Predicting Formulas of Ionic Compounds

In previous examples, we learned that when a metal and a nonmetal react to form an ionic compound, the metal loses one or more electrons to the nonmetal. In Chapter 6, where we learned to name compounds and write formulas, we saw that Group lA metals always form +1 cations, whereas Group 2A form +2 cations. Group 7A elements form -1 anions and Group 6A elements form -2 anions. 

It stands to reason, then, that this pattern is directly related to the stability of the noble gas configuration. Metals lose electrons to attain the electron configuration of a noble gas (the previous one on the periodic table). A nonmetal forms an ion by gaining enough electrons to achieve the electron configuration of the noble gas following it on the periodic table. These observations lead us to an important chemical principle  

In almost all stable chemical compounds of representative elements, each atom attains a noble gas electron configuration. This concept forms the basis for our understanding of chemical bonding. 

We can apply this principle in predicting the formulas of ionic compounds. To predict the formula of an ionic compound, we must recognize that chemical compounds are always electrically neutral. In addition, the metal will lose electrons to achieve noble gas configuration and the nonmetal will gain electrons to achieve noble gas configuration. Consider the compound formed between barium and sulfur. Barium has two valence electrons, whereas sulfur has six valence electrons  

If barium loses two electrons, it will achieve the configuration of xenon. By gaining two electrons, sulfur achieves the configuration of argon. Consequently, a pair of electrons is transferred between atoms. Now we have Ba and Since compounds are electrically neutral, there must be a ratio of one Ba to one S, giving the formula BaS. 

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Predict formulas of ionic compounds from charges of ions. 

O How do we use electron configurations to predict formulas of ionic compounds ... 

Types of Chemical Bonds electronegativity Bond Polarity and Dipole Moments Ions Electron Configurations and Sizes Predicting Formulas of Ionic Compounds Sizes of Ions... 

Predicting Formulas of Ionic Compounds Sizes of Ions... 

OBJECTIVES To learn about Stable electron configurations. To learn to predict the formulas of ionic compounds. 

When Na(s) and Cl2(g) react the ionic compound NaCl(s) forms. Predict the formula of ionic compounds formed from the following combinations of elements ... 

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. 

These chapters introduce you to the two main types of bonding found in nature ionic bonding and cov ent bonding. 1 show you how to predict the formulas of ionic compounds (salts) and how to name them. 1 explain covalent bonding, how to draw Lewis structural formulas, and how to predict the shapes of simple molecules. 1 tell you about chemical reactions and show you the various general types. In addition, 1 cover chemical equilibrium, kinetics, and electrochemistry — batteries, cells, and electroplating. 

In this chapter, I introduce you to ionic bonding, the type of bonding that holds salts together. I discuss simple ions and polyatomic ions how they form and how they combine. 1 also show you how to predict the formulas of ionic compounds and how chemists detect ionic bonds. 

It is important to realize that you cannot predict the formulas of most molecular substances in the same way that you predict the formulas of ionic compounds. That is why we name them using prefixes that explicitiy indicate their composition. Compounds that contain hydrogen and one other element are an important exception, however. These compounds can be treated as if they contained ions. Thus, HCl is hydrogen chloride (diis is die name used for the pure... 

Cations and anions combine in very predictable ways within ionic compounds, always acting to neutralize overall charge. Therefore, the name of an ionic compound implies more than just the identity of the atoms that make it up. It also helps you determine the correct chemical formula, which tells you the ratio in which the elements combine. Consider these two examples, both of which involve lithium ... 

An ionic bond results from the electrostatic attraction of oppositely charged ions. Once we know what ions an element is likely to form, we shall be able to predict the formulas of its compounds and explain some of their properties. 

In Chapter 5, we learned to write formulas for ionic compounds from the charges on the ions and to recognize the ions from the formulas of the compounds. For example, we know that aluminum chloride is AICI3 and that VCI2 contains ions. We cannot make comparable deductions for covalent compounds because they have no ions there are no charges to balance. To make similar predictions for species with covalent bonds, we need to use the concept of oxidation number, also called oxidation state. A system with some arbitrary rules allows us to predict formulas for covalent compounds from the positions of the elements in the periodic table and also to balance equations for complicated oxidation-reduction reactions. 

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. 

If the reactants in an equation are two elements, the only way in which they can react is to form a binary compound, which is composed of two elements. Often, when a metal reacts with a nonmetal, electrons are transferred and an ionic compound is formed. You can use the charges of the ions to predict the formula of the compound formed. Metals in Groups I and 2 lose one electron and two electrons, respectively. Nonmetals in Groups 16 and 17 gain two electrons and one electron, respectively. Using the charges on the ions, you can predict the formula of the product of a synthesis reaction, such as the one in Figure 10. 

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