Nomenclature

1. Nomenclature

Nomenclature is a collection of rules for naming things. Chemical nomenclature is the collection of rules for naming chemicals. Nomenclature is vitally important to the study of chemistry. This lesson describes an approach that is used to name simple ionic and molecular compounds, such as NaCl, CaCO, and NO. The simplest of these are binary compounds—those containing only two elements—but we will also consider how to name ionic compounds containing polyatomic ions and one specific, very important class of compounds known as acids. The nomenclature rules will focus on inorganic compounds—compounds that are composed principally of elements other than carbon.

terms to know

Nomenclature

A collection of rules for naming things.

Binary Compound

A compound that contains only two elements

2. Ionic Compounds

To name an inorganic compound, we need to consider the answers to several questions before determining the exact nomenclature rules for that compound.

brainstorm

First, is the compound ionic or molecular? If the compound is ionic, does the metal form ions of only one type (fixed charge) or more than one type (variable charge)? Are the ions monatomic or polyatomic? If the compound is molecular, does it contain hydrogen? If so, does it also contain oxygen? From the answers to these questions, we can place the compound in an appropriate category and then name it accordingly with the rules for that category of compounds.

2a. Compounds Containing Only Monatomic Ions

The name of a binary inorganic compound containing monatomic ions consists of the name of the cation (the name of the metal) followed by the name of the anion (the name of the nonmetallic element with its ending replaced by the suffix –ide). Some example names of simple binary compounds are shown below.

Example Names of Some Ionic Compounds
NaCl, sodium chloride	NaO, sodium oxide
KBr, potassium bromide	CdS, cadmium sulfide
CaI2, calcium iodide	MgN, magnesium nitride
CsF, cesium fluoride	CaP, calcium phosphide
LiCl, lithium chloride	AlC, aluminum carbide

2b. Compounds Containing Polyatomic Ions

Compounds containing polyatomic ions are named similarly to those containing only monatomic ions—i.e., by naming first the cation and then the anion. The name of the anion is the name of the polyatomic ion. You do NOT change the ending by replacing it with the suffix –ide.

Name	Formula
ammonium	NH
hydronium	HO
peroxide	O
hydroxide	OH
acetate	CHCOO
cyanide	CN
azide	N
carbonate	CO
bicarbonate	HCO
nitrate	NO
nitrite	NO
sulfate	SO
hydrogen sulfate	HSO
sulfite	SO
hydrogen sulfite	HSO
phosphate	PO
hydrogen phosphate	HPO
dihydrogen phosphate	HPO
perchlorate	ClO
chlorate	ClO
chlorite	ClO
hypochlorite	ClO
chromate	CrO
dichromate	CrO
permanganate	MnO4

Some example names of compounds containing polyatomic ions are shown below.

Names of Some Polyatomic Ionic Compounds
KCHO, potassium acetate	NHCl, ammonium chloride
NaHCO, sodium bicarbonate	CaSO, calcium sulfate
Al(CO), aluminum carbonate	Mg(PO), magnesium phosphate

IN CONTEXT

CHEMISTRY IN EVERYDAY LIFE

Ionic Compounds in Your Cabinets

Every day you encounter and use a large number of ionic compounds. Some of these compounds, where they are found, and what they are used for are listed in the table below. Look at the label or ingredients list on the various products that you use during the next few days, and see if you run into any of those in this table or find other ionic compounds that you could now name or write as a formula.

Ionic Compounds	Use
NaCl, sodium chloride	ordinary table salt
KI, potassium iodide	added to “iodized” salt for thyroid health
NaF, sodium fluoride	ingredient in toothpaste
NaHCO, sodium bicarbonate	baking soda; used in cooking (and as antacid)
Na CO, sodium carbonate	washing soda; used in cleaning agents
NaOCl, sodium hypochlorite	active ingredient in household bleach
CaCO, calcium carbonate	ingredient in antacids
Mg(OH), magnesium hydroxide	ingredient in antacids
Al(OH), aluminum hydroxide	ingredient in antacids
NaOH, sodium hydroxide	lye; used as drain cleaner
K PO, potassium phosphate	food additive (many purposes)
MgSO, magnesium sulfate	added to purified water
Na HPO, sodium hydrogen phosphate	anti-caking agent; used in powdered products
Na SO, sodium sulfite	preservative

2c. Compounds Containing a Metal Ion with a Variable Charge

Most of the transition metals and some main group metals can form two or more cations with different charges. Compounds of these metals with nonmetals are named with the same method as compounds in the first category, except the charge of the metal ion is specified by a Roman numeral in parentheses after the name of the metal. The charge of the metal ion is determined from the formula of the compound and the charge of the anion.

EXAMPLE

Consider binary ionic compounds of iron and chlorine. Iron typically exhibits a charge of either 2+ or 3+, and the two corresponding compound formulas are FeCl and FeCl. The simplest name, “iron chloride,” will, in this case, be ambiguous, as it does not distinguish between these two compounds. In cases like this, the charge of the metal ion is included as a Roman numeral in parentheses immediately following the metal name. These two compounds are then unambiguously named iron(II) chloride and iron(III) chloride, respectively. Other examples are provided below.

Compound	Name
FeCl	iron(II) chloride
FeCl	iron(III) chloride
HgO	mercury(I) oxide
HgO	mercury(II) oxide
SnF	tin(II) fluoride
SnF	tin(IV) fluoride

2d. Ionic Hydrates

Ionic compounds that contain water molecules as integral components of their crystals are called hydrates. The name for an ionic hydrate is derived by adding a term to the name for the anhydrous (meaning “not hydrated”) compound that indicates the number of water molecules associated with each formula unit of the compound. The added word begins with a Greek prefix (see below) denoting the number of water molecules and ends with “hydrate.”

EXAMPLE

The anhydrous compound copper(II) sulfate also exists as a hydrate containing five water molecules and is named copper(II) sulfate pentahydrate. Washing soda is the common name for a hydrate of sodium carbonate containing 10 water molecules; the systematic name is sodium carbonate decahydrate.

Formulas for ionic hydrates are written by appending a vertically centered dot, a coefficient representing the number of water molecules, and the formula for water. The two examples mentioned in the previous example are represented by the formulas:

formula to know

copper (II) sulfate pentahydrate

CuSO5HO

sodium carbonate decahydrate

NaCO10HO

term to know

Hydrate

An ionic compound containing water molecules as integral components of their crystals.

3. Molecular (Covalent) Compounds

The bonding characteristics of inorganic molecular compounds are different from ionic compounds, and they are named using a different system as well. The charges of cations and anions dictate their ratios in ionic compounds, so specifying the names of the ions provides sufficient information to determine chemical formulas. However, because covalent bonding allows for significant variation in the combination ratios of the atoms in a molecule, the names for molecular compounds must explicitly identify these ratios.

3a. Compounds Composed of Two Elements

When two nonmetallic elements form a molecular compound, several combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO. Since these are different substances with different properties, they cannot both have the same name (they cannot both be called carbon oxide). To deal with this situation, we use a naming method that is somewhat similar to that used for ionic compounds, but with added prefixes to specify the numbers of atoms of each element.

The name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is first, followed by the name of the more nonmetallic element (the one farther to the right and/or top) with its ending changed to the suffix –ide. The numbers of atoms of each element are designated by the Greek prefixes shown above in the ionic hydrate section.

When only one atom of the first element is present, the prefix mono- is usually deleted from that part. Thus, CO is named carbon monoxide, and CO is called carbon dioxide. When two vowels are adjacent, the –a in the Greek prefix is usually dropped.

Compound	Name		Compound	Name
SO	sulfur dioxide		BCl	boron trichloride
SO	sulfur trioxide		SF	sulfur hexafluoride
NO	nitrogen dioxide		PF	phosphorus pentafluoride
NO	dinitrogen tetroxide		PO	tetraphosphorus decaoxide
NO	dinitrogen pentoxide		IF	iodine heptafluoride

There are a few common names that you will encounter as you continue your study of chemistry.

EXAMPLE

Although NO is often called nitric oxide, its proper name is nitrogen monoxide. Similarly, NO is known as nitrous oxide even though our rules would specify the name dinitrogen monoxide. (And HO is usually called water, not dihydrogen monoxide.) You should commit to memory the common names of compounds as you encounter them.

3b. Binary Acids

Some compounds containing hydrogen are members of an important class of substances known as acids. The chemistry of these compounds is explored in more detail in a later unit of this course. But for now, it will suffice to note that many acids release hydrogen ions, H+, when dissolved in water.

IN CONTEXT

To denote this distinct chemical property, a mixture of water with an acid is given a name derived from the compound’s name. If the compound is a binary acid (comprised of hydrogen and one other nonmetallic element):

1. The word “hydrogen” is changed to the prefix hydro-.
2. The other nonmetallic element name is modified by adding the suffix -ic.
3. The word “acid” is added as a second word.

For example, when the gas HCl (hydrogen chloride) is dissolved in water, the solution is called hydrochloric acid.

Name of Gas	Name of Acid
HF(g), hydrogen fluoride	HF(aq), hydrofluoric acid
HCl(g), hydrogen chloride	HCl(aq), hydrochloric acid
HBr(g), hydrogen bromide	HBr(aq), hydrobromic acid
HI(g), hydrogen iodide	HI(aq), hydroiodic acid
HS(g), hydrogen sulfide	HS(aq), hydrosulfuric acid

terms to know

Acid

A compound containing hydrogen.

Binary Acid

A compound that is comprised of hydrogen and one other nonmetallic element.

3c. Oxyacids

Many molecular compounds containing three or more elements are subject to specialized nomenclature rules. However, we will briefly discuss the important compounds known as oxyacids, compounds that contain hydrogen, oxygen, and at least one other element, and are bonded in such a way as to impart acidic properties to the compound. Typical oxyacids consist of hydrogen combined with a polyatomic, oxygen-containing ion. To name oxyacids:

1. Omit “hydrogen”.
2. Start with the root name of the anion.
3. Replace –ate with –ic, or –ite with –ous.
4. Add “acid”.

EXAMPLE

Consider HCO (which you might be tempted to call “hydrogen carbonate”). To name this correctly, “hydrogen” is omitted; the –ate of carbonate is replace with –ic; and acid is added—so its name is carbonic acid. There are some exceptions to the general naming method (e.g., HSO is called sulfuric acid, not sulfic acid, and HSO is sulfurous, not sulfous, acid).

Formula	Anion Name	Acid Name
HCHO	acetate	acetic acid
HNO	nitrate	nitric acid
HNO	nitrite	nitrous acid
HClO	perchlorate	perchloric acid
HCO	carbonate	carbonic acid
HSO	sulfate	sulfuric acid
HSO	sulfite	sulfurous acid
HPO	phosphate	phosphoric acid

EXAMPLE

Name the following compounds:

(a) FeS (b) CuSe (c) MgSO∙7HO (d) Ti(SO) (e) KO (f) NO (g) ClO (h) HClO

Solution:

According to nomenclature rules, we first need to determine if the compound is ionic or molecular.

Compounds a, b, c, d, and e are ionic because they contain a metal cation. Compounds f, g, and h are molecular

For the ionic compounds, does the metal have a fixed charge or a variable charge?

Compounds c and e have fixed charges, while a, b, and d have variable charges.

For the ionic compounds a through e, the names are:

(a) FeS = iron (III) sulfide (since S has a 2- charge and there are 3 of them = 6- total charge, each iron will be 3+ charge)

(b) CuSe = copper (II) selenide (since Se has a 2- charge, Cu must have a 2+ charge)

(c) MgSO∙7HO = magnesium sulfate heptahydrate

(d) Ti(SO) = titanium (III) sulfate (since SO has a 2- charge and there are 3 of them = 6- total charge, each titanium will be 3+ charge)

(e) KO = potassium oxide

For the molecular compounds f through h, non-acids must include prefixes and acids follow the acid rules. Compounds f and g are non-acids and h is an acid.

For the molecular compounds f through h, the names are:

(f) NO = dinitrogen trioxide

(g) ClO = dichlorine heptoxide

(h) HClO= chloric acid (ClO is chlorate, so chlorate becomes chloric acid)

term to know

Oxyacid

A compound that contain hydrogen, oxygen, and at least one other element, and are bonded in such a way as to impart acidic properties to the compound.

4. Writing Formulas

In every ionic compound, the total number of positive charges of the cations equals the total number of negative charges of the anions. We can use this observation to help us write the formula of an ionic compound. The formula of an ionic compound must have a ratio of ions such that the numbers of positive and negative charges are equal.

EXAMPLE

The formula for magnesium nitride is MgN. Magnesium has a 2+ charge and nitrogen has a 3- charge. For the molecule to be neutral, there must be 3 magnesium atoms (6+) and 2 nitrogen atoms (6-). A simple method is to make the charge of the cation become the subscript of the anion and the charge of the anion to become the subscript of the cation.

If both charges have the same numeric value, then the subscripts are one.

EXAMPLE

The formula for magnesium oxide is MgO. Magnesium has a 2+ charge and oxygen has a 2- charge. But instead of writing MgO, we just write MgO since they have the same magnitude of charge.

Many ionic compounds contain polyatomic ions as the cation, the anion, or both. As with simple ionic compounds, these compounds must also be electrically neutral, so their formulas can be predicted by treating the polyatomic ions as discrete units. We use parentheses in a formula to indicate a group of atoms that behave as a unit.

EXAMPLE

The formula for calcium phosphate, one of the minerals in our bones, is Ca(PO4). This formula indicates that there are three calcium ions (Ca) for every two phosphate (PO) groups.

Molecular compounds use prefixes to indicate how many of each atom there are in the compound. The formula for dinitrogen tetroxide is N O .

make the connection

If you're taking the Introduction to Chemistry Lab course simultaneously with this lecture, it's a good time to try the lab, Ionic and Covalent Bonds in Unit 2 of the Lab course. Good luck!