Naming Compounds

Naming Ionic Compounds

An ionic compound is named first by its cation and then by its anion.

Learning Objectives

Convert between the chemical formula of an ionic compound and its name

Key Takeaways

Key Points

  • Most cations and anions can combine to form neutral compounds (typically solids under normal conditions) that are usually referred to as salts.
  • The net charge of an ionic compound must be zero. Therefore, the number of cations and anions in an ionic compound must be balanced to make an electrically neutral molecule.
  • When naming ionic compounds, the cation retains the same name as the element. The anion’s name is similar to the elemental name, but the ending of the name has been removed and replaced with “-ide.”
  • If a metallic element has cations of different charges, which cation is used has to be indicated by its suffix (an older method) or by Roman numerals in parentheses after its name in writing (the Stock system ).

Key Terms

  • Stock system: A system of naming that includes using Roman numerals to indicate the charge on transition metals.

In chemistry, an ionic compound is a chemical compound in which ions are held together by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ionic compounds have high melting and boiling points, and they tend to be hard and brittle.

Ions can be single atoms, as the sodium and chlorine in common table salt (sodium chloride), or more complex (polyatomic) groups such as the carbonate in calcium carbonate. But to be considered an ion, they must carry a positive or negative charge. Thus, in an ionic bond, one ‘bonder’ must have a positive charge and the other a negative one. By sticking to each other, they resolve, or partially resolve, their separate charge imbalances. Positive to positive and negative to negative ionic bonds do not occur.

Most cations and anions can combine to form solid compounds that are usually known as salts. The one overriding requirement is that the resulting compound must be electrically neutral: therefore the ions Ca2+ and Br combine only in a 1:2 ratio to form calcium bromide, CaBr2. Because no other simpler formula is possible, there is no need to name it “calcium dibromide.” CaBr2 can be named using either the Stock method or the older, classic way of naming.

For example, CuCl2 indicates a molecule where one Cu2+ cation associates with two Cl anions to form a neutral compound. Its systematic name is copper (II) chloride, where copper’s oxidation number is indicated in parentheses. Its older name is cupric chloride.

The Stock Method of Naming

An ionic compound is named first by its cation and then by its anion. The cation has the same name as its element. For example, K+1 is called  the potassium ion, just as K is called the potassium atom. The anion is named by taking the elemental name, removing the ending, and adding “-ide.” For example, F-1 is called fluoride, for the elemental name, fluorine. The “-ine” was removed and replaced with “-ide.” To name a compound, the cation name and the anion named are added together. For example, NaF is also known as sodium fluoride.

If either the cation or the anion was a polyatomic ion, the polyatomic ion name is used in the name of the overall compound. The polyatomic ion name stays the same. For example, Ca(NO3)2 is called calcium nitrate.

For cations that take on multiple charges (typically transition metals), the charge is written using Roman numerals in parentheses immediately following the element name. For example, Cu(NO3)2 is copper (II) nitrate, because the charge of two nitrate ions (NO3−1) is 2(-1) = -2. Since the net charge of the ionic compound must be zero, the Cu ion has a 2+ charge. This compound is therefore, copper (II) nitrate. The Roman numerals in fact show the oxidation number, but in simple ionic compounds this will always be the same as the metal’s ionic charge.

The Old, Classic, or Common Way of Naming

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Names of some ionic compounds: Common, or trivial, names of compounds are sometimes used in informal conversations between chemists, especially older chemists. Systematic names are formal names that are always used in print.

Since some metallic elements form cations that have different positive charges, the names of ionic compounds derived from these elements must contain some indication of the cation charge. The older method uses the suffixes -ous and -ic to denote the lower and higher charges, respectively. In the cases of iron and copper, the Latin names of the elements are used (ferrous/ferric, cuprous/cupric). This system is still used, although it has been officially supplanted by the more precise, if slightly cumbersome, Stock system. In both systems, the name of the anion ends in -ide.

Naming Compounds – Part 1 – YouTube: This video explains how to name covalent and ionic compounds.

Naming Molecular Compounds

Molecular compounds are named using a systematic approach of prefixes to indicate the number of each element present in the compound.

Learning Objectives

Apply the rules for naming molecular compounds

Key Takeaways

Key Points

  • In nomenclature of simple molecular compounds, the more electropositive atom is written first and the more electronegative element is written last with an -ide suffix.
  • The Greek prefixes are used to dictate the number of a given element present in a molecular compound.
  • Prefixes can be shortened when the ending vowel of the prefix “conflicts” with a starting vowel in the compound.
  • Common exceptions exist for naming molecular compounds, where trivial or common names are used instead of systematic names, such as ammonia (NH3) instead of nitrogen trihydride or water (H2O) instead of dihydrogen monooxide.

Key Terms

  • nomenclature: A set of rules used for forming the names or terms in a particular field of arts or sciences.
  • electronegative: Tending to attract electrons within a chemical bond.
  • electropositive: Tending to not attract electrons (repel) within a chemical bond.

Chemical Nomenclature

The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity concerning to what chemical compound the name refers. Each chemical name should refer to a single substance. Today, scientists often refer to chemicals by their common names: for example, water is not often called dihydrogen oxide. However, it is important to be able to recognize and name all chemicals in a standardized way. The most widely accepted format for nomenclature has been established by IUPAC.

Molecular compounds are made when two or more elements share electrons in a covalent bond to connect the elements. Typically, non-metals tend to share electrons, make covalent bonds, and thus, form molecular compounds.

Rules for Naming Molecular Compounds:

  1. Remove the ending of the second element, and add “ide” just like in ionic compounds.
  2. When naming molecular compounds prefixes are used to dictate the number of a given element present in the compound. ” mono-” indicates one, “di-” indicates two, “tri-” is three, “tetra-” is four, “penta-” is five, and “hexa-” is six, “hepta-” is seven, “octo-” is eight, “nona-” is nine, and “deca” is ten.
  3. If there is only one of the first element, you can drop the prefix. For example, CO is carbon monoxide, not monocarbon monoxide.
  4. If there are two vowels in a row that sound the same once the prefix is added (they “conflict”), the extra vowel on the end of the prefix is removed. For example, one oxygen would be monooxide, but instead it’s monoxide. The extra o is dropped.

Generally, the more electropositive atom is written first, followed by the more electronegative atom with an appropriate suffix. For example, H2O (water) can be called dihydrogen monoxide (though it’s not usually). Organic molecules (molecules made of C and H along with other elements) do not follow this rule.

Examples of Molecular Compound Names:

  • SO2 is called sulfur dioxide
  • SiI4 is called silicon tetraiodide
  • SF6 is called sulfur hexafluoride
  • CS2 is called carbon disulfide

Naming Compounds – Part 2 – YouTube: This video explains how to use a chemical name to write the formula for that compound.

Naming Acids and Bases

Acid names are based on the anion they form when dissolved in water; base names follow the rules for ionic, organic, or molecular compounds.

Learning Objectives

Convert between the structure of an acid or base and its chemical name

Key Takeaways

Key Points

  • Acids are named based on their anion — the ion attached to the hydrogen. In simple binary acids, one ion is attached to hydrogen. Names for such acids consist of the prefix “hydro-“, the first syllable of the anion, and the suffix “-ic”.
  • Complex acid compounds have oxygen in them. For an acid with a polyatomic ion, the suffix “-ate” from the ion is replaced with “-ic.”
  • Polyatomic ions with one extra oxygen (as compared to the typical polyatomic ion) have the prefix “per-” and the suffix “-ic.”
  • Polyatomic ions with one fewer oxygen have the suffix “-ous”; ions with two fewer have the prefix “hypo-” and the suffix “-ous.”
  • Strong bases with “-OH” (hydroxide) groups are named like ionic compounds. Weak bases are named like molecular compounds or organic compounds.

Key Terms

  • polyatomic ion: A charged species (ion) composed of two or more atoms covalently bonded. Also known as a molecular ion.

Naming Acids

Acids are named by the anion they form when dissolved in water. Depending on what anion the hydrogen is attached to, acids will have different names.

Simple acids, known as binary acids, have only one anion and one hydrogen. These anions usually have the ending “-ide.” As acids, these compounds are named starting with the prefix “hydro-,” then adding the first syllable of the anion, then the suffix “-ic.” For example, HCl, which is hydrogen and chlorine, is called hydrochloric acid.

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Nomenclature of common acids: This chart provides the nomenclature of some common anions and acids

More complex acids have oxygen in the compound. There is a simple set of rules for these acids.

  1. Any polyatomic ion with the suffix “-ate” uses the suffix “-ic” as an acid. So, HNO3 will be nitric acid.
  2. When you have a polyatomic ion with one more oxygen than the “-ate” ion, then your acid will have the prefix “per-” and the suffix “-ic.” For example, the chlorate ion is ClO3. Therefore, HClO4 is called perchloric acid.
  3. With one fewer oxygen than the “-ate” ion, the acid will have the suffix “-ous.” For example, chlorous acid is HClO2.
  4. With two fewer oxygen than the “-ate” ion, the prefix will be “hypo-” and the suffix will be “-ous.” For example, instead of bromic acid, HBrO3, we have hypobromous acid, HBrO.

Naming Bases

Most strong bases contain hydroxide, a polyatomic ion. Therefore, strong bases are named following the rules for naming ionic compounds. For example, NaOH is sodium hydroxide, KOH is potassium hydroxide, and Ca(OH)2 is calcium hydroxide. Weak bases made of ionic compounds are also named using the ionic naming system. For example, NH4OH is ammonium hydroxide.

Weak bases are also sometimes molecular compounds or organic compounds because they have covalent bonds. Therefore, they are named following the rules for molecular or organic compounds. For example, methyl amine (CH3NH2) is a weak base. Some weak bases have “common” names. For example, NH3 is called ammonia; its name isn’t derived from any naming system.

Naming Hydrates

The name of a hydrate follows a set pattern: the name of the ionic compound followed by a numerical prefix and the suffix -hydrate.

Learning Objectives

Generate the chemical formula and systematic name of a given inorganic hydrate

Key Takeaways

Key Points

  • Hydrates are named by the ionic compound followed by a numerical prefix and the suffix “-hydrate. ” The “· nH2O” notation indicates that “n” (described by a Greek prefix) number of loosely bonded water molecules are associated per formula unit of the salt.
  • An anhydride is a hydrate that has lost water. A substance that does not contain any water is referred to as anhydrous.
  • In organic chemistry, a hydrate is a compound of water, or its elements, with another molecule. Glucose, C6H12O6, was originally thought of as a carbohydrate (carbon and water), but this classification does not properly describe its structure and properties.

Key Terms

  • hydrate: A solid compound containing or linked to water molecules.
  • carbohydrate: A sugar, starch, or cellulose that is a food source of energy for an animal or plant; a saccharide
  • anhydride: Any compound formally derived from another (or from others) by the loss of a water molecule; a molecule with no water.

Inorganic Hydrates

“Hydrate” is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains loosely bonded water. The name of a hydrate follows a set pattern: the name of the ionic compound followed by a numerical prefix and the suffix “-hydrate.” For example, CuSO4 · 5 H2O is “copper(II) sulfate pentahydrate.” The notation of hydrous compound · nH2O, where n is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The “[latex]\cdot[/latex]” indicates that the water is loosely bonded to the ionic compound. The “n” is usually a low integer though it is possible for fractional values to exist. The prefixes are the same Greek prefixes used in naming molecular compounds. Therefore, in a monohydrate “n” is one; in a hexahydrate “n” is 6, and so on.

The Greek prefixes used in naming hydrates for numbers 1/2 through 10 are as follows:

  • 1/2: hemi-
  • 1: mono-
  • 2: di-
  • 3: tri-
  • 4: tetra-
  • 5: penta-
  • 6: hexa-
  • 7: hepta-
  • 8: octa-
  • 9: nona-
  • 10: deca-

A hydrate that has lost water is referred to as an anhydride. An anhydride can normally lose water only with significant heating. A substance that no longer contains any water is referred to as anhydrous.

Organic Hydrates

In organic chemistry, hydrates tend to be rarer. An organic hydrate is a compound formed by the addition of water or its elements to another molecule. For example, ethanol, CH3–CH2–OH, can be considered a hydrate of ethene, CH2=CH2, formed by the addition of H to one C and OH to the other C. Another example is chloral hydrate, CCl3–CH(OH)2, which can be formed by the reaction of water with chloral, CCl3–CH=O.

Molecules have been labeled as hydrates for historical reasons. Glucose, C6H12O6, was originally thought of as C6(H2O)6 and was described as a carbohydrate, but this is a very poor description of its structure given what is known about it today. Methanol is often sold as “methyl hydrate,” implying the incorrect formula CH3OH2. The correct formula is CH3–OH.

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Cobalt(II) chloride hexahydrate: CoCl2·6H2O has the systematic name cobalt(II) chloride hexahydrate.

Naming Familiar Inorganic Compounds

Familiar inorganic and organic compounds are often known by their common, or “trivial,” names.

Learning Objectives

Recognize when it is appropriate to use a common chemical name

Key Takeaways

Key Points

  • Many frequently used chemicals have familiar common names. A single substance can have several such names.
  • Some common names for chemical substances have historical roots and have been used for thousands of years.
  • Common chemical names are used in spoken or informal written communication by chemists. For some simple compounds, their systematic and common names are the same.

Key Terms

  • common name: The name by which a species is known to the general public, rather than its taxonomic or scientific name.

Common Names v. Systematic Names

Many chemicals are so much a part of daily life that people know them by their familiar names. Ordinary cane sugar, for example, is more formally known as sucrose, but asking for it at the dinner table by that name will likely be a conversation stopper. Now imagine using its systematic name in the same context: “Please pass the α-D-glucopyranosyl-(1,2)-β-D-fructofuranoside!” But saying “sucrose” would be quite appropriate if you needed to distinguish this particular sugar from the hundreds of other named sugars. And the only place you would come across a systematic name such as the rather unwieldy one mentioned above would be in scientific documentation in reference to a sugar that has no simple common name. 

Many common chemical names have very old and intriguing origins, as the following two examples illustrate.

Most people associate the name ammonia (NH3) with a gas with a pungent odor. While its systematic name, “nitrogen trihydride” (which is rarely used), tells you its formula, what it will not tell you is the interesting history of its discovery. Smoke from burning camel dung (the staple fuel of North Africa) condenses on cool surfaces to form a crystalline deposit, which the ancient Romans first noticed on the walls and ceiling of the temple that the Egyptians had built to the sun god Amun in Thebes. They named the material “sal ammoniac,” meaning “salt of Amun.” In 1774, Joseph Priestly (the discoverer of oxygen) found that heating sal ammoniac produced a gas with a pungent odor, which T. Bergman named “ammonia” eight years later.

Arabic alchemy has given us a number of chemical terms. For example, alcohol is believed to derive from the Arabic al-khwl or al-ghawl, which originally referred to a metallic powder used to darken women’s eyelids (kohl). Alcohol entered the English language in the 17th century with the meaning of a “sublimated” substance, then changed to mean the “pure spirit” of anything, and only became associated with “spirit of wine” in 1753. Finally, in 1852, it become a part of chemical nomenclature that denoted a common class of organic compound. But, it is still common practice to refer to the specific substance CH3CH2OH as “alcohol” rather than by its systematic name, ethanol.

General Practices in Naming

The general practice among chemists is to use the more common chemical names whenever it is practical to do so, especially in spoken or informal written communication. Many of the “common” names are known and used mainly by the scientific community. Chemical substances that are employed in the home, the arts, or in industry have acquired traditional or “popular” names that are still in wide use. Many, like sal ammoniac mentioned above, have fascinating stories behind their names.

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Sulfuric Acid: The historical name for sulfuric acid is “oil of vitriol”. Medieval European alchemists prepared it by roasting “green vitriol” (iron (II) sulfate) in an iron retort. Its chemical formula is H2SO4.