Empirical formulas of ionic compounds

We normally write the empirical formulas of ionic compounds without showing the charges. The + and — are shown here to emphasize the transfer of electrons. 

Write the empirical formulas of ionic compounds, given the chaiges of their component ions. (Section 2.7)... 

IONS AND IONIC COMPOUNDS We learn that atoms can gain or lose electrons to form ions. We also look at how to use the periodic table to predict the charges on ions and the empirical formulas of ionic compounds. 

For transition metals, the lanthanides, and the actinides, no such simple rule exists. If we accept the ions charges as chemical facts, we can still write the empirical formulas for ionic compounds so that the net (overall) charge is zero. If we had Fe2+ and O2-, the compound would require a minimum of one of each of the elements for a neutral formula, FeO, whereas Fe3+ and O2- would have the formula Fe2C>3. 

Molecular formulas give all the information that empirical formulas do, plus the ratio of the number of moles of each element to the number of moles of the compound. (Molecular formulas are used only for molecular substances, not ionic substances.) A molecular formula can be determined from the empirical formula of the compound and its formula mass First, divide the formula mass by the mass in amu of one empirical formula unit, which will result in a small integer. Then, multiply each subscript of the empirical formula by that integer. (Section 7.5)... 

To predict the formula of the ionic compound, we simply recognize that chemical compounds are always electrically neutral—they have the same quantities of positive and negative charges. In this case we must have equal numbers of Ca2+ and 02 ions, and the empirical formula of the compound is CaO. 

Before beginning, we need to understand the distinction between two types of chemical formulas, empirical formulas and molecular formulas. When the subscripts in a chemical formula represent the simplest ratio of the kinds of atoms in the compound, the formula is called an empirical formula. Most ionic compounds are described with empirical formulas. For example, chromium(III) oxide s formula, Cr203, is an empirical formula. The compound contains two chromium atoms for every three oxide atoms, and there is no lower ratio representing these relative amounts. 

If you know the formula of a compound, you can calculate its percent composition. Just the reverse can be done too. If you know the percent composition of a compound, you can calculate a formula for the compound. A formula calculated from percent composition data is called an empirical formula (one calculated from experimental data). The formulas of ionic compounds are always empirical formulas. The formulas of molecular compounds may be the same as their empirical formulas or they may be some whole-number multiple of it. You will learn how to do composition-from-fbrmula and fbrmula-from-composition calculations in this chapter. 

There are two broad classes of formulas for compounds empirical formulas and molecular formulas. The empirical formula shows the simplest ratio of elements in a compound and uses the smallest possible set of subscript numbers. Empirical formulas are also called simple formulas. The formulas of a// ionic compounds are empirical formulas. Since ionic compounds do not exist as molecules, their formulas are not molecular formulas. The formulas calculated from percent composition data are empirical formulas. (We ll get to these calculations later.)... 

The formulas of ionic compounds are always the same as their empirical formulas because ionic compounds do not consist of discrete molecular units. For example, a solid sample of sodium chloride (NaCl) consists of equal numbers of Na and Cl ions arranged in a three-dimensional network (Figure 2.11). In such a compound there is a 1 1 ratio of cations to anions so that the compound is electrically neutral. As you can see in Figure 2.11, no Na" ion in NaCl is associated with just one particular d ion. In fact, each Na ion is equally held by six surrounding Cl ions and vice versa. Thus NaCl is the empirical formula for sodium chloride. In other ionic compounds the actual structure may be different, but the arrangement of cations and anions is such that the compounds are all electrically neutral. Note that the charges on the cation and anion are not shown in the formula for an ionic compound. 

In order for ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero. If the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. If the charges are numerically equal, then no subscripts are necessary. This rule follows from the fact that because the formulas of ionic compounds are empirical formulas, the subscripts must always be reduced to the smallest ratios. Let us consider some examples. 

Explain why the chemical formulas of ionic compounds are always the same as their empirical formulas. 

The unit cell of a binary compound of copper and oxygen is shown here. Given this image and the ionic radii rc + = 0.74 A and ro>- = 1.26 A, (a) determine the empirical formula of this compound, (b) determine the coordination numbers of copper and oxygen, (c) estimate the length of the edge of the cubic unit cell, and (d) estimate the density of the compound. 

In the following index the compounds are listed by their empirical formulas in the order of increasing carbon content. Formulas of ionic compounds are given in brackets ions as well as components of solvates and adducts are separated by a period. 

Ionic compounds are compounds that are formed from cations and anions. Such compounds are held together by the electrostatic attraction between opposite charges. An example of an ionic compound is sodium chloride (NaCl), common table salt. The formulas of ionic compounds are usually the same as their empirical formulas because ionic compounds do not consist of discrete molecular units. For example, a solid sampk dium cl iid j((NagJ ccn s numbers of Na and Q ions... 

The formulas of ionic compounds are empirical formulas (Chapter 1). For example, sodium chloride consists of a lattice containing a large number of sodium and chloride ions in a 1 1 ratio. Each sodium ion is attracted to every chloride ion each chloride ion is attracted to every sodium ion. However, no molecules are present and thus only an empirical formula, NaCl, can be written. If the ionic nature is to be emphasized, then the formula may be written as [lSla Cl ]. 

The formula of an ionic compound is always an empirical formula, for example the empirical formula of aluminium oxide is AI2O3. 

As you recall, molecules are comprised of atoms chemically bonded into a discrete and identifiable unit. There are no ionic molecules, because every cation is attracted to every anion, so there are no identifiable ion pairs that belong exclusively to each other. Therefore, the formula of an ionic compound is an empirical formula. That is, the formula of an ionic compound lists the simplest ratio of cations to anions necessary to achieve electrical neutrality. For example, calcium oxide contains Ca2+ ions and O2- ions. While the formulas CaO and Ca202 both represent electrically neutral combination of ions, the subscripts in the latter formula have a common factor of 2. Therefore, Ca202 is not the simplest ratio of calcium cations to oxide anions. Therefore, the formulation of calcium oxide as Ca202 is incorrect. The correct empirical formula of calcium oxide is CaO. 

Through the use of chemical symbols and numerical subscripts, the formula of a compound can be written. The simplest formula that may be written is the empirical formula. In this formula, the subscripts are in the form of the simplest whole number ratio of the atoms in a molecule or of the ions in a formula unit. The molecular formula, however, represents the actual number of atoms in a molecule. For example, although CH20 represents the empirical formula of the sugar, glucose, C6H1206, represents the molecular formula. For water, H20, and carbon dioxide, C02, the empirical and the molecular formulas are the same. Ionic compounds are generally written as empirical formulas only for example, common table salt is NaCl. 

Empirical Formulas Ionic compounds form lattices that have an almost endless number of ions bonded together. Because it is impossible to count every ion in a sample, the lowest ratio of the elements present in the compound is used. A great example of this is NaCl. In every sample of NaCl there is one sodium ion to every chlorine ion. This does not mean that empirical formulas are limited to only ionic compounds. For example, if you examine the empirical formula of glucose C6H 206 and see that it is CH20, then you can better understand the term carbohydrate to mean hydrated carbon. ... 

Which of the following formulas identify ionic compounds but are not empirical formulas ... 

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