Chemical Nomenclature

Naming Simple Binary Compounds
Binary compounds 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.
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.
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 and  I. (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.
Stop here and do Nomenclature Exercise #1

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

Stop here and do Nomenclature Exercise #2

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 electropostive. (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.

Stop here and do Nomenclature Exercise #3

-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


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).

Stop here and do Nomenclature Exercise #4
 

Oxidation States and the Stock System

The nomenclature work that you have done so far is all that is needed if only one binary compound of the two elements exists. Many combinations of two elements, however, can result in more than one compound. For example, iron and chlorine react to produce both FeCl2 and FeCl3, which have demonstrably different properties, and so the name iron chloride is ambiguous. Whenever two or more compounds of the same two elements are possible, one of two approaches to modifying the name is used. Both approaches are valid, although one may be more appropriate with a particular compound than the other. Very few compounds are named using both approaches.

The first of these approaches taken when two or more different compounds of the same elements exist is the oxidation state approach, also called the Stock system (after the chemist A.T. Stock). It can be rationalized in the following simple discussion, which is a brief summary of a more elaborate discussion we will take up later. 

 
The oxidation state of an element in a compound is denoted by placing a Roman numeral after the name of the element. Oxidation states are given if and only if they are necessary to make the name unambiguous.

Example. In the case of sodium chloride, oxidation states are unnecessary because only one binary compound of these two elements is known to be +1. It is always +1 and nothing else.  In the case of iron chloride, oxidation states should be used. For FeCl2, the correct name is iron (II) chloride, and, for FeCl3, the correct name is iron (III) chloride. The oxidation state of elemental iron is iron(0); that of Fe2+ is iron (II); and that of Fe3+ is iron (III). The iron in FeCl2 or FeO is also iron (II), while that in FeCl3 or Fe2O3 is iron (III).

Calculation of the oxidation state of an element when it is combined in a compound or ion has many different approaches. It usually depends on who is teaching it and how they first learned it themselves from their teachers. It can be done by the application of one definition and a few general properties of some of the common elements. The definition is as follows:

The sum of the oxidation states of all the elements in a compound is zero; the sum of the oxidation states in an ion (positively or negatively-charged species) is equal to the net charge on the ion.

The properties of the elements used to determine oxidation state are, in order of precedence:
# - The oxidation state of the alkali metals (Group 1) in compounds is normally +1 and the oxidation state of the alkaline earths (Group 2) is normally +2. (There are virtually no exceptions.)
# -  The oxidation state of oxygen in compounds is normally -2. (The only significant exceptions are the peroxides, such as H2O2 and Na2O2, in which the oxygen is in the -1 oxidation state.)
# - The oxidation state of hydrogen in compounds is normally +1. (The only significant exceptions are the saline hydrides, such as LiH, in which hydrogen is in the -1 oxidation state.)
# - The oxidation state of the halogens in compounds is normally -1. (The only significant exceptions are those compounds of the halogens which also include oxygen, such as NaClO4.)
# - The oxidation state of most other elements in chemical compounds can vary, and is obtained by difference using the definition and the general properties of these common elements. Most of the transition metals have several different oxidation states.
# - Transition elements that have only one valence such as Ag+1, Sc+3, Y+3, Zr+4, Tc+7, or Cd+2 do not need brackets indicating their charge. Putting them in is not incorrect but it would be a chemical social faux paus.
 
Example. To name the compound NiO2, first name the elements: nickel oxygen.  Second, change the ending: nickel oxide. Third, calculate the oxidation state of nickel: 0 (compound) = x + 2 (-2); 0 = x - 4;x = +4. This gives the name: nickel (IV) oxide. The oxidation state of oxygen need not be specified.

Example. To name the compound V2O5, first name the elements: vanadium oxygen. Second, change the ending: vanadium oxide. Third, calculate the oxidation state of vanadium: 0 = 2x + 5 (-2);
0 = 2x - 10;   10 = 2x;   x = +5.        This gives the name: vanadium (V) oxide.

There is an older nomenclature for compounds with variable oxidation state traceable to Lavoisier, in which the higher oxidation state of an element is designated by an -ic ending on the element name and the lower by an -ous ending, as vanadous oxide (VO) and vanadic oxide (V2O3). Since this system fails when more than two oxidation states are known for the same element, and the numeric oxidation state designated by -ic or -ous endings changes from element to element, it is obsolete. Modern chemists no longer use it for binary compounds, although traces of it still remain in the nomenclature of less simple compounds. Some commercial manufacturers, however, still label their products in the old way.

The older method was made up based on the few metals that were known in the past. The name also is derived from the old name of the elements.
Element New Name Old Name Possible valences
Fe Iron Ferrum +2,+3
Cu Copper Cuprum +1,+2
Sn Tin Stannum +2,+4
Au Gold Aurum +1,+3
Hg Mercury Hydroargentum +1,+2
Pb Lead Plumbum +2,+4
Sb Antimony Stibbum +3,+5
The "ous"-"ic" System
As you can see above these metals only had two possible valences. A system that distinguishes between only two is fine and that is what the "ous"-"ic" system did.

The lower valence metal had its named changed to end in "ous" while the higher valence metal had its names changed to end in "ic".
Example: Fe+2  ferrous   Fe+3 ferric

  Cu+1 cuprous  Cu+2 cupric

  Sn+2 stannous  Sn+4 stannic

   Au+1 aurous  Au+3  auric*

Pb+2 plumbous  Pb+4 plumbic

  Sb+3      stibbous   Sb+5   stibbic

  Hg+1    mercurous      Hg+2    mercuric**

 
*  Chemistry trivia time: In the James Bond movie Goldfinger who was the villian?   Auric Goldfinger
 
    What was the license plate number on Goldfinger's Rolls Royce?                           AU3
 
    What was the name of Goldfinger's business establishment?                                   Auric Enterprises
 
** Mercury's name was changed because hydroargentous and hydroargentic would be just to much to handle.

This system work's quite well with elements possessing 2 possible valences.   The problem lies in the fact that a few elements have more than two valences.   Vanadium has 4 possible valences (+5, +4, +3, and +2) and manganese has 5 (+7, +6, +4, +3, and +2). Using the "ous"-"ic" system we could only name the first two lowest valences.   It is for this reason that the Stock system is in prominent use today.

Stop here and do Nomenclature Exercise #5

Counting in Greek

This approach is used when naming compounds that are made up of non-metal elements only. The numeric or numbering approach is used when the oxidation state approach either does not unambiguously name the compound . We can't use it with non-metal to non-metal compounds.

Example: NO2 and N2O4,  both of which exist, could both be named nitrogen(IV) oxide.
 
In either case, the alternative naming approach is to give the numbers of atoms of each element in the molecule, using Greek prefixes.
 
The first ten Greek prefixes are:
1    mono
2    di
3    tri
4    tetra
5    penta
6    hexa
7    hepta
8    octa
9    nona
10  deca

The prefix nona is Latin rather than Greek. The Greek prefix, used only in very specialized chemical nomenclature, would be ennea.

Example. The compound CO is carbon mono oxide, or carbon monoxide; CO2 is carbon di oxide, or carbon dioxide; and SO3 is sulfur tri oxide, or sulfur trioxide.

Stop here and do Nomenclature Exercise #6