Chemical Nomenclature: Part I – IONIC COMPOUNDS
Chemical nomenclature simply means the naming of chemical compounds. Prior to about 1800, when the first systematic approach to giving names to chemical compounds was developed by Antoine L. Lavoisier and his colleagues in France, every known compound had its own specific name. Some were named for their color, such as martial ethiops, which is the black oxide of iron, Fe3O4. Others had Arabic names, such as alcohol; some had Latin names, such as sal ammoniac, NH4Cl. Some were named after the men who discovered them (Glauber's salt, Na2SO4), and some were named after their physical properties (butter of antimony, SbCl3, from its consistency).Principles of Chemical Nomenclature
Modern chemical nomenclature is derived from Lavoisier's rules and follows his major principles, which are:
To each and every compound there can correspond one and only one correct name; to each and every name there can correspond one and only one compound.
The proper name of a compound is the simplest systematic name which can unambiguously specify the compound being spoken of.
Chemical elements, which are the simplest forms into which materials can be resolved by chemical means, react with each other to yield chemical compounds containing various proportions of two or more elements. Therefore the names of compounds should be derived from the names of the elements of which they are composed.
These three principles were originally fundamental to all chemical nomenclature. Later, as chemistry evolved into the two traditional areas of organic and inorganic chemistry, the third principle was retained only for inorganic compounds, for the following reasons.
Organic chemistry is the study of the compounds based on the elements carbon and hydrogen. Organic compounds are named by giving the chain or other structure of the carbon-hydrogen atoms and substituting into that structure the names of any other elements or groups of elements and the particular places at which they are present. It is a position and place naming system. Organic chemistry, including the nomenclature of organic compounds, will be dealt with in these notes much later.
Inorganic compounds are still named according to the principle that the name of an inorganic compound is derived from the names of the chemical elements which make up that compound. There are many more types of inorganic compounds, even though the number of known organic compounds is very much greater than the number of known inorganic compounds.
Naming Binary Inorganic Compounds
Inorganic nomenclature is simplest for binary compounds, those compounds which contain two and only two chemical elements. The name of a compound is derived from the names of its elements and some general agreements among chemists. This gives us the following procedures:
The name of an inorganic compound is made up of the names of the elements of which it is composed.
The elements are always named in the order most metallic to least metallic.
A binary compound could have two possible names depending upon the order in which the two element names were written. Since it is not possible to have two different correct names for the same compound, some (any) one order must be used consistently. Chemists have therefore agreed to always write the names of the elements in the order of most metallic to least metallic (most electropositive to least electropositive). The relative metallic character of an element can most easily be determined by its position in the periodic chart. Elements become more metallic as you move left in the same row and down in the same column. Thus, you can tell that cesium (Cs) is very metallic because it is one of the left-most and lowest elements, while fluorine (F) is least electropositive because it is one of the right-most and highest elements; germanium (Ge) is more metallic than carbon (C) but less metallic than tin (Sn) or lead (Pb). This was the basis for the Pauling scale of electronegativities, which expresses the same thing in quantitative numeric form, and so Pauling electronegativity values can also be used to order elements. The element with the greatest Pauling electronegativity value is the least metallic and the least electropositive. Pauling electronegativities can be found in your textbook or in the Databook Tables.
The name of the second element in a binary compound is modified to end in -ide. The ending - IDE replaces the previous ending of the element name and is not simply added to it; it would therefore be incorrect to write "sodium chlorineide".
Naming Molecules of Simple Binary Compounds
Naming compounds is fairly easy. Every elements symbol starts with a capital and sometimes has a second lower-case letter after it. Look at the molecule and divide it up into the positive and negative elements. Name the positive element first, followed by the negative element ending in IDE.
Example: NaCl is made up of Na and Cl. Na is sodium, Cl is chlorine but in a compound it is modified to chloride. So the name is Sodium chloride.
Example: SrI2 is made up of Sr andI. (The number of I is not important for naming purposes, at least not yet). Sr is strontium and I is iodine which is modified to be iodide. So the name is strontium iodide.
Creating formulas from names
This process is the reverse of the naming process. The steps are:
Identify the elements with their symbols. Write the positive element first followed by the second element.
Look on the periodic table and find the valences of the elements and write them in above and to the right of the symbols as superscripts.
Cross multiply the valences and place the numbers as subscripts below and to the right of the symbol.
Stop and check that the total positive charges and total negative charges balance out to zero (0).
If the numbers generated are divisible by a common denominator then divide them to get the lowest possible numbers.
Erase the superscripts and any ones (1) because a "1" is always assumed.
Example: What is the formula for calcium chloride?
Get the symbols: Ca Cl
Get the valences: Ca+2 Cl-1
Cross multiply: Ca+21 Cl-12
Stop and check: (1 X +2) + (2 X -1) = +2 -2 = 0 Okay!
Erase the superscripts: CaCl2
The formula of calcium chloride is CaCl2
Example: What is the formula for Sodium nitride?
Get the symbols: Na N
Get the valences: Na+1 N-3
Cross multiply: Na+13 N-31
Stop and check: ( 3 X +1) + ( 1 X -3 ) = +3 -3 = 0 Okay!
Erase the superscripts: Na3N
The formula of sodium nitride is Na3N
Polyatomic Ions
These ions are made up from 2 or more different types of atoms. Quite often they have a non-metal atom which is capable of different valences. It is probably best at this time for you to commit these to memory and worry about how they are formed until later.
+1 ions
NH4+ ammonium (Positive ions are always written first in a formula name)
H3O+ hydronium
Example: Ammonium chloride NH4Cl
The rules for putting these together remains the same. There is one additional rule. If you need more than 1 of a polyatomic ion, indicate this with brackets and a subscript.
Example: Ammonium phosphide (NH4)3P
This indicates that you need 3 complete ammonium ions and one phosphide ion.
The superscripts which indicated charges are gone. They have been replaced by subscripts that give the numbers of each kind of ion. The overall charge on the entire molecule is zero (0).
Note: The positive ion is written and named first. This is because it is the most electropositive. The negative ion is written and named second because it is not as electropositive. (As a matter of fact it is very electronegative).
-3 ions
PO43- phosphate
BO33- borate
AsO43-arsenate
The above ions have a -3 charge. Treat them as if they were a single atom ion by placing brackets around them when necessary.
Example: Sodium phosphate Na3PO4
This shows that you need 3 Na+1 ions to balance out the -3 charge of the single phosphate ion.
Example: Magnesium phosphate Mg3(PO4)2
This shows that you need 2 Mg2+ ions for a total positive charge of +6 to balance out the 2 phosphate ions' charges of -6.
-2 ions
B4O72- tetraborate
CO32- carbonate
SO42- sulphate
CrO42- chromate
Cr2O72- dichromate
HPO42- monohydrogen phosphate
Treat these just like you did the -3 ions.
Example: Strontium sulphate SrSO4
If the rules for cross multiplying are followed you could end up with Sr2(SO4)2 as an answer. This is a case where you have a common number between the two groups that can be easily divided. In this case by 2. The strontium ion being +2 exactly balances out the -2 charge on the SO42- ion, so we really only need one of each. No brackets are needed or should be included.
Example: Ammonium dichromate (NH4)2Cr2O4
We need 2 ammonium ions, each with a charge of +1 to balance out the -2 charge on the dichromate ion. Since ammonium is polyatomic we must put brackets around it indicating that we need two complete ammonium ions.
Example: Boron monohydrogen phosphate B2(HPO4)3
Boron ions have a +3 valence and monohydrogen phosphate has a -2 valence. We need 2 boron ions for a total charge of +6 to balance out the 3 X -2 = -6 charge of the monohydrogen phosphate.
-1 ions
BrO3- bromate
OH- hydroxide
CN- cyanide
OCN- cyanate
NO3- nitrate
ClO3- chlorate
MnO4- permanganate
CH3COO- acetate (also written as C2H3O2-)
HCO3- bicarbonate (also known as hydrogen carbonate)
HSO4- bisulphate (also known as hydrogen sulphate)
H2PO4- dihydrogen phosphate
IO3- iodate
definition: ternary - made up of three elements
One class of ternary compounds, the cyanides, are a unique historical anomaly. The cyanide ion, CN-, is very difficult to break apart into its constituent elements. It is also chemically very similar to the halide ions, and for many years it was believed to be another halide ion. As a consequence, even now its salts retain the -ide ending of binary compounds. The compound KCN is universally called potassium cyanide although it actually is a ternary compound. Potassium cyanate is KOCN, a quite different (and quaternary) compound.
The same rules apply for making up formulas and names. Positive ions are written and named first followed by the negative ions.
Example: Sodium cyanide NaCN One sodium +1 ion balances the -1 charge of the CN- ion.
Example: Boron acetate B(CH3COO)3 One boron ion with its charge of +3 will need 3 acetate ions each with a charge of -1 in order to balance to give the overall charge of zero (0).
Example: Strontium permanganate Sr(MnO4)2 One strontium ion with a charge of +2 will need 2 permanganate ions each with a charge of -1 to balance to an overall charge of zero (0).