Saturday, July 30, 2011

Naming Ioning Compounds

Nomenclature of Compounds


Naming Ionic Compounds
An ion is a single atom or a group of atoms with a net charge.  A positively charged ion is called a cation.  A negatively charged ion is called an anion.  An ion occurs when the total numbers of electrons and protons are not equal.  For example, a neutral magnesium atom has 12 protons and 12 electrons.  The magnesium ion, however, has 12 protons, but only 10 electrons.  The net charge is 2+ and the formula for the ion is written as Mg2+.  This is an example of a monatomic ion (single atom ion).  Groups of covalently bonded atoms can also have a net charge.  For example, the hydroxide ion consists of one atom each of oxygen and hydrogen.  The group as a whole has 9 protons and 10 electrons giving a net charge of 1-.  The formula of this polyatomic ion ("many" atom ion) is written as OH-.  Most polyatomic ions are anions; the ammonium ion, NH4+, is an important polyatomic cation.

Naming monatomic ions follows some simple rules.  The ionic forms of some transition metals and the names and formulas of the polyatomic ions, however, must be memorized. 

There are some helpful hints and trends for the monatomic ions to make learning easier.  Metals tend to form cations and nonmetals tend to form anions.  The common charges of monatomic main group ions are related to their position on the periodic table.  All elements in Group IA (the first column - the alkali metals) form cations with a 1+ charge.  All elements in Group IIA (the second column - the alkaline earth metals) form cations with a 2+ charge.  Metals in Group IIIA form cations with a 3+ charge. Elements in Group VIIA (the halogens) form anions with a 1- charge.  Elements in Group VIA (the chalcogens) form anions with a 2- charge.

Most main group elements have only one monatomic ionic form.  In such cases, the monatomic cations are simply named with the element name followed by the word "ion".  For example Na+ is the sodium ion.  The monatomic anions are named by changing the suffix of the element name to "ide" and adding the word "ion".  For example F- is the fluoride ion and O2- is the oxide ion.

Most transition metals (and even some heavier main group elements), however, have more than one possible monatomic ionic form.  For example, iron has two ionic forms, Fe2+ and Fe3+.   These two forms are distinguished in the written names of the ions by writing the charge as a Roman numeral in parentheses after the name of the element (without a space).  Thus Fe2+ is the iron(II) ion (pronounced "iron two ion").  This ion was formerly called the ferrous ion from the Latin name for iron, ferrum. Many other transition metals also have old names that are still used.  For more examples see Table 3.1.

Ionic compounds consist of oppositely charged ions present in a ratio to yield a uncharged substance.  Usually only two kinds of ions are present.  Occasionally three kinds are present, but this is uncommon. 

The nomenclature of the ionic compounds is simple.  By convention, the name of the cation is written first and the name of the anion is written second.  The word "ion" is omitted and the names are separated by a space.  The same order (cation first, anion second) applies to writing the chemical formulas of ionic compounds.  The chemical formula, however, also indicates the smallest whole number ratio between the ions as subscripts.  For example, table salt is composed of a 1:1 ratio of Na+ and Cl- ions.  Its name is sodium chloride and the formula is NaCl.  (Note that omitted charge numbers and subscripts imply a "1".)

Table 3.1

Name of Ion
Formula of Ion
Classic Name of Ion
copper(I) ion
cuprous ion
copper(II) ion
cupric ion
iron(II) ion
ferrous ion
iron(III) ion
ferric ion
lead(II) ion
plumbous ion
lead(IV) ion
plumbic ion
gold(I) ion
aurous ion
gold(III) ion
auric ion

Consider the following examples:

Example 1:  Write the formula for magnesium chloride.
Solution:  The cation is Mg2+, and the anion is Cl-.  When the anion and the cation are combined, the charges must "cancel".  In other words, the total number of positive charges has to be the same as the total number of negative charges.  The smallest whole number ratio of cations to anions to cancel the charges can be found mathematically using the least common multiple method.  However, an easier way of accomplishing this is to write the cation followed by the anion, and criss-cross the numbers where the number (without its sign) of the charge on the cation becomes the subscript for the anion, and the number of the charge on the anion becomes the subscript for the cation as shown below:

Mg2+     Cl1-      à     Mg1Cl2     à     MgCl2

Note that in writing the final formula, if there is no subscript, then the number is understood to be 1. 

Example 2:  Write the formula for calcium phosphate.
Solution:  The cation is Ca2+ and the anion is PO43-.  Use the criss-cross method to determine the subscripts.

Ca2+        PO43-     à    Ca3(PO4)2

Note that the polyatomic ion is in parentheses.  Whenever you have more than one of a polyatomic ion in a formula, enclose the polyatomic ion formula in parentheses and put the subscript outside the parentheses.

Example 3:  Write the formula for tin(IV) sulfide.
Solution:  The cation is Sn4+ (remember the Roman numeral tells what the charge is on the cation) and the anion is S2-.  Use the criss-cross method to determine the subscripts.

Sn4+             S2-    à    Sn2S4    à   SnS2

Notice that both of the subscripts are divisible by two.  In writing formulas for ionic compounds, you need to write the empirical formula with respect to the cation and the anion.  In other words, the subscripts must be in the lowest whole number ratio.  In order to accomplish this for tin(IV) sulfide divide the subscripts by 2 to get SnS2.

Example 4:  Write the formula for iron(III) phosphate.
Solution:  The cation is Fe3+ and the anion is PO43-.  You can do the criss-cross method if you wish, but that is not necessary here.  Since the numbers on the charges are the same for the positive and the negative charge, you only need one of each (in other words, no subscripts).  Therefore, the formula is FePO4.

Example 5:  Name the compound K2S.
Solution:  The key to naming the compound is to identify the cation and anion in the compound.  If you cannot find a distinct anion and a distinct cation in the compound, then the compound is not ionic.  Another way to determine if the compound is NOT ionic is that all of the atoms involved are non-metals or metalloids (unless it begins with NH4).

When attempting to identify the cation and the anion, remember that the cation is always written first, and except for ammonium (NH4+), the cation only consists of one atom.  Here, the cation is K+, and the anion is S2-.  In this case, the charges are deduced by the positions of potassium and sulfur on the periodic chart.

Once you have identified the cation and the anion, simply name the cation first, then the anion (do not write the word "ion" in the name of the ionic compound).  The name of K2S is potassium sulfide.

Example 6:  Name the compound Ba(NO3)2 .
Solution:  Let's identify the cation and the anion.  There are more than two different atoms here.  Since "NH4" is not in the formula, it is the anion that is polyatomic.  The cation is Ba2+, and the anion is NO3-.  The charge on barium must be 2+ since it is an alkaline earth metal.  The charge on nitrate must be memorized.  The name of this compound is barium nitrate.

Example 7:  Name the compound CuBr2
Solution:  Identifying the anion is easy.  The anion is Br-.  Since the cation is a transition metal (or heavy main group metal), we need to look at the charge from the anion to help us determine the charge on the cation.  Each Br- anion has a 1- charge.  Since there are two bromide ions, the total negative charge is 2-.  This means that the cation(s) must have a total charge of 2+.  Since there is only one cation, the charge for each cation is 2+.  Therefore, the cation must be Cu2+, and the compound is named copper(II) bromide, or cupric bromide. 

A list of ions is provided for you to help you with this exercise.

Cations with only one possible charge:

Li+       lithium ion                                          Al3+     aluminum ion
Na+      sodium ion                                          NH4+   ammonium ion
K+        potassium ion                                      Zn2+     zinc(II) ion
Mg2+    magnesium ion                                    Ag+      silver(I) ion
Ca2+     calcium ion                                          Ni2+     nickel(II) ion
Sr2+      strontium ion                                       Ba2+     barium ion

Cations which can have more than one charge:

Fe2+     iron(II) ion or ferrous ion                    Sn2+     tin(II) ion or stannous ion      
Fe3+     iron(III) ion or ferric ion                     Sn4+     tin(IV) ion or stannic ion
Cu+      copper(I) ion or cuprous ion                Pb2+     lead(II) ion or plumbous ion
Cu2+     copper(II) ion or cupric ion                 Pb4+     lead(IV) ion or plumbic ion
Co2+     cobalt(II) ion or cobaltous ion                        Mn2+    manganese(II) ion or manganous ion
Co3+     cobalt(III) ion or cobaltic ion              Mn3+    manganese(III) ion or manganic ion


H-                    hydride ion                              C2H3O2-          acetate ion
F-                    fluoride ion                             MnO4-             permanganate ion
Cl-                   chloride ion                             SO42-               sulfate ion
Br-                   bromide ion                             HSO4-             hydrogen sulfate ion
I-                     iodide ion                                SO32-               sulfite ion
O2-                  oxide ion                                 Cr2O72-            dichromate ion
O22-                 peroxide ion                            CrO42-             chromate ion
S2-                   sulfide ion                               PO43-               phosphate ion
N3-                  nitride ion                                C2O42-             oxalate ion
NO3-               nitrate ion                                CN-                 cyanide ion
NO2-               nitrite ion                                 OH-                 hydroxide ion
CO32-              carbonate ion                           ClO3-               chlorate ion
HCO3-             hydrogen carbonate ion          ClO-                hypochlorite ion

So far, only the naming of ionic compounds has been illustrated.  Now let's discuss how to name compounds where there are no distinct cations or anions, the covalent compounds.

Naming Covalent Compounds
Covalent compounds are composed of atoms joined by the sharing of pairs of electrons, called covalent bonds.  Binary compounds, substances composed of two nonmetallic elements, are the simplest covalent compounds. Usually, one of the elements is more electronegative than the other.  Fluorine is the most electronegative element.  Generally, the closer an element is to fluorine on the periodic table, the greater its electronegativity.  Thus, electronegativity tends to increase as you go up any given column or to the right on any given row in the periodic table (ignoring noble gases). 

In writing formulas and names, the less electronegative element is written first, followed by the more electronegative element.  In the name, the more electronegative element is written as if it were an anion, so it has the "ide" suffix. The names of the elements are separated by a space.

By the law of multiple proportions two elements may combine in more than one ratio to form different compounds.  For example, carbon and oxygen can form either CO or CO2. These compounds must have different names.  We use Latin prefixes to indicate the number of atoms of each element type as shown in the table below (Table 3.2):

Table 3.2

Number of Atoms

If there is only one atom of the first element in the name / formula, the prefix "mono-" is omitted.  So CO is carbon monoxide and CO2 is carbon dioxide.  Note the "o" in "mono" is dropped before the word oxide.

Some compounds have a "common" name.  For example, trihydrogen nitride is ammonia, and dihydrogen monoxide is called water.

Example 8:  Write the formula for carbon tetrafluoride.
Solution:  Since the first atom has no prefix, there is only 1 carbon atom.  The prefix "tetra-", followed by fluoride, means there are four fluorine atoms.  Therefore the formula is written CF4.

Example 9:  Write the formula for dinitrogen trioxide.
Solution:  The first atom has a prefix "di-" meaning two, so there are two nitrogen atoms that are written first.  The prefix "tri-" means there are three oxygen atoms, so the formula is written N2O3.

Example 10:  Name the compound, IF3.
Solution:  There is only one iodine, so we do not need the "mono-" prefix.  For fluorine, the prefix "tri-" is used since there are three of them.  Also, fluorine is written like the anion, fluoride.  So the name of this compound is iodine trifluoride.

Example 11:  Name the compound, P4S6.
Solution:  There are four P atoms written first, so the first part of the name is tetraphosphorus.  The prefix "hexa-" is used since there are six of the second atom.  Since the second atom is sulfur, it is written as the anion, sulfide.  So the name of this compound is tetraphosphorus hexasulfide.

The tricky part about naming these compounds is classifying them as ionic or covalent.  Once you do that, you know which set of rules to apply.  Use the following guidelines to help you choose. 

                        Composed of only nonmetals or metalloids   Covalent
                        and no ammonium, NH4+ , is present 

                        Metal and nonmetal                                        Ionic

                        Ammonium and a nonmetal                            Ionic

                        Metal and a polyatomic ion                            Ionic

                        Ammonium and a polyatomic ion                   Ionic

                        Name contains prefixes like "mono",              Covalent
                        "di", "tri", etc.

                        Name has no prefixes and is not a                  Ionic
                        special name like water.

If you can distinguish metals from nonmetals and can recognize the polyatomic ion groups as formulas and names, the easier this will be.

Experiment 3 - Nomenclature of Compounds


Name________________________________________      Section _________

25 bottles are set out for you with either the name or the formula on the label.  Record this on the table below.  Then fill in the rest of the missing information.  All of the compounds on this table are ionic.  See the example below.

sodium chloride
copper(II) bromide





barium nitrate

silver(I) acetate







potassium nitrite





ammonium sulfate

chromium(III) chloride

cobalt(II) nitrate



potassium Iodide

cobalt(II) sulfate

iron(III) chloride



ammonium phosphate



mercury(II) chloride

copper(I) sulfide



iron(II) sulfate

Again, fill in the blanks in the table.  This is a list of compounds that are found in the "real world".  Some of the compounds below are ionic and others are covalent.  You must be able to tell the difference.

Real World Use

self tint sunglasses
dinitrogen monoxide

laughing gas
diphosphorus pentoxide

drying agent
lithium carbonate

medication for bipolar disorder

baking soda
tin (II) fluoride

fluoride in toothpaste

used in road flares

magnesium sulfate

Epsom salts

root eater
cobalt(II) chloride

humidity indicator

smelling salts

pollutant in coal burning plants
barium sulfate

nasty drink before X-rays

used in breathalyzers
dihydrogen monosulfide

rotten eggs smell
chromium(III) oxide

dye used to color money
lead(II) sulfate

used in lead acid batteries


magnesium supplement
sodium hypochlorite

calcium sulfate

plaster of Paris

bubbles in soft drinks

generic iron supplement
manganese(IV) oxide

used in flashlight batteries
tetraphosphorus trisulfide

used in match heads
silicon dioxide

sand (life's a beach)


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