Writing formulas and naming compounds

I.  Chemical Formulas

      A. definition:  a combination of element symbols used to represent the composition of a compound

      B. the formula can reveal:

            1. how many different elements are present

            2. what the different elements are

            3. by use of a subscript, how many atoms of each element are present

            4. how many total atoms are present

            5. the combining ratio of the elements

      C. binary compound:  composed of only two elements

      D. polyatomic compound:  composed of three or more elements of which some are in a polyatomic ion

II. Oxidation Numbers

      A. represent the apparent charge on an atom or group of atoms (what charge does it ACT like it has in          the compound?)

      B. an ion is a charged atom or group of atoms

            1.  a single atom with a charge is a monatomic ion

            2.  two or more atoms as a group with a charge is a polyatomic ion

            3.  an ion can be positively or a negatively charged

            4. the charge on an ion is the oxidation number (if it is a single element) or sum of the oxidation                      numbers (if it is a group of elements)

      C. oxidation numbers are found by experiments to determine the combining ratio of the elements

III. Using Oxidation Numbers to Write Formulas using International Union of Pure and Applied Chemistry       System (IUPAC)

      A. made of positive and negative "ions"

            1. in general, the positive ion is written first

      ­      2. in general, the negative ion is written second

      B. compounds are electrically neutral, that means the net or overall charge must equal zero, or

                                                        total positive charge  +  total negative charge  =  zero

                                                      # (+)       +              # (–)       =  0

                (subscript of pos. ion) . (charge of pos. ion) + (subscript of neg. ion) . (charge of neg. ion) = 0                                                 

      C. subscripts are used to balance the positive and negative charges

            1. H2  means 2(H),  just like 2x

            2. (CN)2  means 2(C + N),  just like 2(x + y)

            3. (NO3)2  means 2(N + 3O),  just like 2(x + 3y)

      D. when "ions" don't combine one-for-one

            1. find the common multiple of the absolute value of their charges

            2. common multiple divided by the absolute value of the positive charge equals the number of                     positive "ions" needed which is the subscript for the positive "ion"

            3. common multiple divided by the absolute value of the negative charge equals the number of                     negative "ions" needed which is the subscript for the negative "ion"

            4. example:    

                        chromium(III) oxide

                              Cr +3                       O –2                              common multiple is 6

                        6 ÷ |+3| = 2           6 ÷ |–2| = 3               need two Cr+3's and three O–2's

                        therefore the formula is:  Cr2O3

IV. Writing Formulas using the Greek prefix system

      A.  the prefix on the positive part tells you the number for the subscript

      B.  the prefix on the negative part tells you the number for the subscript

      C.  mono = 1,  di = 2,  tri = 3,  tetra = 4,  penta = 5,  hexa = 6,  hepta or septa = 7,  octa = 8, 

            nona = 9,  deca = 10,  etc.

      D.  examples:  CO  carbon monoxide,   SO2  sulfur dioxide,  N2O5  dinitrogen pentoxide   

V. Naming Compounds using IUPAC system

      A. in general, positive "ions" are named first:

            1. monatomic positive "ion":

                  a. single oxidation number for the element: use the name of the element

                  b. more than one oxidation number for the element:  use a Roman numeral after the element's                            name (in parenthesis) to distinguish,  the  Roman numeral is equal to the charge on the "ion",                            iron (III) = Fe+3

            2. polyatomic positive "ion":  use the name the "ion", 

                  the only one we really use is ammonium, NH4+1

      B. in general, negative "ions" are named second:

            1. monatomic negative "ion": change ending of element name to -ide   (this is already done on your                oxidation numbers sheet, so all you do is use the name listed on the oxidation numbers sheet)

            2. polyatomic negative "ion":  use name of "ion"

      C. to determine the Roman numeral in the name go back to   #(+)  +  #(–) = 0 and solve for the unknown

            example:  N2O5 ,    2(x) + 5(–2) = 0,   x = +5,  so the compound is nitrogen(V) oxide

VI. Naming Compounds using the Greek prefix system

      A.  the Greek prefix system is the naming prefered when a compound is made from two nonmetals

      B.  the positive part is always the name of the element

      C.  the negative part the name of the element with the ending changed and always ends in   -ide

      D.  greek prefix is used to indicate the number of atoms present i.e. the subscript

      E.  mono = 1,  di = 2,  tri = 3,  tetra = 4,  penta = 5,  hexa = 6,  hepta or septa = 7,  octa = 8, 

            nona = 9,  deca = 10,  etc.

      F.  mono on the positive is often not used, if there is no prefix it is understood to be mono

      G.  examples:  carbon dioxide  CO2,   sulfur trioxide  SO3,     dinitrogen tetraoxide  N2O4

VII. Exceptions

      A. diatomic molecules:  there are seven diatomic molecules, MEMORIZE THEM,  these are:

                  H2            N2    O2                F2                  Cl2            Br2   I2

            or remember by the rhyme   HCl BrI FNO          or  H BrON ClIF            or  BrINCl HOF

      B. acids:  MEMORIZE the names and formulas

                  HCl            HNO3        H2SO4        H2CO3        CH3COOH  or  HC2H3O2

            (hydrochloric,   nitric,          sulfuric,             carbonic,          acetic)

      C.  common names:  many compounds are known by there common name

            examples:  ammonia  NH3,  water H2O

      D.  old names:  many compounds made with positive ions with Roman numerals are called by  -ous for              the lower number and  -ic for the higher number

            examples:  ferrous oxide  FeO instead of iron (II) oxide,  ferric oxide  Fe2O3 instead of iron (III) oxide

      E. organic compounds: compounds based on carbon are named differently,

            examples:  CH4 is methane, CH3CH2CH3 is propane, CH3CH2CH2CH3 is butane