Chemical Nomenclature
Chemical Nomenclature is the systematic naming of chemical compounds. Here we examine just a few rules for chemical substances that can be named relatively easily.
Ionic Compounds
- Name the cation (the more electropositive one) first and the anion (the more electronegative one) second.
- Monoatomic cations take their name from the element name.
- Monoatomic anions take their names from the first
part of the element name and then add
"-ide".
Cation Name H+ Hydrogen Na+ Sodium Al3+ Aluminum Anion Name H- Hydride Cl- Chloride O2- Oxide What are the names for NaCl, MgO, and HBr?
NaCl = sodium chloride
MgO = magnesium oxide
HBr = hydrogen bromide
- When elements form more than one type of ionic
compound Roman numerals are used to indicate the charge
on the cation.
Fe2+ and O2- give FeO = Iron(II) Oxide Fe3+ and O2- give Fe2O3 = Iron(III) Oxide Sn2+ and Cl- give SnCl2 = Tin(II) Chloride Sn4+ and Cl- give SnCl4 = Tin(IV) Chloride There is an older system of naming these compounds that uses the Latin name of the metal with the suffices of "-ic" and "-ous" to designate the higher and lower charge of the metal, respectively.
So the examples above would be, under the older system,
FeO = Ferrous Oxide Fe2O3 = Ferric Oxide SnCl2 = Stannous Chloride SnCl4 = Stannic Chloride Here are some other examples
Ion Latin Name Systematic Name Au+ Aurous Gold(I) Au3+ Auric Gold(III) Cu+ Cuprous Copper(I) Cu2+ Cupric Copper(II)
- For polyatomic ions, you should memorize the names
below.
Ion Name NH4+ ammonium OH- hydroxide CN- cyanide C2O42- oxalate Cr2O72- dichromate NO3- nitrate SO42- sulfate PO43- phosphate ClO- hypochlorite ClO3- chlorate MnO4- permanganate HSO4- hydrogen sulfate or bisulfate Hg22+ Mercury(I) C2H3O2- acetate SCN- thiocyanate CrO42- chromate NO2- nitrite SO32- sulfite CO32- carbonate AsO43- arsenate ClO2- chlorite ClO4- perchlorate HSO3- hydrogen sulfite or bisulfite HCO3- hydrogen carbonate or bicarbonate -
Binary Covalent Compounds
When a pair of elements form more than one type of covalent compound, Greek prefixes are used to indicate how many of each element are in a compound. For example:
| Compound | Name |
|---|---|
| N2O | dinitrogen monoxide |
| NO | nitrogen monoxide |
| N2O3 | dinitrogen trioxide |
| N2O5 | dinitrogen pentoxide |
Some of the Greek prefixes are given in the table below:
| Prefix | Number of Particular Element |
|---|---|
| mono | 1 |
| di | 2 |
| tri | 3 |
| tetra | 4 |
| penta | 5 |
| hexa | 6 |
| hepta | 7 |
| octa | 8 |
Rules for Binary Covalent Compounds
- The prefix mono is never used for naming the first element of a compound.
- The final
o
ora
of a prefix is often dropped when the element begins with a vowel.
For example, for CO the name will be carbon
monoxide, and the final o
of mono is dropped.
Remember, it's only the final o
or a
. So, the
name of ClO2 will be chlorine
dioxide, and no vowels
are dropped.
How do you know which element goes first? The element that comes first in the following list "goes" first.
B, Si, C, Sb, As, P, N, H, Te, Se, S, I, Br, Cl, O, F
Finally, H2O, which according to the rules should be called dihydrogen monoxide is always called water, and NH3, or nitrogen trihydride, is always called ammonia.
Naming Acids, Oxyacids and Their Salts
- If the anion does not contain oxygen, then the acid
is named with the prefix hydro- and the
suffix -ic.
- For example, when gaseous HCl is dissolved in H2O, it forms hydrochloric acid.
- HCN in H2O is hydrocyanic acid.
Before we learn the rule for naming oxyacids, let's learn the rules for naming oxyanions. What are oxyanions? They are anions formed from oxygen and a nonmetal. Here are some examples: ClO4-, ClO3-, ClO2-, ClO-, SO42-, SO32-.
There are two rules for naming these:
- If there are only two members in the same series,
then the anion with the least number of oxygens ends in
-ite, and the anion with the
most ends in -ate.
- For example, SO32- is sulfite and SO42- is sulfate.
- When there are more than two oxyanions in a series,
hypo- (less than) and per- (more than) are used as
prefixes. Here are some examples:
- ClO- is hypochlorite
- ClO2- is chlorite
- ClO3- is chlorate
- ClO4- is perchlorate
- If the anion name ends in -ate, then the acid name ends in -ic or -ric.
- If the anion name ends in -ite, then the acid name ends in -ous.
Here are examples of the last three rules:
| Acid | Anion | Acid Name |
|---|---|---|
| HClO | hypochlorite | hypochlorous acid |
| HClO2 | chlorite | chlorous acid |
| HClO3 | chlorate | chloric acid |
| HClO4 | perchlorate | perchloric acid |