Unit 4.2 Inorganic Binary Compounds Type I

How does shorthand work? Shorthand was a very popular way of recording speech, especially in dictating letters and in court testimony. Instead of trying to write out all the words, the person taking the dictation would use a set of symbols that represented syllables or words. The pages above show a shorthand version of “A Christmas Carol” written by Charles Dickens. Unless you know shorthand, the passage is meaningless. But knowing shorthand allows you to read this classic story.

Different professions also use a type of shorthand in communication to save time. Chemists use chemical symbols in combination to indicate specific compounds. There are two advantages to this approach:

1. The compound under discussion is clearly described so there can be no confusion about its identity. 2. Chemical symbols represent a universal language that all chemists can understand, no matter what their native language is.

Writing Formulas for Binary Ionic Compounds

If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charge to cancel each other out. Consider the compound aluminum nitride. The ions are:

Al+3 N-3 Since the ions have charges that are equal in magnitude, one of each will be the lowest ratio of ions in the AlN formula.

Remember that the total charge must be equal to zero. (+3) + (-3) = 0

The ions for the compound lithium oxide, two lithium ions are required to balance out the charge of one oxide ion.

Li+1 O-2

Li2O

Again the total charge must be equal to zero. 2(+1) + (-2) = 0

An alternative way to writing a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped. Shown below is the crisscross method for aluminum oxide. The red arrows indicate that the 3 from the 3+ charge will cross over to Al+3 O-2 become the subscript of the O. The 2 from the 2− charge will cross over Al2O3 to become the subscript of the Al.

The total charge must be equal to zero. 2(+3) + 3(-2) = 0

Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions. In the case of aluminum nitride, the crisscross method would yield a formula of Al3N3, which is not correct. It must be reduced to AlN. Following the crisscross method to write the formula for lead (IV) oxide would involve the following steps:

+4 -2 Pb O The crisscross first yields Pb2O4 for the formula, but that must be reduced to the lower ratio and Pb2O4 PbO2 is the correct formula.

PbO2

The total charge must be equal to zero. (+4) + 2(-2) = 0

Inorganic Binary Compounds Type I

An inorganic binary compound type I is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. Monatomic meaning that it is a one atom ion. These would include ions such as those made from Alkali metals, alkali Earth metals, or Halogens. It may include other ions.

Naming Binary Ionic Compounds When examining the formula of a compound in order to name it, you must first decide what kind of compound it is. For an inorganic binary compound type I, it must be made of two types of atoms only! A metal will always be the first element in the formula, while a nonmetal will always be the second.

When naming, the metal cation is named first. Remember that the positive metal ions keep the name given to the metal. For example the sodium ion, Na+1, is called sodium.

The nonmetal anion is named second. Monatomic, or one atom negative ions always drop the ending and add –ide to the name. For example the ion made from chlorine, Cl-1, is called chloride.

The compound made from one sodium ion and one chloride ion, NaCl, is called sodium chloride.

Subscripts are used to balance the charge to equal zero net charge for the molecule. Subscripts in the formula do not affect the name. The following chart shows three examples.

Naming Inorganic Binary Compounds Types I Formula Name Net charge KF Potassium fluoride (+1) + (-1) = 0

Na3N Sodium nitride 3(+1) + (-3) = 0

Ca3P2 Calcium phosphide 3(+2) + 2(-3) = 0

Notice that in each of the formulas above, the overall charge of the compound is zero.

Potassium ion is K+1, while fluoride ion is F-1. Since the magnitude of the charges is equal, the formula contains one of each ion. K+1 F-1 (+1) + (-1) = 0 KF This would also be the case for a compound such as magnesium sulfide, MgS, in which the ions are Mg2+ and S2-. Mg+2 S-2 (+2) + (-2) = 0 MgS

In order to make a neutral compound, three of the +1 sodium ions are required in order to balance out the single -3 nitride ion. Na+1 N-3 The crisscross method is used with subscripts to balance the compound to zero net charge. 3(+1) + (-3) = 0 Na3N

So the Na is given a subscript of 3. For calcium phosphide, the calcium ion is Ca+2, while the phosphide ion is P-3. The least common multiple of 2 and 3 is 6 (3 x +2 = 6, and 2 x -3 = 6). To make the compound neutral, three calcium ions have a total charge of +6, while two phosphide ions have a total charge of -6. The Ca is given a subscript of 3, while the P is given a subscript of 2.

+2 -3 Ca P 3(+2) + 2(-3) = 0

Ca3P2 Summary

 A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion.  Monatomic ion is a one atom ion.  Formulas for binary compounds begin with the metal followed by the non-metal.  The metal cation keeps the name of the metal.  The non-metallic anion is named by dropping the end of the element and adding –ide.  Positive and negative charges must cancel each other out.  For binary compounds, it is not necessary to indicate the number of ions in the compound using subscripts.  Ionic compound formulas are written using the lowest ratio of ions.

Review

1. What is a binary compound? 2. Which of the following is not a binary compound? a. NaCl

b. KH2PO4 c. KBr 3. Name the following compounds: a. NaBr

b. MgCl2 c. LiI d. CaO 4. Write formulas for the binary ionic compounds formed between the following pairs of elements: a. cesium and fluorine b. calcium and sulfur c. aluminum and chlorine d. zinc and nitrogen

Answers

1. A molecule made of only two types of atoms.

2. KH2PO4 has more than two types of atoms. 3. a. NaBr Sodium bromide

b. MgCl2 Magnesium chloride c. LiI Lithium iodide d. CaO Calcium oxide 4. Write formulas for the binary ionic compounds formed between the following pairs of elements: a. cesium and fluorine CsF Cesium fluoride b. calcium and sulfur CaS Calcium sulfide

c. aluminum and chlorine AlCl3 Aluminum chloride

d. zinc and nitrogen Zn3N2 Zinc nitride