S ECTION 6–3: N AMING C OMPOUNDS AND W RITING F ORMULAS

O BJECTIVES Recognize and describe binary ionic compounds, metals with multiple ions, and polyatomic ions. Name and determine chemical formulas for ionic and molecular compounds.

D ESCRIBING I ONIC C OMPOUNDS The name of an ionic compound must distinguish the compound from other ionic compounds containing the same elements. The formula of an ionic compound describes the ratio of the ions in the compound. We must be able to differentiate between the different types of compounds that contain the same elements. Red copper oxide Black copper oxide

B INARY I ONIC C OMPOUNDS A compound that contains only two elements is called a binary compound. We can easily name an ionic binary compound by using the name of the cation and the name of the ion. If we have a compound that contains sodium and chlorine would be called “sodium chloride.” It doesn’t matter how many atoms of each there are.

B INARY I ONIC C OMPOUNDS Sodium chloride  NaCl Aluminum oxide  Al 2 O 3 Beryllium iodide  BeI 2 Magnesium sulfide  Mg 2 S Calcium nitride  Ca 3 N 2 Boron fluoride  BF 3

M ETALS WITH M ULTIPLE I ONS Most transition metals have more than one charge. For instance, copper can have a 1+ or a 2+ charge. Iron can have a 2+ or 3+ charge. ALL transition metals will have a positive charge! When we name a compound with a transition metal that has more than one charge, we indicate the charge with a roman numeral: Copper (I) sulfide  Cu 2 S Copper (II) sulfide  CuS

P OLYATOMIC I ONS A covalently bonded group of atoms that has a positive or negative charge and acts as a unit is a polyatomic ion. For example, (OH) - is the hydroxide ion. It has a -1 charge.

W RITING F ORMULAS FOR I ONIC C OMPOUNDS In writing ionic compounds, you place the symbol for the cation first, followed by the symbol for the anion. Use subscripts to show the ratio of the ions in the compound. All compounds are neutral, the total charges on the cations and the anions must add up to zero.

W RITING F ORMULAS FOR I ONIC C OMPOUNDS Read and understand what the problem is telling you. Plan and solve how to do the problem. Look back and check your answer.

P ROBLEM #1 What is the formula for Calcium Chloride? Ca has a +2 charge Cl has a -1 charge It takes 2 of the -1 charges to balance the +2 charge. Thus 2 chloride ions for each calcium ion. CaCl 2 is the formula.

P ROBLEM #2 What is the formula for calcium oxide?

P ROBLEM #3 What is the formula for copper(I) sulfide?

D ESCRIBING M OLECULAR C OMPOUNDS The name and formula of a molecular compound describe the type and number of atoms in a molecule of the compound. Naming molecular compounds the general rule is that the most metallic element appears first in the name. The name of the second element is changed to end in the suffix –ide as in carbon dioxide.

D ESCRIBING M OLECULAR C OMPOUNDS CONT. Writing the formula for the molecular compound is easy. Write the symbols for the elements in the order that the elements appear in the name. The prefixes indicate the number of atoms of each element in the molecule. The prefixes appear as subscripts in the formulas. Example: diphosphorus tetrafluoride. The formula will be P 2 F 4