General Chemistry/Naming Substances
Some compounds have common names, like water for H2O. However, there are thousands of other compounds that are uncommon or have multiple names. Also, the common name is usually not recognized internationally. What looks like water to you might look like agua or vatten to someone else. To allow chemists to communicate without confusion, there are naming conventions to determine the systematic name of a chemical.
Naming Ions and Ionic Compounds
Ions are atoms that have lost or gained electrons. Note that in a polyatomic ion, the ion itself is held together by covalent bonds. Monoatomic cations (positive) are named the same way as their element, and they come first when naming a compound. Monoatomic anions (negative) have the suffix -ide and come at the end of the compound's name.
- Examples of ionic compounds
- NaCl - Sodium chloride
- MgCl2 - Magnesium chloride
- Ca3N2 - Calcium nitride
Polyatomic ions have special names. Many of them contain oxygen and are called oxyanions. When different oxyanions are made of the same element but have a different number of oxygen atoms, prefixes and suffixes are used to tell them apart. The chlorine family of ions is an excellent example.
The -ate suffix is used on the most common oxyanion (like sulfate SO42- or nitrate NO3-). The -ite suffix is used on the oxyanion with one less oxygen (like sulfite SO32- or nitrite NO2-). Sometimes there can be a hypo- prefix, meaning one less oxygen than the -ite. There is also a per- prefix, meaning one more oxygen than the -ate.
||Occasionally, you will see a bi- prefix. This is an older prefix, it means the compound can both take up and lose a proton. It is often replaced with the word hydrogen. In either case, the oxyanion will have a hydrogen in it, decreasing its charge by one. For instance, there is carbonate (CO32-) and bicarbonate or hydrogen carbonate (HCO3-).|
One last prefix you may find is thio-. It means an oxygen has been replaced with a sulfur within the oxyanion. Cyanate is OCN-, and thiocyanate is SCN-.
- Examples of polyatomic ions
- NH4Cl - Ammonium chloride
- K(HCO3) - Potassium hydrogen carbonate
- AgNO3 - Silver nitrate
- CuSO3 - Copper (II) sulfite
In the last example, copper had a roman numeral 2 after its name because the transition metals can have more than one charge. The charge on the ion must be known, so it is written out for ions that have more than one common charge. Silver always has a charge of 1+, so it isn't necessary (but not wrong) to name its charge. Zinc always has a charge of 2+, so you don't have to name its charge either. Aluminum will always have a charge of +3. All other metals (except the Group 1 and 2 elements) must have roman numerals to show their charge.
Common polyatomic ions that you should know are listed in the following table
Further explanation of the roman numerals is in order. Many atoms (especially the transition metals) are capable of ionizing in more than one way. The name of an ionic compound must make it very clear what the exact chemical formula is. If you wrote "copper chloride", it could be CuCl or CuCl2 because copper can lose one or two electrons when it forms an ion. The charge must be balanced, so there would be one or two chloride ions to accept the electrons. To be correct, you must write "copper(II) chloride" if you want CuCl2 and "copper (I) chloride" if you want CuCl. Keep in mind that the roman numerals refer to the charge of the cation, not how many anions are attached.
Common metal ions are listed below and should be learned:
|Lead (II)/Plumbous (most common)||Pb2+|
|Mercury (I) (Note: Mercury (I) is a polyatomic ion)||Hg22+|
There are two systems of naming molecular compounds. The first uses prefixes to indicate the number of atoms of an element that are in the compound. If the substance is binary (containing only two elements), the suffix -ide is added to the second element. Thus water is dihydrogen monoxide. A prefix is not necessary for the first element if there is only one, so SF6 is 'sulfur hexafluoride'. The prefix system is used when both elements are non-metallic.
||If the last letter of the prefix is an a and the first letter of the element is a vowel, the a is dropped. That makes V2O5 divanadium pentoxide (instead of pentaoxide). Similar dropping occurs with mono- and elements beginning with o, as in the case of monoxide. This does not, however, happen with di- and tri-. A molecule containing three iodine atoms would be named triiodide.|
The second system, the stock system, uses oxidation numbers to represent how the electrons are distributed through the compound. This is essentially the roman numeral system that has already been explained, but it applies to non-ionic compounds as well. The most electronegative component of the molecule has a negative oxidation number that depends on the number of pairs of electrons it shares. The less electronegative part is assigned a positive number. In the stock system, only the cation's number is written, and in Roman numerals. The stock system is used when there is a metallic element in the compound. In the case of V2O5, it could also be called vanadium(V) oxide. Knowing that oxygen's charge is always -2, we can determine that there are five oxygens and two vanadiums if we were given the name without the formula.
If an acid is a binary compound, it is named as hydro[element]ic acid. If it contains a polyatomic ion, then it is named [ion name]ic acid if the ion ends in -ate. If the ion ends in -ite then the acid will end in -ous. These examples should help.
- Examples of acid names
- HCl - Hydrochloric acid
- HClO - Hypochlorous acid
- HClO2 - Chlorous acid
- HClO3 - Chloric acid
- HClO4 - Perchloric acid