Periodic Table Worksheet s1

Name ______Naming Molecular Compounds

As you remember, molecular compounds are relatively small groups of covalently bonded nonmetal atoms, that have weak to moderate intermolecular forces. There are two methods for naming binary molecular compounds. The most commonly used method is to use prefixes (Table 1) to indicate how many of each type of atom there is in the molecular formula. In addition, the suffix –ide is added to the root of the final element’s name. However, if there is only one atom for the first element in the formula we do not use the prefix mono-, but we do use the prefix mono- if there is only one atom for the second element in the formula. As examples, Cl2O7, CF4 and CO are named dichlorine heptaoxide, carbon tetrafluoride, and carbon monoxide, respectively. Exceptions include the binary molecular compounds of hydrogen and the halogens, HF, HCl, HBr and HI, which do not use the prefix mono-, e.g. hydrogen fluoride. There are a number of binary compounds with traditional names that do not follow this system, with the most important examples being water (H2O) and ammonia (NH3). In addition, organic compounds, molecules containing carbon bonded to hydrogen, have their own unique naming system. The International Union of Pure and Applied Scientists (IUPAC) have tried to standardize the methods for naming compounds, but are faced with years and years of tradition associated with the naming of chemical formulas.

Table 1. Prefixes used for naming binary molecular compounds.

Prefix Number mono- 1 di- 2 tri- 3 tetra- 4 penta- 5 hexa- 6 hepta- 7

An alternate method1 is the “stock system,” whose approach is identical to that described for ionic compounds. The utility of this system is that we do not need to remember separate methods for ionic (metal and nonmetals) and molecular (all nonmetals) compounds. The drawback is that we then need to be able to identify the oxidation numbers or the charges of each individual atom in a formula. In the case of molecular compounds it is assumed that the more electronegative element, which will usually be the second element in the formula, has a negative oxidation number (charge) and the more electropositive element, which will be the first element in the formula, has a positive oxidation number. It is also important to remember that for molecular compounds, having not net charge, the sum of the positive and negative oxidation numbers must equal zero. For example, carbon tetrafluoride, CF4, would be carbon (IV) fluoride (reverse the 4+ - criss-cross to give C and 4 F ) using the stock system. For carbon dioxide, CO2, which has two oxides (2 O2-), the oxidation number of carbon must be 4+, giving the stock name of carbon (IV) oxide. In contrast, carbon monoxide, CO (C2+ and O2-), is carbon (II) oxide in the stock system.

1 Primarily used on the New York State Chemistry Regents exam and nowhere else, to my knowledge.

Fill in the blanks for following molecular compounds and their names.

Formula Common Name Stock Name

1. CO2 ______

2. ______carbon monoxide ______

3. ______sulfur (VI) oxide

4. SO2 ______

5. ______dinitrogen monoxide ______

6. ______nitrogen (II) oxide

7. NO2 ______

8. ______dinitrogen trioxide ______

9. N2O4 ______

10. N2O5 ______

11. ______phosphorus trichloride ______

12. ______phosphorus (V) chloride

13. NH3 ______

14. H2O ______

15. ______sulfur (VI) fluoride

16. ______sulfur tetrafluoride ______

17. P2O5 ______

18. ______carbon (IV) chloride

19. CS2 ______

20. ______oxygen (II) fluoride

Answers:

1. carbon dioxide, carbon (IV) oxide 2. CO, carbon (II) oxide 3. SO3, sulfur trioxide 4. sulfur dioxide, sulfur (IV) oxide 5. N2O, nitrogen (I) oxide 6. NO, nitrogen monoxide 7. nitrogen dioxide, nitrogen (IV) oxide 8. N2O3, nitrogen (III) oxide 9. dinitrogen tetroxide, nitrogen (IV) oxide 10. dinitrogen pentoxide, nitrogen (V) oxide 11. PCl3, phosphorus (III) chloride 12. PCl5, phosphorus pentachloride 13. ammonia, ammonia 14. water, water 15. SF6, sulfur hexafluoride 16. SF4, sulfur (IV) fluoride 17. diphosphorus pentoxide, phosphorus (V) oxide 18. CCl4, carbon tetrachloride 19. carbon disulfide, carbon (IV) sulfide 20. OF2, oxygen difluoride