anion and uses either all of the name or the first part of the element name plus the ending "ic".  An example is hydrochloric acid (HCl).  Ternary acids contain oxygen as a part of a polyatomic anion.   They never start with "hydro" but instead begin with the name of the middle element most of the time and end in either "ic" or "ous" depending on whether or not there are two different amounts of oxygen in two similar ions.  "ous" is inferior to "ic" and indicates less oxygen.  For example, the two related ions, SO₄^-2 and SO₃^-2 produce the two acids known as sulfuric acid (H₂SO₄) and sulfurous acid (H₂SO₃) which are derived from the anions 'sulfate' and 'sulfite'.  It gets a little trickier when there are even more possibilities for the amounts of oxygen.  In cases where there are as many as four different amounts of oxygen in a group of related ions, i.e. ClO^-1, ClO₂^-1, ClO₃^-1, and ClO₄^-1, then we must use the naming scheme which allows us to indicate the number of oxygens present.  To solve this,  IUPAC has decided to still use the "ic" and "ous" endings for two of  these and then introduce the prefixes, "hypo" which means under and "per" (short for "hyper") which means above or higher than.
With these ideas in mind, it should make sense that the following acids are thus named:
HClO   -   hypochlorous acid
HClO₂  -  chlorous acid
HClO₃  -  chloric acid
HClO₄  -  perchloric acid
(Others in the halogen group are similarly named)

Now, it's time to try your luck.  What are the names of the following compounds?

1.  HBr (aqueous)    4.  HNO₂           7.  KIO₂            10.  SiO₂             13.  FeCl₃
2.  NaOH                 5.  (NH₄)₃PO₄    8.  Al(O₂)₃        11.  HgO             14.  CaO₂
3.  HFO₄                  6.  N₂O                9.  PCl₃            12.  Hg₂Cl₂         15.  KNO₃
 
>ANSWERS<          .

Chemical Nomenclature
http://www.woodrow.org/teachers/chemistry/links/chem1/Chapter6.html