22 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

  • : 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  \text{Ca}^{2+} and  \text{Br}^- combine only in a 1:2 ratio to form calcium bromide,  \text{CaBr}_2 . Because no other simpler formula is possible, there is no need to name it “calcium dibromide.”  \text{CaBr}_2  can be named using either the Stock method or the older, classic way of naming.

For example,  \text{CuCl}_2 indicates a molecule where one  \text{Cu}^{2+} cation associates with two  \text{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,  \text{K}^{+1}  is called  the potassium ion, just as  \text{K} is called the potassium atom. The anion is named by taking the elemental name, removing the ending, and adding “-ide.” For example,  \text{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,  \text{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, \text{Ca(NO}_3\text{)}_2 is called calcium nitrate.

“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 ( \text{NH}_3 ) instead of nitrogen trihydride or water ( \text{H}_2\text{O} ) instead of dihydrogen monoxide.

Key Terms

  • : A set of rules used for forming the names or terms in a particular field of arts or sciences.
  • : Tending to attract electrons within a chemical bond.
  • : 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, nonmetals 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, “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,  \text{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,  \text{H}_2\text{O} (water) can be called dihydrogen monoxide (though it’s not usually). Organic molecules (molecules made of  \text{C} and  \text{H} along with other elements) do not follow this rule.

Examples of Molecular Compound Names:

  •  \text{SO}_2  is called sulfur dioxide
  •  \text{SiI}_4  is called silicon tetraiodide
  •  \text{SF}_6  is called sulfur hexafluoride
  •  \text{CS}_2  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 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

  • : 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 ( \text{NH}_3 ) 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 Priestley (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  \text{CH}_3\text{CH}_2\text{OH} 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 \text{H}_2\text{SO}_4 .

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