Lecture notes: CHEM103 December 2, 2008

 

 

 

NOW, THE COMPLETE RULES - using "Pauling Electronegativities"

                                                                                                                          

1)    count # of valence electrons for each (neutral) atom; include electrons for polyatomic ions à add them up

2)    position atoms so that the least electronegative (non-H) atom(s) is in the center

3)    draw a skeleton structure w/ single bonds between each atom (-2 per bond from total)

4)    add lone pairs to outer atoms until used up (-2 per pair from total)

5)    add any remaining electrons (as lone pairs) to central atom(s) (-2 per pair from total)

6)    if central atom is still short of an octet, shift lone pair(s) to form a multiple bond to central atom(s)

later…

7)    calculate formal charges to check structures (expanded/deficient octets valid?)

8)    look for “resonance structures”

 

 

methane, CH4 (step 3)

 

                   freon, CCl2F2 (step 4)

 

                   phosphine, PH3 (step 5)

 

                   carbon disulfide, CS2 (step 6)

 

                   phosphorous pentachloride, PCl5 (step 7)

                  

nitrate anion (step 1)

 

 

 

 

MORE EXAMPLES:

         

hydrogen sulfide (H2S)

 

methyl iodide (CH3I)

 

sulfur tetrachloride (SCl4)

 

carbon dioxide (CO2)

 

ethylene (C2H4)

 

acetylene (C2H2)

 

ethanol (C2H5OH)

 

ammonium cation

 

acetate anion

 

 

 

 

 

 

 

BUT THERE ARE EXCEPTIONS (to the octet “rule”)!

 

       I.            expanded (or deficient) octet

 

    II.            formal charges

 

 III.            hydrocarbons

 

 IV.            resonance structures

 

    V.            radicals

 

 

 

 

 

     I.            Expanded/deficient octets:

 

KEY: deficient – for atoms in group I, (II), III – deficient based on formal charge considerations

 

KEY: expanded – for atoms in period 3 OR lower… you may “expand” the octet to satisfy the formal charge

 

examples:

 

SF6

BH3

 

 

09t7

 

 

 

 

     I.            Formal charge:

 

Definition: a useful calculation made for each atom which expresses the “share” of electrons each atom has…

(USEFUL IN DEALING WITH EXCEPTIONS!!!)

 

 

FORMAL CHARGE RULES

 

CALCULATION:

 

Calculate formal charge for EACH atom in the molecule (separately) as:

 

                             # of valence electrons

--        {the total of all electrons surrounding atom except [(1 for each bond) and (2 for each lone pair)]}

 

                   =       Formal charge

 

                   or – valence # minus all lone pair electrons and half of all bonded electrons

 

 

 

 

APPLICATION:

 

1.    minimize the size of any single formal charge;

 

2.    minimize the number of formal charges;

 

3.    allow negative formal charges to remain on more electronegative atoms (and vice versa)

 

examples:

 

                             NH3, HOCl, HOCN, NO2 +, N2O (nitrous oxide), NO3 – (nitrate)

 

 

 

 

                   (note: why NOT N≡N=O?)