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Names of Chemical Compounds

>> Parts of this equation/concept include:

A - Binary Molecular Compounds
B - Binary Ionic Compounds
>> Cations
>> Anions
>> Combining Cations and Anions
C - Acids

 

When naming or determining the name of a chemical compound, you must first decide what system to use. The system depends on the type of compound. In addition to the systematic names, some compounds are normally known by common names. The common names do not indicate anything about the formula but are used nevertheless. An example of a common name is water, for H2O. (It's systematic name is dihydrogen oxide, but no one calls it anything but water.) Ammonia, NH3, and acetic acid, HC2H3O2, are two other common names of very common compounds.

Since the systems are based on the elements, knowing the names and symbols of common elements is expected. The common elements are those through at least atomic number 20. Other common elements are atomic numbers 24-30, all but the last element in the halogens (group 7A), the noble gases (group 0), alkali metals (group 1A), and alkali earth metals (group 2A), and the elements Ag, Au, Cd, Hg, Sn, Sb, Pt, Pb, and Bi.

Molecular compounds are normally a combination of nonmetals. Although they are molecular compounds, organic compounds are classified differently. Organic compounds are based on carbon. Typically, combinations consisting mostly of carbon and hydrogen are named with the organic naming system.

Ionic compounds are normally a combination of a metal, which forms the cation, and a nonmetal or combination of nonmetals, which form the anion. One common cation that is not a metal is NH4+, ammonium ion. Compounds containing the ammonium ion are still ionic, even though they do not contain a metal.

Making hydrogen the first element of the chemical formula indicates that it is an acid. This is standard for inorganic (not organic) acids. Organic acids are named with the organic naming system. An important type of organic acid has a formula that ends with COOH.

The names of chemical compounds are not capitalized. When you write a formula, the chemical symbols must be exact, including upper and lower case, for the formula to be correct. There are exceptions to most rules, but the following work for commonly encountered compounds.

 

A. Binary Molecular Compounds

Binary molecular compounds are composed of two types of nonmetals. The nonmetals are normally ordered with the element leftmost on the periodic table first. If both elements are in the same column, then the element lower on the periodic table is first. The order is the same for the formula and the name.

Because the nonmetals can combine in many ways, prefixes are used to express how many atoms of each element are in the atom. Memorize the following list of prefixes.

1 mono*       2 di       3 tri       4 tetra       5 penta
6 hexa       7 hepta       8 octa       9 nona       10 deca

The prefix for one is starred because its use is optional for the second element. If there is only one atom of the first element, no prefix is used.

To name the compound, start with the name of the first element, adding the prefix if there is more than one atom of that element. After a space, use the appropriate prefix for the second element, the root of the element name and the ending ide.

>> Example 1

Name the following binary molecular compounds.

  1. NI3
  2. N2O
  3. ClF5
  4. CCl4
  5. P4O10

Solution:

  1. There is only one nitrogen, so the first element does not have a prefix. The root of iodine is iod and the prefix for 3 is tri. nitrogen triiodide
  2. There are two nitrogen atoms this time, so the prefix di will be used. The root of oxygen is ox. There is only one, so the prefix mono is optional. When the prefix mono is used with oxide, it is acceptable to drop the last o of mono. However, it is not required that you do so.

    There are three acceptable answers for this question:

    dinitrogen oxide
    dintirogen monoxide
    dinitrogen monooxide

  3. No prefix for the first element. The prefix for 5 is penta; the root of fluorine is fluor.

    chlorine pentafluoride

  4. No prefix for the first element. The prefix for 4 is tetra; the root of chlorine is chlor.

    carbon tetrachloride

  5. tetraphosphorus decaoxide.

>> Example 2

What are the formulas of the following binary molecular compounds?

  1. oxygen difluoride
  2. tetranitrogen decaoxide
  3. arsenic tribromide
  4. boron trichloride
  5. iodine heptafluoride

Solution:

  1. Di refers to two. No prefix on the first element means there is only one oxygen, so OF2.
  2. Tetra refers to 4; deca, to 10, so N4O10.
  3. No prefix on arsenic; tri is three, so AsBr3.
  4. No prefix and tri is still 3, so BCl3.
  5. No prefix and hepta is 7, so IF7.

 

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B. Binary Ionic Compounds

Binary ionic compounds may contain more that two elements but are binary because they contain two ions. The cation, a positively charged ion, is written first in both the formula and the name. The number of cations and anions (negatively charged ion) is not expressed in the name, since this value is always the simplest ratio that will make the entire compound neutral (uncharged). The compounds are named by naming the cation, then the anion.

Since the name of the compound does not express the ratio of cation and anion, knowing the charge of the commonly used ions is essential to determining the formula. If the charge on one ion is known, it can be used to determine the charge on the other ion, since the entire compound must be neutral.

The names of monatomic (one-atom) ions can be determined systematically. Many polyatomic (many-atom) ions have a system for naming, but normally learning the system is more difficult than just memorizing the names of the ions. Common ions and their names are listed in Table 4.1.

>> Cations

Most cations are monatomic metals. Two important exceptions are ammonium ion, NH4+, and mercury(I) ion, Hg22+. An uncharged metal can be differentiated from the ion in the formula because the ion will either be combined with an anion or have a superscripted (to the right) charge. Likewise, the metal is known to be an ion if its name is combined with the name of an anion or if its name is followed by the term ion. If it is uncharged, the name is commonly followed by term metal.

The name of metal ions with only one common charge is the same as the name of the metal. Two groups of metals with only one common charge are the alkali metals (group 1A) and the alkali earth metals (group 2A). Alkali metals always have a charge of +1, which is, not coincidentally, the group number and the number of electrons that, when lost, give the metal a noble gas electron configuration. When writing the formula of the ion, do not include the 1 as part of the charge. Alkali earth metals always have a +2 charge. In addition to these two groups, aluminum is always Al3+, zinc is Zn2+, and silver is Ag+.

Other metals can form different ions with different charges. Therefore the name must reflect charge. It does this by following the name of the metal with the charge, as a roman numeral, in parenthesis. (The "+" is assumed and not part of the Roman numeral.) An older way of designating charge is to use the Latin name and the suffix ic for the common ion of that metal with the highest charge and the suffix ous for the common ion with the lowest charge. Ions for which the older system is still commonly (although not exclusively) used are Fe3+, ferric ion; Fe2+, ferrous ion; Cu2+, cupric ion; and Cu+, cuprous ion.

>> Example 3

What are the names of the following cations?

  1. Mn2+
  2. K+
  3. Cr3+
  4. Zn2+
  5. Cu+

Solution:

  1. It is not group 1A or 2A or one of the three exceptions, so manganese(II) ion.
  2. It is a group 1A, so no Roman numeral: potassium ion.
  3. It is not group 1A, 2A, or an exception: chromium(III) ion.
  4. It is one of the exceptions, so no Roman numeral: zinc ion.
  5. It is not one of the exceptions, so copper(I) ion. An equally acceptable answer is cuprous ion.

>> Example 4

What are the formulas of the following ions?

  1. lithium ion
  2. magnesium ion
  3. bismuth(V) ion
  4. ferric ion
  5. silver ion

Solution:

  1. It's a group 1A ion, so Li+.
  2. It's a group 2A ion, so Mg2+.
  3. It tells you the charge, so Bi5+.
  4. This is why it's good to know the old system. You don't have to name things with it, but you must understand when others do. Fe3+
  5. This is an exception that needs to be memorized. Ag+

>> Anions

Monatomic anions are formed from nonmetals. These ions are named with the root element name and the suffix ide. The charge on these ions is the number of electrons needed for a noble gas configuration (or number of spaces needed to reach a noble gas). Therefore monatomic ions of the halogens (group 7A) all have a –1 charge (don't write the 1 in the formula) and are collectively called halides.

Polyatomic anions are very common. These are normally combinations of nonmetals but can be combinations of metals and nonmetals. If there are two metals in a compound, the first is probably a cation and the next part of a polyatomic anion. Although there is some system to naming polyatomic ions, it is probably easier to memorize the common ones.

Sometimes a hydrogen is added to multiply charged anion. That changes its name by either adding the word hydrogen before the ion's name or by adding a bi prefix to the name. Each hydrogen adds +1 to the charge.

>> Example 5

What are the names of the following anions?

  1. Cl–
  2. P3–
  3. NO2–
  4. AsO43–
  5. HCO3–

Solution:

  1. Monatomic, so just change suffix and it's chloride ion.
  2. Monatomic. The root element is phosph, so it's phosphide (or alternate spelling phosfide) ion.
  3. This is a polyatomic ion. There are two common combinations of nitrogen and oxygen. This has fewer oxygens than the other choice, so it has an ite ending—nitrite ion.
  4. Another polyatomic with a two oxygen/element combination, this common combination has the higher number of oxygens (the highest number varies from element to element), so arsenate ion.
  5. This is a hydrogen variation of the common ion carbonate (ate means three is the highest number of oxygens carbon combines with). So this ion can be called either hydrogen carbonate ion or bicarbonate ion.

>> Example 6

What are the formulas of the following anions?

  1. sulfite ion
  2. sulfide ion
  3. dihydrogen phosphate ion
  4. perchlorate ion

Solution:

  1. Sulfite has fewer oxygens than sulfate. The formula of sulfite is SO32–.
  2. Sulfide has the ide ending so it doesn't combine with oxygen; it is ide added to the root for sulfur, so must be S2–.
  3. Recalling the other system, di means two, so two hydrogens with a phosphate ion. The phosphate ion is PO43– (ate is the "lots-of-oxygen" ending). So dihydrogen phosphate ion is H2PO4–.
  4. The ate of the ending implies many oxygens; however, chlorine has more than two oxygen combinations. The combination of the per and the ate implies that it has the most possible oxygens. Its formula is ClO4–.

Notice that by the time you have memorized the rules, you have memorized the ions.

>> Combining Cations and Anions

An ionic compound is named by naming the cation followed by the anion, just drop the term ion after each. It only requires recognizing each ion. Determining the formula from the name is more difficult because the appropriate ratio of ions must be determined. A useful trick is that the charge on one ion can become the subscript of the other ion. This trick does not always work because the simplest ratio must be used, so always double-check.

If the ion is polyatomic and there is more than one of that ion, the ion should be enclosed in parentheses and a subscript expressing how many of those ions are needed should be put outside the parentheses. Parentheses are only used if there is more than one polyatomic ion!

>> Example 7

What are the names of the following ionic compounds?

  1. Na2CO3
  2. Co(NO3)2
  3. SnS2

Solution:

  1. The ions are Na+ and CO32–. Sodium is in group 1A, so no Roman numeral. The name is sodium carbonate.
  2. Cobalt is a metal with several possible charges. However, nitrate (NO3–) is always –1. For the compound to be electrically neutral, the cobalt must have a +2 charge, so its name is cobalt(II) nitrate.
  3. Tin is another metal with variable charges. However, sulfur always gains two electrons to make S2–. Therefore the charge on tin must be +4. The name is tin(IV) sulfide.

>> Example 8

What are the formulas of the following compounds?

  1. potassium iodide
  2. aluminum sulfate
  3. lead(II) sulfite
  4. ferrous hydroxide
  5. ammonium phosphate
  6. sulfur dioxide

Solution:

  1. Potassium is K+ and iodide is I–. The formula is KI.
  2. Aluminum is Al3+ and sulfate is SO42–, so the formula is Al2(SO4)3. Notice that the charges, without the + or – sign, are the subscripts on the other ion. Because there is more than one sulfate ion, the sulfate ion is in parentheses and the subscript tells how many of these ions there are.
  3. The cation is Pb2+. You can tell the charge from the Roman numeral. The formula for sulfite ion is SO32–. So the compound is PbSO3. The charges are the same, so the simplest ratio is 1:1, not 2:2, which is suggested by the charge/subscript method.
  4. The ferrous ion is Fe2+ and hydroxide is OH-. Therefore the formula is Fe(OH)2.
  5. Ammonium ion is NH4+ and phosphate is PO43–, so the formula is (NH4)3PO4. Since there is only one phosphate, it does not have parentheses.
  6. SO2. Notice the prefixes? This was a molecular compound. Don't forget that system!

>> Example 9

Fill in the following table.

Formula Cation Anion Name
CaO      
  Pb4+ F–  
      sulfur dichloride
      copper(II) hydroxide
XeF4      

Solution:

Row 1. Calcium is a metal and oxygen is a nonmetal, so this is an ionic compound. Calcium, as a group 2 metal, has a +2 charge, Ca2+. Oxygen, as a group 6A nonmetal, has a –2 charge, O2–. Since both ions have the same charge, they exist in a 1:1 ratio, CaO.

Row 2. For a neutral compound to exist, 4 F– ions are needed for each Pb4+, PbF4. Lead can have multiple charges, so it must be named with a Roman numeral. Fluorine atom becomes fluoride ion.

Row 3. The prefixes are a tip-off that this is a molecular compound. Therefore there are no ions. According to the prefixes, there is one sulfur and two chlorides.

Row 4. Copper(II) is the name of the Cu2+ ion. Hydroxide is OH–. Two hydroxides will be needed to neutralize the charge on the copper, so Cu(OH)2.

Row 5. Xenon and fluorine are both nonmetals, so this is a molecular compound. There are no ions. Tetra is the appropriate prefix for 4, so xenon tetrafluoride.

Formula Cation Anion Name
CaO Ca2+ O2– calcium oxide
PbF4 Pb4+ F– lead(IV) fluoride
SCl2 — — sulfur dichloride
Cu(OH)2 Cu2+ OH– copper(II) hydroxide
XeF4 — — xenon tetrafluoride

 

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C. Acids

Since acids are substances that release H+ in water, it is traditional to write the hydrogen atom first in the formula. There are two major categories of inorganic acids. Binary acids contain two elements. They are named as hydro{element root}ic acid. Oxoacids contain oxygen. The names of these acids are based on the anion the acid came from. (Hydrogen acts as a cation, H+. Although acids are molecular compounds, they react with water to form ions.) If the anion has an ate ending, the ate is changed to ic and the word acid added. If the anion has an ite ending, the ite is changed to ous and is followed by acid.

>> Example 10

What are the names of the following acids?

  1. HBr
  2. HNO2
  3. H3PO4
  4. H2S

Solution:

  1. It is a binary acid. The root of bromine is brom, so the name is hydrobromic acid.
  2. It is an oxoacid associated with the NO2– (nitrite ion). So the name is nitrous acid.
  3. It is an oxoacid associated with PO43– (phosphate ion), so it is phosphoric acid. (To follow the system rigorously it should be phosphic acid, but phosphoric acid is what they call it.)
  4. A binary acid. Remember, binary, in the context of naming, refers to two types of elements (or ions), not two atoms. So hydrosulfuric acid.

>> Example 11

What are the formulas of the following acids?

  1. hydroiodic acid
  2. carbonic acid
  3. sulfurous acid
  4. perchloric acid

Solution:

  1. The hydro tips it off as a binary acid. Iod is the root for iodine, which makes I– ions. Therefore it needs one hydrogen. The formula is HI.
  2. The ic says that it came from an ate name. Changing the ic back to ate, it becomes carbonate, CO32–. Since this ion has a –2 charge, it needs two hydrogens. H2CO3.
  3. The ous implies it came from ite. So the base anion must be sulfite, which is SO32–. The acid needs two hydrogens. H2SO3.
  4. Changing the ic to ate, you get perchlorate, which is ClO4–. Adding one hydrogen to neutralize the charge, you get HClO4.

 

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