General Chemistry/Naming Substances

Substances with carbon and hydrogen are organic compounds. They have special names that are beyond the scope of this book. For more information, see the Organic Chemistry Wikibook.

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

Notice that there is no need to write how many ions there are. Between the periodic table and our knowledge of ionic bonding, we can determine the number of ions, based on which elements are used.

Polyatomic Ions

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, then prefixes and suffixes are used to tell them apart. The chlorine family of ions is an excellent example.

Name	Formula
Chloride	Cl-
Hypochlorite	ClO-
Chlorite	ClO2-
Chlorate	ClO3-
Perchlorate	ClO4-

The -ate suffix is used on the most common oxyanion (like sulfate SO₄^2- or nitrate NO₃^-). The -ite suffix is used on the oxyanion with one oxygen atom fewer (like sulfite SO₃^2- or nitrite NO₂^-). Sometimes there can be a hypo- prefix, meaning one oxygen atom fewer than for -ite. There is also a per- prefix, meaning one more oxygen atom than an -ate molecule has.

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 most of 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

Name	Formula
Acetate	C2H3O2-
Ammonium	NH4+
Carbonate	CO32-
Cyanide	CN-
Cyanate	CNO-
Thiocyanate	CNS-
Hypochlorite	ClO-
Chlorite	ClO2-
Chlorate	ClO3-
Perchlorate	ClO4-
Hypobromite	BrO-
Bromite	BrO2-
Bromate	BrO3-
Perbromate	BrO4-
Hypoiodite	IO-
Iodite	IO2-
Iodate	IO3-
Periodate	IO4-
Nitrite	NO2-
Nitrate	NO3-
Peroxide	O22-
Permanganate	MnO4-
Sulfite	SO32-
Sulfate	SO42-

In older texts, ions were assigned names based on their Latin root and a suffix. Common ions with this naming system include "plumbous/plumbic" for lead(II)/lead(IV) and "ferrous/ferric" for iron(II)/iron(III). These Latin-based names are outdated, so it's not important to learn them. We now use the Stock system instead.

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:

Name	Formula
Iron(II)/Ferrous	Fe2+
Iron(III)/Ferric	Fe3+
Copper(I)/Cuprous	Cu+
Copper(II)/Cupric	Cu2+
Tin (II)/Stannous	Sn2+
Tin (IV)/Stannic	Sn4+
Lead (II)/Plumbous (most common)	Pb2+
Lead (IV)/Plumbic	Pb4+
Mercury (I) (Note: Mercury (I) is a polyatomic ion)	Hg22+
Mercury (II)	Hg2+

Naming molecules in chemistry

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.

Number	Prefix
1	Mono-
2	Di-
3	Tri-
4	Tetra-
5	Penta-
6	Hexa-
7	Hepta-
8	Octa-
9	Nona-
10	Deca-
11	Undeca-
12	Dodeca-

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 could determine that there were five oxygens and two vanadiums, if we were given the name without the formula.

Naming acids

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