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Formulas

10/07/03

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The combination of symbols that represent  the composition of the compounds is called a chemical formula.  Formulas are a form of chemical shorthand which indicates the proportions in which the elements occur in a compound.

H2O Is written as the formula of a molecule of water.  The subscript 2 after the H tells us that there are two atoms of hydrogen in the molecule.  The subscript after the O tells us that only one oxygen atom is present in the water molecule.  The atomic ratio of hydrogen to oxygen in the water molecule is two to one (2:1).

it is customary when writing a formula to put the positive ion first.  Thus, in sodium chloride the Na + proceeds the Cl-.  In writing the formula for any compound, the sum of the charges must total zero.  The final formula of the compound must be electrically neutral.  For example, in NaCl, the 1+charge of Na added to the 1- charge of Cl equals zero.  The assigned charge is called the atom's oxidation number.

When naming positively charged ions (cations) of the elements use the name of the metal, i.e., Na+1 is the sodium ion. Sodium can't be anything else except 1+.  Iron can have either a 2+ or 3+ charge. For this reason we'll talk about iron (II) or iron (III). These would be pronounced "iron two" or "iron three", respectively. Notice that we've added the charge in Roman Numerals in parenthesis at the end of the metal's name. This is the simplest way to eliminate the ambiguity of which iron ion we're talking about.

Naming negatively charged ions (anions) requires some additional work. For the simple anions of the elements we add the suffix "ide" to the name of the element. Often we also have to alter the spelling and pronunciation to allow for ease of pronunciation. For example, Cl- is chloride while O2- is oxide.

There are a few cases in which an element can form simple positive and negative ions. The most common case of this is hydrogen which can be either H+ or H-. In it's 1+ form we will refer to it as the hydrogen ion while in its 1- form it is hydride.

 

Simple Anions
Group 15 Group 16 Group 17
N3- = nitride O2- = oxide F1- = fluoride
P3- = phosphide S2- = sulfide Cl1- = chloride
As3- = arsenide Se2 = selenide Br1- = bromide
  Te2- = telluride I1- = iodide

Simple Binary Ionic Compounds

Binary compounds are compounds formed from two elements. In the present case we are interested in the ionic binary compounds, so these will be compounds of a metal and a nonmetal. Metals tend to form cations and nonmetals tend to form anions. When we combine cations and anions to form molecules the total charge must be zero. Also, convention requires that the subscripts must be expressed the lowest whole numbers.

 

Binary Ionic Compounds
Compound formula Name
NaCl sodium chloride
CaBr2 calcium bromide
AlCl3 aluminum chloride
Ba3N2 barium nitride
Al2S3 aluminum sulfide

Notice in each case the sum of the positive charges equals the sum of the negative charges. For example, in barium nitride the barium cations each have a 2+ charge so that the total positive charge is 3 x (2+) = 6+. The anions have a 3- charge so the total negative charge is 2 x (3-) = 6-. The net result is a total combined charge of zero.

In the case of the cations in the examples above, there is no ambiguity since these ions have only a single common charge. For those cases where there may be more than one charge, we must specify the charge on the metal. The table below gives examples of these cases.

Ionic compounds
FeC3 iron(III) chloride
FeCl2 iron(II) chloride
Cr2O3 chromium(III) oxide
CuCl copper(I) chloride
CuCl2 copper(II) chloride

Polyatomic ions

Oxyanions

Oxygen reacts with many elements to form oxyanions that contain two elements and a number of atoms.  To name these compounds we will use the suffixes "ate" and "ite" in which the "ate" species will have more oxygens than the "ite".  Refer to table E of the reference table for names of oxyanions.

Polyatomic cations

There are only two polyatomic cations that will commonly appear in an introductory course. These are H3O+1, hydronium, and NH4+1, ammonium. These ions are formed by adding a hydrogen ion to water and ammonia, respectively.

Ionic compounds with polyatomic ions

We can form ionic compounds simple ions or polyatomic ions. When we use polyatomic ions we must remember that these ions are units. If we have two nitrate ions in the compound, we must indicate this by putting parenthesis around the nitrate formula and placing a "subscript 2" after the parenthesis. For example, calcium nitrate is Ca(NO3)2. Nitrate has a 1- charge and there must be two of them to combine with the Ca2+ ion. The table below has a number of examples of compounds with several combinations of ions.

Ionic compounds
NaClO3 sodium chlorate Ca(ClO4)2 calcium perchlorate
Al(NO3)3 aluminum nitrate FeSO4 iron(II) sulfate
(NH4)2S ammonium sulfide Cu2CO3 copper(I) carbonate
NaH2PO4 sodium dihydrogenphosphate CaCO3 calcium carbonate

 

Acids

Simple acids are compounds formed by adding one or more H+ to an anion to form a neutral compound. In these cases the resulting compound is called an acid. In the nomenclature of acids, the suffix on the anion changes in a predictable manner. Thus, anions ending in "ide" become "hydro-oric" acids; anions ending in "ate" become "ic" acids; anions ending in "ite" become "ous" acids.

Remember:

He ate some acid.  It tastes icky.  Riteous dude!

 

Bases

The common ionic bases are all very simple in that they are combinations of a cation with the hydroxide anion. The only common base that is different than this is ammonia, NH3.

Check Table K of the reference table for common acids and bases.

 

Naming of Covalent Compounds

Compounds formed by the combination of two or more nonmetals are generally covalent in nature.  The major difference between ionic and covalent compounds for the purposes of nomenclature is that there is only one way for two ions to be combined into a neutral ionic compound, while there may be many possible combinations of the same elements that give covalent compounds. A simple example of this would be combinations of carbon with oxygen. In this case these two elements may form CO, CO2, and C2O3 as well as other less common combinations. Our nomenclature must be able to distinguish between these various compounds.

In naming covalent compounds, we must first assign one of the elements the ending "ide" just as we did for simple ionic compounds. The simple rule is that the element furthest to the right and top of the periodic table is assigned the "ide" ending. Oxygen takes precedence over all elements except fluorine.

Since there may be several combinations of the same two elements, we must have a way to distinguish between them. For binary compounds it is customary to use the prefixes mono, di, tri, tetra, penta, hexa, hepta, octa, nona, and deca to indicate the numbers of each element present in the compounds. In many cases "mono" is not used if there is no confusion in the compounds. Thus we name CO, carbon monoxide (the monocarbon is assumed), and CO2 is carbon dioxide. C2O3 would be named dicarbon trioxide by this convention. Similarly, NO is nitrogen oxide and N2O5 is dinitrogen pentaoxide.

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