IDENTIFYING MULTIPLE BONDS IN POLYATOMIC MOLECULES Like diatomic molecules polyatomic molecules can contain double or triple bonds To identify multiple bonds we can draw electron dot configurations around atoms in molecules and figure out whether they have an octet POLYATOMIC MOLECULES CONTAINING ELEMENTS BELOW THE SECOND ROW Elements below the 2nd row and on the right side of the periodic table have a tendency to not follow the octet rule They are larger and can fit more than 4 atoms around them They contain empty d orbitals in the valence shell GENERAL RULES FOR DRAWING MOLECULAR STRUCTURES Step 1 Find the total number of valence electrons for ALL atoms If molecule is an anion ann electrons to account for charge for cations subtract electrons to account for charge Step 2 Decide how atoms should be connected and draw lines to indicate these bonds each bond 2 electrons Sometimes connectivity is given sometimes you have to decide Hydrogens and halogens usually only form one bond 2nd row elements usually form number of bonds given in table 7 3 3rd row and higher elements are often the central atom around which the other atoms are arranged Another way to double check Central atom usually has the lowest EN Step 3 Assign electrons to the terminal atoms subtract the number of electrons used for bonding from the total number of valence electrons calculated in step 1 Finish the octet of all terminal atoms by adding lone pairs except for H which does not form an octet Subtract the number of electrons used to finish the octet from the number of electrons left in step 3 Step 4 If step 3 left residual electrons pace them on the central atom as lone pairs Step 5 If step 3 did not leave residual electrons but the central atom does not have an octet convert lone pairs to bonding pairs ELECTRON DOT STRUCTURES AND RESONANCE In some compounds there are several equivalent bonds one of which needs to be a multiple bond to give an octet to all atoms However there is no way to decide which bond should be the multiple bond they are equivalent It has been found that in such cases all bonds are indeed equivalent and the characteristics are between single and double or double and triple bond E g bond length bond strength etc Think of these bonds as having a partial bond order like 1 5 Such structures need to be represented by several resonance structures and the real structure is an average of these FORMAL CHARGES We have talked about ions in chapter 6 and assigned them ionic charges Formal charges help identify the most reasonable resonance structures Atoms with high EN are more likely to carry a negative charge Separating charges cost energy
View Full Document
Unlocking...