Chemical Nomenclature

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.

It would be false to give you the impression that there is one and only one rigid system of naming either organic or inorganic compounds. Since names are derived from an understanding of the nature and the structure of the compounds, nomenclature systems used must allow for improvements in understanding and also in names. On the other hand, nomenclature systems cannot permit excessive changes in names. Imagine the confusion if sequoia trees were called zorkmid trees from January 1, 1960 to January 1, 1970 and oak trees thereafter. It would take a generation or more to sort out the mess. This is exactly the kind of mess we currently have with an older generation who grew up with the pound and the Fahrenheit.  You, who have only known the kilogram and Celsius have no idea of the confusion and frustration these people have gone through. The chemical nomenclature described here is the current nomenclature. It reflects that of the past and you should expect slight alterations in the future.

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.
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


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).

Stop here and do Nomenclature Exercise #4


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.

Oxidation States and the Stock System

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**


For a more complete table click on this link.
*  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.

Writing Formulas for Molecular Compounds
The formulas of many molecular compounds can be predicted using a method similar to the one used for ionic compounds.  The number of electrons that metals and non-metals transfer to become stable ions can be  aclue to th formula of an ionic compound.  Similarily, the number of electron that a non-metal needs to share to become stable is a clue to the number of covalent bonds it can form.  The combining capacity of a non-metal is a measure of the number of covalent bonds that it will need to form a stable molecule.  the combining capacities are listed below.
Combining Capacities of Non-Metal Atoms
4 3 2 1



H
C N O F
Si P S Cl

As Se Br



I
 Carbon has four electrons in its outer valence shell.  Since carbon tends to share rather than gain or lose electrons it needs four more eletrons to fill its shell.  Therefore it has a combining capapcity of 4.  Hydrogen has only 1 electron in its outer valence shell.  Hydrogen needs 1 more electron to fill its shell.  Therefore H has a combining capacity of 1. Then carbon shares its 4 electrons with 1 electron from each of four hydrogens we end up with a formula of CH4.    You can use the combining capacity to write the formulas of molecular compounds without having to consider electronic structure. 

How  would you write the formula of a compound formed between carbon and sulphur?
Rule 1:  Write the symbols, with the left-hand element from the above table first, with the combining capacities.
4       2
C      S

Rule 2:  Crisscross the combining capacities to produce subscripts.
C2S4

Rule 3:  Reduce the subscripts if possible.

The formula C2S4 can be reduced to C1S2


Rule 4:  Any "1" subscript is not needed.
The correct formula is CS2.
 

Stop here and do Nomenclature Exercise #6


PolyAtomic Derivatives
Those inorganic compounds containing (at least) three elements are called ternary compounds. Unfortunately, there is no single system of nomenclature in use for them. The fully systematic I.U.P.A.C. method is not used for most of the common compounds. There are two systems of nomenclature for ternary compounds in use other than the I.U.P.A.C. system. These are the oxygen-acid system devised by De Morveau and Lavoisier used for the majority of the common compounds and the system used for coordination compounds. The first of these will be taken up in the following section, but the latter will be deferred to later in senior chemistry.

The basic premises of the systematic I.U.P.A.C. nomenclature of ternary compounds are similar to the ones for binary compounds. Again, the names of the compounds are made up of the names of the elements of which they are composed. Again, the elements are named in the order most metallic to least metallic. However, while the name ending for a binary compound is -ide, the name ending for a more complex inorganic (ternary, quaternary, etc.) compound is -ate.

The I.U.P.A.C. method for naming ternary compounds is to take a binary-type name using two elements, alter the ending to -ate to designate it as a non-binary compound, and then specify the number and type of added atoms as a prefix to the name or names of the element(s) to which they are attached. The number of atoms is given using the Greek prefixes. A few examples suffice to give this method.

Example. The compound NaClO4 is named sodium tetraoxochlorate (VII) because the number of oxygens is four, giving tetra oxo; the oxidation state of chlorine should be specified as it is here. Other examples are NaAlCl4, sodium tetrachloroaluminate (III); NaAlCl3F, sodium monofluorotrichloroaluminate (III); Na2SO4, sodium tetraoxosulfate (VI); and Na2SO3, sodium trioxosulfate (IV).

Prefixes of other elements used in this way generally end in -o; a few prefixes (aqua, H2O; hydroxo, OH-) give names of simple multi-element groups. This is the system used in naming complex co-ordination structures and as you can see it's cumbersome.

Ternary Inorganic Compounds: Oxygen Acids

The older oxygen acid method of naming inorganic compounds described in this section is much more commonly used than is the systematic I. U. P. A. C. method. It differs from the I. U. P. A. C. method in that the number of oxygens is not given directly (no oxo substituents), and oxidation states are not given. Instead, endings change from -ate, and prefixes are introduced, depending upon the relative numbers of oxygen atoms present.

Virtually all of the common ternary compounds have oxygen as one of their components. Lavoisier believed that all acids, and therefore all salts of acids as well, contained oxygen, and his system of nomenclature was unfortunately based on this erroneous idea. The other elements in the compound were therefore named but the oxygen was not. Instead, the amount of oxygen present was denoted by changes in the form of the name.

The hypo-ite, ite, ate, per-ate system
Take a polyatomic ion. For example ClO3which is the chlorate ion.

It's name ends in "ate".   Add an oxygen to it and you get ClO4-.   Its name is adjusted to indicate that a new oxygen has been added by putting a prefix "per" on the name.  So ClO4-  is perchlorate.

If we take an oxygen away from the ClO3-  ion we get ClO2-    the chlorite ion.

If we take another oxygen away from ClO2- we get ClO- the hypochlorite ion.

Note that the ions did not change their charge. Only the number of oxygens changed and the name changed to reflect it.

If you know one of the "ate" or "ite" or even "per-ate" ions then you can manufacture the other three.  (The other three may not exist in nature, but you can still create them on paper for naming purposes.)

Example: AsO5-3 is the perarsenate ion. It is "per-ate" so you can't add more oxygen. But you can subtract 3 oxygens one at a time and get three new ions.

AsO4-3 would be arsenate;  AsO3-3 would be arsenite and AsO2-3 would be hypoarsenite.

Try it on your own:  Bromate is BrO3- Name and write the other three ions based on it.

Compounds can be named using these new ions and following the general rules of combining ions.

Example. The sodium salts of the oxyacids of chlorine are named as given below:
Remember that Na is a +1 ion and the chlorine oxy-acids are all -1.
NaClO4, sodium perchlorate (higher oxygen content)
NaClO3, sodium chlorate (normal oxygen content)
NaClO2, sodium chlorite (lower oxygen content)
NaClO, sodium hypochlorite (even lower oxygen content)

The compound name changes its ending and/or adds a prefix to denote the relative oxygen content. The prefix per- means higher oxygen content and the prefix hypo- means lower oxygen content. The oxygen content as specified is relative, not absolute; thus NaClO4 with four oxygens is sodium perchlorate while Na2SO4 with four oxygens is sodium sulfate (and so Na2SO3 is sodium sulfite).

Unfortunately, chemists rarely use the I.U.P.A.C. system for the common compounds but retain this older oxygen-acid nomenclature. The I.U.P.A.C. system is reserved for compounds less well known or of more complex structure. Since the oxygen-acid nomenclature of ternary compounds does not give the absolute number of oxygens involved, this must be derived from experience.

Example. Some of the more common sodium salts are named as follows:
NaNO2, sodium nitrite;          NaNO3, sodium nitrate
NaPO3, sodium phosphite;  Na3PO4, sodium phosphate
Na3AsO3, sodium arsenite; Na3AsO4, sodium arsenate
Na2SO3, sodium sulfite;       Na2SO4, sodium sulfate
Na2CO3, sodium carbonate (no carbonite is known)
Na3BO3, sodium borate       (no borite is known)
Na4SiO4, sodium silicate     (no silicite is known)

The salts of the halogens fluorine, bromine, and iodine generally follow the pattern of the chlorine salts given earlier. In general, salts of elements follow the same pattern going down a column of the periodic chart - that is, elements of the same group follow the same pattern. While this is generally true it is not always so; phosphorus, arsenic, and antimony follow the same pattern, but nitrogen does not, and silicon and germanium follow the same pattern, but carbon does not.

Manganese and ruthenium do not follow these rules. Their negative ions are named as follows:
KMnO4 is potassium permanganate, 
K2MnO4 is potassium manganate;
KRuO4 is potassium perruthenate, 
and K2RuO4 is potassium ruthenate.

Although the number of oxygens does not rise in going to these "per" compounds the oxidation state of the central transition metal atom does.

The compounds of chromium are unusual in a different way; K2CrO4 is potassium chromate and K2CrO3 is potassium chromite, but K2Cr2O7 is called potassium dichromate.   We also name KOCN as potassium cyanate by analogy with the cyanide ion, and K2C2O4 is potassium oxalate because it is the potassium salt of the organic acid called oxalic acid.

Stop and do Nomenclature Exercise #7

The Binary Acids and Oxyacids
Acids are compounds that contain hydrogen. Hydrogen acts like a metal because it tends to lose its one lone electron very easily.  It is a gas, not a solid like other metals, only because of its very low atomic mass.

The binary acids are those that contain hydrogen and one other element only. Name it normally then drop the "ide" ending and add "ic acid" to get the proper name.

Example:    HF     Hydrogen fluoride     becomes    hydrofluoricacid

Example:    HCN   Hydrogen cyanide    becomes   hydrocyanic acid


The oxyacid compounds, can either be named as hydrogen salts (Example: HClO4, hydrogen perchlorate), a correct modern system which has been endorsed by I.U.P.A.C.    The older oxygen acid method in which the names of the acid and its salts are related. The two systems of names would give, for the oxyacids of chlorine, the following list (older oxygen acid method first):

This is the "hypo-ous, ous, ic, per-ic" system for acids.

If a compound should be named "per-ate" change its name to "per-ic acid"; if the compound just ends in "ate" then the name changes to "ic acid". The pattern follows in the examples below.

  • HClO is named hypochlorous acid OR hydrogen hypochlorite
  • HClO2 is named chlorous acid OR hydrogen chlorite
  • HClO3 is named chloric acid OR hydrogen chlorate
  • HClO4 is named perchloric acid OR hydrogen perchlorate

  • Have you noted the essential difference between naming acids and everything else?
    The only difference between the names is the replacement of hydrogen -ate by -ic acid and the replacement of hydrogen -ite by -ous acid.

    Since sulfur and oxygen are similar in their chemistry, sulfur can sometimes replace oxygen in a ternary compound. The replacement of one oxygen with one sulfur is denoted by the additional prefix thio. Multiple replacement, which is rare, is denoted by additional numeric prefixes as dithio or trithio.

    Example. The compound NaOCN, sodium cyanate, becomes NaSCN, sodium thiocyanate, on replacement; Na2SO4, sodium sulfate, becomes Na2S2O3, sodium thiosulfate.

    Stop here and do Nomenclature Exercise #8