7.6 – NOTES Lewis Structures

How to draw Lewis structures: A. Structural formulas How to draw Lewis structures: 1. Determine positions. H and the halogens are always on the outside of a molecule since it forms 1 bond only. Least electronegative atom is often the central atom. 2. Find the total number of electrons available for bonding. Add up the valence electrons, and then adjust for the charge if it is a polyatomic ion. 3. Place a single covalent bond between all pairs of atoms. Add up the e- used so far. 4. Place rest of e- in pairs around the outside atoms. If all available e- are used and every atom has an octet (and H has 2 e-), you are finished. 5. If all available e- are used and some atoms do not have an octet, make double or triple bonds as necessary, using e- pairs from the atoms. C, N, O and sometimes S have the ability to from double and triple bonds. B, H and the halogens NEVER form multiple (double or triple) bonds.

CO2 = C will be in the middle because it has lowest EN   O C O

C brings 4 electrons and have 1 atom of C, O brings 6 electrons and have 2 atoms: Total # of electrons – 1(4) + 2(6) = 16 e-

Place a single covalent bond between all pairs of atoms.   O C O

Place the rest of e- around the outside atoms Place the rest of e- around the outside atoms. Start with one of the hydrogen atom, complete the octet then proceed to the second oxygen.   1st - O C O 2nd - O C O *have used 10 of 16 e- ** have used all 16 e-

All available e- have been used so will need to move a lone pair from one of the outside atoms to between the C and O so the pair becomes a bonded pair and counts for both atoms. Move e- until every atom has an octet   O C O *could move one pair of e- from each outside atom or two pairs from one atom