Writing Covalent Compound Formulas

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

Naming Molecular (CoValent) Compounds and Writing Their Formulas... 

The azide ion has an ionic radius of 148 pm and forms many ionic and covalent compounds that are similar to those of the halides, (a) Write the Lewis formula for the azide ion and predict the N—N—N bond angle, (b) On the basis of its ionic radius, where in Group 17 would you place the azide ion ... 

You have seen how Lewis structures can help you draw models of ionic, covalent, and polar covalent compounds. When you draw a Lewis structure, you can count how many electrons are needed by each atom to achieve a stable octet. Thus, you can find out the ratio in which the atoms combine. Once you know the ratio of the atoms, you can write the chemical formula of the compound. Drawing Lewis structures can become overwhelming, however, when you are dealing with large molecules. Is there a faster and easier method for writing chemical formulas ... 

You can use valences to write chemical formulas. This method is faster than using Lewis structures to determine chemical formulas. As well, you can use this method for both ionic and covalent compounds. In order to write a chemical formula using valences, you need to know which elements (or polyatomic ions) are in the compound, and their valences. You also need to know how to use the zero sum rule For neutral chemical formulas containing ions, the sum of positive valences plus negative valences of the atoms in a compound must equal zero. 

In section 3.4, you learned how to name ionic and covalent compounds. You also learned how to write their formulas. In Chapter 4, you will learn how compounds and elements interact in nature, in the laboratory, and in everyday life. These interactions are responsible for the tremendous variety of substances and materials found on Earth. 

Q The formula for hydrogen peroxide is H202. Explain why it is not correct to write the formula for this covalent compound as HO. 

Since covalent compounds do not produce ions in aqueous solution, write their molecular formulas. Water is a common example, because it dissociates only very slightly into ions. When a reaction involves a gas, always include the gas in the net ionic equation. 

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. 

Section 16.1 introduces the concept of oxidation number and how to calculate the oxidation number of an element from the formula of the compound or ion of which it is a part. Section 16.2 describes how to use the oxidation numbers to name compounds, formalizing and extending the rules given in Chapter 6. Section 16.3 shows how to predict possible oxidation numbers from the position of the element in the periodic table and how to use these oxidation numbers to write probable formulas for covalent compounds. Section 16.4 presents a systematic method for balancing equations in which oxidation numbers change. 

B. Write the formula of a covalent compound of element X with element Y if X has an oxidation number +6 and Y has an oxidation number 2. 

Now that you understand chemical formulas, you can look back at the other chemical compounds shown earlier in this chapter, and write their chemical formulas. A water molecule contains one oxygen atom and two hydrogen atoms, so its formula is H2O. Ammonia, shown in Figure 22, is a covalent compound that contains one nitrogen atom and three hydrogen atoms. Its chemical formula is NH3. 

Write the Lewis formula for gaseous beryllium chloride, BeCl2, a covalent compound. 

This covalent compound of boron is an example of limitation type B. As we follow the steps in writing the Lewis formula, we use six electrons as the number needed by boron in step 2. Steps 3 and 4 should show only three pairs of electrons for boron. 

Write the Fewis formula for phosphorus pentafluoride, PF5, a covalent compound. 

Write the Lewis formula for each of the following covalent compounds. Which ones contain at least one atom with a share in less than an octet of valence electrons (a) BeBr2 (b) BBrj (c) NCI3 (d) AICI3. 

Now that you know that two major types of compounds exist, you may wonder where else you come into contact with these compounds on a day-to-day basis. In Chapter 5, you ll look at more examples of ionic and covalent compounds, including compounds more complex than the simple ones used as examples in this chapter. You ll also learn the important practical skill of naming and writing the formulas of compounds, as well as how to identify a few special categories of compounds such as acids, bases, and organic compounds. 

Toothpaste Ingredients Examine the ingredient label on a tube of toothpaste. Write formulas for as many of the chemical names listed as you can. List whether each ingredient is an ionic or a covalent compound. 

Now let s go the other way and convert from systematic names to chemical formulas. The first step in writing formulas when given the systematic name of a binary covalent compound is to recognize the name as representing a binary covalent compound. It will have one of the following general forms. 

Follow these steps for writing formulas for binary covalent compounds when you are given a systematic name. Notice that they are the reverse of the steps for writing names from chemical formulas. 

Example 3.5- Writing Formulas for Binary Covalent Compounds... 

The compound represented by the hall-and-stick model to the left is used in the processing of nuclear fuels. Although bromine atoms most commonly form one covalent bond, they can form five bonds, as in the molecule shown here, in which the central sphere represents a bromine atom. The other atoms are fluorine atoms. Write this compound s chemical formula and name. List the bromine atom first in the chemical formula. 

For each of the following pairs of elements, state whether the binary compound they form is likely to be ionic or covalent. Write the empirical formula and name of the compound (a) B and F, (b) K and Br. 

We need to learn how to properly name and write formulas for ionic and covalent compounds. We should become familiar with some of their properties and be able to relate these properties to the structure and bonding of the compounds. 

Which of the following sets of information allows you to obtain the molecular formula of a covalent compound In each case that allows it, explain how you would proceed (write a solution Plan ). 

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