Naming and Writing Formulas

1. Naming and Writing Formulas

2. Classifying Compounds The system for naming an ionic compound is different from that for naming a covalent compound, so before a compound can be named, it must be classified as ionic or covalent. Classifying a compound is not an easy task, but for the purposes of naming them, we employ a simple test: Is there a metal or a polyatomic ion present? If the answer is yes, use the system for naming ionic compounds. If the answer is no, use the system for naming covalent compounds.

3. Naming Ionic Compounds Ionic compounds are named simply by naming the ions present. There are, however, two complicating factors: I. Some metals form more than one ion. II. Identifying polyatomic ions I. Metals that form more than one ion, such as iron, add a Roman numeral to the name to indicate the charge: Fe2+ is called iron (II) and Fe3+is called iron (III) Assume a Roman numeral is required for any metalexcept 1. metals in groups IA and IIA on the periodic table 2. aluminum, cadmium, silver, and zinc

4. Stock System • Some elements form more than one ion • Use roman numerals to distinguish the ion formed Fe2+ Fe3+ Iron (II) Iron (III) Only for metals that form more than one ion

5. Naming using the stock system • Name of the cation + Roman numeral indicating charge then the name of the anion CuCl2 copper (II) chloride

6. Rules for Naming Polyatomic Ions • No good rules • Since most polyatomic ions contain oxygen, we look at the number of oxygen atoms • One less oxygen than –ate is an –ite NO3- NO2- Nitrate Nitrite

7. One more oxygen than –ate add per- to the beginning of the name ClO3- ClO4- Chlorate Perchlorate One less oxygen than –ite add hypo- to the beginning of the name ClO2- ClO- Chlorite Hypochlorite

8. Naming Ionic Compounds (continued) If a Roman numeral is required, the charge on the metal ion must be determined from the charge on the negative ion. Helpful Rules to Remember A metal ion is always positive. The Roman numeral indicates the charge,not the subscript. The positive and negative charges must cancel (total charge must = 0). Nonmetals are always negative & can never form more than one monatomic ion. Examples

9. Naming Ionic Compounds (continued) II. Polyatomic ions each have specific names which must be memorized so they can be recognized on sight. (At this point, if you are asked to name any compound that contains more than two elements, it will contain at least one polyatomic ion.) A few of the more common polyatomic ions

10. Naming Monatomic Ions • Monatomic cations – just use the name Li+ Lithium • Monatomic anions – drop the ending and add – ide Br Br- Bromine Bromide

11. Naming Ionic Compounds:Examples Na2SO4 sodium sulfate Fe(NO3)2 iron (II) nitrate AlCl3 aluminum chloride PbI4 lead (IV) iodide (NH4)3PO4 ammonium phosphate Mg3N2 magnesium nitride AgC2H3O2 silver acetate

12. Naming Covalent Compounds Covalent compounds are named by adding prefixes to the element names. The compounds named in this way are binary covalent compounds. ‘Binary’ means that only two atom are present. ‘Covalent’ (in this context) means both elements are nonmetals. A prefix is added to the name of the first element in the formula if more than one atom of it is present. (The less electronegative element is typically written first.) A prefix is always added to the name of the second element in the formula. The second element will use the form of its name ending in ‘ide’.

13. Naming Covalent Compounds Prefixes Note: When a prefix ending in ‘o’ or ‘a’ is added to ‘oxide’, the final vowel in the prefix is dropped.

14. Naming Binary Covalent Compounds:Examples N2S4 dinitrogen tetrasulfide NI3 nitrogen triiodide XeF6 xenon hexafluoride CCl4 carbon tetrachloride P2O5 diphosphorus pentoxide SO3 sulfur trioxide

15. Classifying Compounds Classifying a compound using its name is not as difficult as using its formula. The names of covalent compounds will be easily recognized by the presence of the prefixes (mono-, di-, tri-, etc.). If no prefixes are present in the name, the compound is ionic. (Exception: some polyatomic ion names always contain prefixes (such as dichromate) but those will be memorized and recognized as ions.)

16. Writing Formulas for Ionic Compounds Formulas for ionic compounds are written by balancing the positive and negative charges on the ions present. The total positive charge must equal the total negative charge because the number of electrons lost by one element (or group of elements) must equal the number gained by the other(s). Polyatomic ion names must still be recognized from memory (e.g. ammonium nitrate), but metals will have a Roman numeral associated with them if there is the possibility of more than one ion (e.g. copper (I) chloride or copper (II) chloride). The Roman numeral indicates the charge on the ion not the number of ions in the formula.

17. Writing Formulas for Ionic Compounds (continued) Helpful Rules to Remember A metal ion is always positive. The Roman numeral indicates the charge,not the subscript. The positive and negative charges must cancel (total charge must = 0). If more than one polyatomic ion is needed, put it in parentheses, and place a subscript outside the parentheses. Examples

18. When a polyatomic ion such as nitrate or sulfate combines with other elements We follow the same rules for writing formulas

19. What is the formula for the ionc compound formed by sodium and a sulfate ion? Rule 1: write the symbols of the metal and of the polyatomic group Na SO4 Rule 2: write the ionic charges 1+ 2- Na SO4

20. Crisscross rule: crisscross the ionic charges 1+ 2- Na SO4 Note that polyatomic ions do not ”reduce” . Formula cannot be simplified Na1SO2 because SO4 is a group The formula is Na2SO4

21. Try this: what is the formula of lead(IV) carbonate?

22. Writing Formulas for Covalent Compounds The names of covalent compounds contain prefixes that indicate the number of atoms of each element present. If no prefix is present on the name of the first element, there is only one atom of that element in the formula (its subscript will be 1). A prefix will always be present on the name of the second element. The second element will use the form of its name ending in Remember: • The compounds named in this way are binary covalent compounds (they contain only two elements, both of which are nonmetals). • When in covalent compounds, atoms do not have charges. Subscripts are determined directly from the prefixes in the name.

23. Writing Formulas for Binary Covalent Compounds:Examples nitrogen dioxide NO2 diphosphorus pentoxide P2O5 xenon tetrafluoride XeF4 sulfur hexafluoride SF6

24. Naming Acids • If the name of the anion ends in –ide, the acid name begins with the prefix hydro- The stem of the anion is given then the suffix -ic is added and is followed by the word acid. H2S (anion sulfur)  hydro + stem + ic + acid Hydrosulfuric Acid

25. Naming Acids • If the name of the anion ends in –ite, the acid name is the stem of the anion with the suffix –ous and is followed by the word acid. H2SO3 (anion Sulfite)  stem + ous + acid Sulfurous Acid

26. Naming Acids • If the name of the anion ends in –ate, the acid name is the stem of the anion with the suffix –ic and is followed by the word acid. H2SO4 (anion Sulfate)  stem + ic + acid) Sulfuric Acid

27. Naming Acids Does the formula contain Oxygen? No Yes Hydro “stem”ic acid Anion end in __ ? “ate” “ite” “stem”ic acid “stem”ous acid

28. Writing Formulas for Acids • If the name starts with “hydro” Hydrosulfuric Acid Write the hydrogen ion with charge. H+1 Write the anion with the proper charge. S-2 Balance the charges using subscripts. H2S

29. Writing Formulas for Acids • If the name contains the suffix –ous Sulfurous Acid Write the hydrogen ion with charge. H+1 Look up the polyatomic ion (sulfite) and write it with the correct charge. SO3-2 Balance the charges using subscripts. H2SO3

30. Writing Formulas for Acids • If the name contains the suffix –ic without the prefix hydro Sulfuric Acid Write the hydrogen ion with charge. H+1 Look up the polyatomic ion (sulfate) and write it with the correct charge. SO4-2 Balance the charges using subscripts. H2SO4

31. Writing Formulas/Naming Acids • Remember the following statements… “I ate it and it was icky.” -ate becomes -ic “Riteous” -ite becomes -ous (Righteous)

32. Naming Bases • Bases are named using the traditional ionic naming system. Metal name + polyatomic ion name Examples: Ca(OH)2 = calcium hydroxide NaOH = sodium hydroxide Al(OH)3 = aluminum hydroxide

33. Writing Base Formulas • Base formulas are written using the traditional ionic system. Look up the metal ion. Write the symbol with the proper charge. Ca+2 Look up the polyatomic ion. With bases, this will always be hydroxide, OH-1. Balance the charges using subscripts. Ca(OH)2

34. The rules for naming and writing formulas for compounds are possible because of 2 laws: • 1. The Law of Definite Proportions: samples of any chemical compound, the masses of the elements are always in the same proportions. • 2. The Law of Multiple Proportions: Whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.