Ionic Compounds

Networks

The nature of ionic bonding is such that the ions attract one another from all directions with each positive ion surrounded by negative ions and each negative ion surrounded by positive ions in such a way that a regular, repeating, three-dimensional pattern of alternating positive and negative ions is set up. This pattern is called a crystal lattice.

Diagram of two-dimensional ionic network.

This model shows the three-dimensional aspect. The red balls represent the negative ions and the silver balls represent the positive ions. Notice that each negative ion has positive ions above, below, to the left, to the right, in front and in back of it. All of the ions in the model are surrounded by oppositely charged ions. This model does not show the proper sizes of the positive and negative ions. The positive ions are actually much smaller than the negative ones because the positive ions have lost electrons and the negative ions have gained additional electrons.

Notice that the nature of ionic bonding results in a network type of material. There's a network of alternating positive and negative ions that goes all the way through the entire crystal. Ionic bonding holds each ion to all of its neighbors and those in turn to all of their neighbors. The ionic bonding continues from ion to ion in three dimensions all the way through the entire crystalline material.

Notice that the shape of the actual ionic crystal is reflected in the pattern of ions in the model. This (NaCl, left) is a very simple crystal pattern, others are more complex (CaCO3, right). But even in them the pattern of the ions shows up in the crystal faces. An even better way of seeing this is to look at some of the models of ionic crystals that we have in the lab. Go over and take a look at those now if you are in the lab. If not, be sure to study them when you come to the lab.

Calcite Crystal with Model in Background

Compounds - Fixed Ratios of Ions

Within that crystalline network of ions, the ratio of cations to anions remains constant. That ratio depends solely on the charges of the cations and anions. The ratio of the number of cations to anions will be just opposite the ratio of charges on the ions, so that the amount of positive and negative charge is equalized. Because the ratio of ions is fixed, ionic bonding results in the formation of compounds, which have fixed composition.

Ions	Ca²⁺	with	Cl^- Cl^-
Charge Ratio	2	to	1
Ion Ratio	1	to	2
Formula	CaCl₂

Determining formulas

It is possible to figure out or predict the formulas of ionic compounds just from knowing which elements are involved by considering the charges on the ions.

On the following pages are several examples of how to determine formulas. (They are also shown in example 9 of your workbook.) Each tries to model the step-by-step thought process you can use to figure out ionic formulas and names for the compounds. In sequence, each presents an additional factor or level of complexity. It will be handy to have a periodic table with you as you go through this. Note as you go through these examples that the metal or positive ion is always written first in the formulas. Study these examples (and the section on "Comment on Naming Ionic Compounds") as needed, then try your hand at predicting ionic formulas and naming ionic compounds in the "Practice" section that follows "Comment on Naming Ionic Compounds".

First Example - sodium and chlorine

Sodium is in the first column of the periodic table, so it loses one electron and becomes a sodium ion with a +1 charge. Chlorine is a nonmetal in the next to the last column. Because it is a nonmetal, it will tend to gain electrons, and it will gain as many electrons as it needs to fill up its outermost energy level. There's room for only one more electron so it will take on a -1 charge and is called a chloride ion. So we have a +1 charge for sodium and a -1 charge for chlorine. The charges are opposite but equal; therefore, there is a 1:1 ratio of sodium ions to chloride ions and NaCl is the formula of the compound. The name of this ionic compound is sodium chloride because it is made of sodium ions and chloride ions.

Ions	Na⁺	Cl^-
Charge ratio	1	1
Ion ratio	1	1
Formula	NaCl
Name	sodium chloride

Second Example -barium and chlorine

Barium is in Group II-A and so it will have a +2 charge when it forms an ion. The ion is called a barium ion. Chlorine, again has a -1 charge on its chloride ion. With a +2 charge and a -1 charge, we will need two of the chloride ions to match the amount of charge on one barium ion. Therefore, we will need two chlorines for every barium. The formula for this compound is BaCl₂. Note that the charge ratio is two-to-one, so the ion (or atom) ratio is one-to-two. The name of this ionic compound is barium chloride because it is made of barium ions and chloride ions. Note that it is not called barium dichloride.

Ions	Ba²⁺	Cl^-
Charge ratio	2	1
Ion ratio	1	2
Formula	BaCl₂
Name	barium chloride

Third Example - iron and chlorine

Iron is a transition metal. With transition metals you cannot count on knowing the ionic charge just from its position in the periodic table. For most transition metals, you will have to look up what charges actually exist for that element. You can look these up in your textbook or you can look back at the list in your workbook. However, you should have memorized the charges for copper and iron. Iron has an ion with a +2 charge called iron(II) ion or ferrous ion, and one with a +3 charge called iron(III) ion or ferric ion. Chlorine, of course, has a -1 charge.

Because there are two types of cations, two different compounds can be formed. When the Fe²⁺ combines with chloride, you will get FeCl₂. FeCl₂ is named iron(II) chloride because it is made of iron(II) ions and chloride ions. It can also be called ferrous chloride.

If the iron +3 ion combines with chloride, then that will give you FeCl₃ because three chloride ions are needed to neutralize the charge on a +3 ion. FeCl₃ is named iron(III) chloride because it is made of iron(III) ions and chloride ions. It can also be called ferric chloride.

Ions

Fe²⁺

Cl^-

Fe³⁺

Charge ratios

Ion ratios

Formulas

FeCl₂

FeCl₃

Names

iron(II) chloride

iron(III) chloride

Again note that in in these compounds the charge ratios can be reversed to get the atom ratios.

Fourth Example - magnesium and oxygen

Magnesium is a Group II-A element; therefore, it will have a +2 charge on the magnesium ion. Oxygen is in Group VI-A. It has six electrons in its outer shell. It has room for two more so when it forms an ion, it has a -2 charge and it is called an oxide ion. Here again, we have equal charges on the two ions: a +2 on the magnesium and a -2 charge on the oxygen. So they will combine in a 1:1 ratio and the formula for that compound is MgO. It is called magnesium oxide because it is made of magnesium ions and oxide ions.

Ions	Mg²⁺	O^2-
Simplified charge ratio	1	1
Ion ratio	1	1
Formula	MgO

Fifth Example - aluminum and oxygen

Aluminum is in Group III-A. It has three electrons in the outermost energy level. It is a metal. Metals lose electrons. It has three electrons in the outermost energy level; therefore, it loses three electrons and forms a +3 ion called an aluminum ion. Oxygen, of course, takes on a -2 charge to form and oxide ion. The charge ratio is 3:2 therefore the atom ratio will be 2:3. Since two aluminum ions each with +3 charge will neutralize the charge on three oxide ions with -2 charge, the formula will be Al₂O₃. The name is aluminum oxide because it is made from aluminum ions and oxide ions.

Ions	Al³⁺	O^2-
Charge ratio	3	2
Ion ratio	2	3
Formula	Al₂O₃

Comment on Naming Ionic Compounds

Before continuing I want to emphasize that when naming ionic compounds, you simply identify the ions present and name them. Then put the names together, positive first, negative last, with a space in between. If a nonmetal needs an "-ide" ending, use it. If a metal needs a Roman numeral or an "-ous" or "-ic" ending to show which charge it has, use it. If not, don't.

Practice with Determining Names and Formulas for Ionic Compounds

Now work through the following exercise (also shown in exercise 10 in your workbook) and check your answers on the next page or with the instructor. Then continue with the lesson.

DETERMINING IONIC FORMULAS AND NAMING IONIC COMPOUNDS

For each of the following pairs of elements, use the periodic table to decide the charge on both the cation and anion and determine the formula of the compound(s) formed in each case. When writing the formulas put the cation first. For transition metals the common ionic charges are given. Name each of the compounds formed.

Elements		Compound Formula	Compound Name
Mg	Br
K	S
Cl	Al
S	Cu (+1, +2)
F	Zn (+2)
O	Co (+2, +3)
magnesium	nitrogen
calcium	iodine

Answers

Elements		Compound Formula	Compound Name
Mg	Br	MgBr₂	magnesium bromide
K	S	K₂S	potassium sufide
Cl	Al	AlCl₃	aluminum chloride
S	Cu (+1, +2)	Cu₂S	copper(I) sulfide
S	Cu (+1, +2)	CuS	copper(II) sulfide
F	Zn (+2)	ZnF₂	zinc sulfide
O	Co (+2, +3)	CoO	cobalt(II) oxide
O	Co (+2, +3)	Co₂O₃	cobalt(III) oxide
magnesium	nitrogen	Mg₃N₂	magnesium nitride
calcium	iodine	CaI₂	calcium iodide

Formulas Containing Polyatomic Ions

The examples you just worked with did not include any polyatomic ions. There is one additional bit of information you need before working with formulas that contain polyatomic ions.

If we were to combine aluminum ion and carbonate ion to make aluminum carbonate, we would need to have a 2:3 ratio of aluminum to carbonate to offset the 3:2 charge ratio. When putting a subscript with a polyatomic ion, put the polyatomic ion in parentheses to show that the subscript applies to that entire ion.

Ions	Al³⁺	CO₃^2-
Charge ratio	3	2
Ion ratio	2	3
Formula	Al₂(CO₃)₃

If no additional subscript is needed for the polyatomic ion, like in calcium carbonate (a 1:1 ratio), then no parenetheses are used.

Ions	Ca²⁺	CO₃^2-
Simplified charge ratio	1	1
Ion ratio	1	1
Formula	CaCO₃

Practice with Formulas Containing Polyatomic Ions

Now try your hand at the following exercise (also shown in your workbook as exercise 11). When you have done that, check your answers on the next page or with the instructor.

DETERMINING NAMES AND FORMULAS OF IONIC COMPOUNDS
WITH POLYATOMIC IONS

Determine the names and formulas of the compounds formed from these combinations of ions.

Cation	Anion	Formula	Name
Na⁺	CO₃^2-
Ca²⁺	PO₄^3-
Al³⁺	OH^-
K⁺	ClO₃^-
Fe²⁺	SO₄^2-
Fe³⁺	NO₃^-
Cu²⁺	C₂H₃O₂^-
NH₄⁺	NO₃^-

Answers

Cation	Anion	Formula	Name
Na⁺	CO₃^2-	Na₂CO₃	sodium carbonate
Ca²⁺	PO₄^3-	Ca₃(PO₄)₂	calcium phosphate
Al³⁺	OH^-	Al(OH)₃	aluminum hydroxide
K⁺	ClO₃^-	KClO₃	potassium chlorate
Fe²⁺	SO₄^2-	FeSO₄	iron(II) sulfate
Fe²⁺	SO₄^2-	FeSO₄	ferrous sulfate
Fe³⁺	NO₃^-	Fe(NO₃)₃	iron(III) nitrate
Fe³⁺	NO₃^-	Fe(NO₃)₃	ferric nitrate
Cu²⁺	C₂H₃O₂^-	Cu(C₂H₃O₂)₂	copper(I) acetate
Cu²⁺	C₂H₃O₂^-	Cu(C₂H₃O₂)₂	cupric acetate
NH₄⁺	NO₃^-	NH₄NO₃	ammonium nitrate

Questions about Naming Ionic Compounds

If at this time you are having trouble naming ionic compounds, I think you need to think through the basic procedure involved. The following questions (which are also given in exercise 12 in your workbook) should help you to do that. Even if you are not having trouble naming ionic compounds this would be a good exercise because it will help you distinguish between the process used for ionic compounds and a different process that will be used to name covalent compounds a little later in this lesson.

After you have established that you are dealing with and ionic compound, you need to identify and name the ions present in the compound. How do you do that? How do you separate the positive and negative parts of the formula? How do you determine the charge on the cation and on the anion? What factors need to be taken into account when naming a cation? What factors need to be taken into account when naming an anion? Your answers to these questions will constitute the guidelines you need for naming ionic compounds.

If you need help answering these questions, ask an instructor (in lab or on line) for help.

Practice Using Name to get Formula

Your familiarity with the names of ions should allow you to use the names of ionic compounds to determine their formulas. take some time now to get some practice figuring out the formulas of the few ionic compounds listed below (and also in exercise 13 in your workbook). Answers follow on the next page.

PRACTICE: NAMES TO FORMULAS

Determine the formula for each of the following compounds.

a. aluminum hydroxide

b. barium chloride

c. cobaltic carbonate

d. iron(II) sulfate

Answers

a. aluminum hydroxide - Al(OH)₃

b. barium chloride - BaCl₂

c. cobaltic carbonate - Co₂(CO₃)₃

d. iron(II) sulfate - FeSO₄

Lab Work

To help you get additional practice naming and figuring out the formulas of ionic compounds, there is a computer program for you to work with in the lab. Actually, there are two computer programs that you will work with. One program is for covalent compounds and the other is for ionic compounds. These programs will not teach you how to name compounds or figure out their formulas, but they will provide you with plenty of practice. These particular programs are not available in web format, so you will not be able to do them on-line. However, if you wish, you can bring a blank diskette to the lab and have a copy made. (Be sure to get instructions for its use, as well.) If you got a copy of the "Balancing Equations" program, these programs should have been copied along with it.