Lewis Structures Two Ways

2 Drawing Lewis Structures 1) Count the total valence electrons for the molecule: To do this, find the number of valence electrons for each atom in the molecule, and add them up. Example: CO 2 Valence electrons: C = 4, (O = 6)x2  16 To start: Draw Lewis symbols of all individual atoms in formula

3 Drawing Lewis Structures 2) Write the symbols for each atom –the least electronegative atoms goes in the center –If there are 2 elements: central atom is first –more than 2: order of connected atoms as written in formula 3) Add in each atoms valence electrons O CO

4) Make bonds between unpaired electrons O C O **Redraw** O = C = O ¨ ¨ ¨ ¨

5) Make sure that each atom is fulfilling the octet or duet Rule 6) Count number of electrons and make sure that they match original total valence electrons

AP CHEM Method The Quicker way

7 Drawing Lewis Structures 1) Count the total valence electrons for the molecule: To do this, find the number of valence electrons for each atom in the molecule, and add them up. Example: CO 2 Valence electrons: C = 4, (O = 6)x2  16 Total number of valence electrons is important, not where they came from. To start: Draw Lewis symbols of all individual atoms in formula

8 Drawing Lewis Structures 2) Write the symbols for each atom –the least electronegative atoms goes in the center –attach atoms with bonds –If there are 2 elements: central atom is first –more than 2: order of connected atoms as written in formula Use a single dash to show bonds between atoms: CO 2 : 2 atoms, C is centrals, both O are attached to C by single bond O—C—O

9 Drawing Lewis Structures 3)Complete octets around all atoms bonded to central atom by adding electrons. (H does not get an octet!) CO 2 :.... : O – C – O :.... Count electrons now: 16 valence electrons shown, both O have octet, C?

10 Drawing Lewis Structures 4) Any leftover valence electrons after step 3? Place them on the central atom. CO 2 : no leftover valence electrons

11 Drawing Lewis Structures 5) If central atom does not have an octet of electrons after step 4: Try multiple bonds! Use unshared electrons already shown in step 3 and 4 and move them between central atom and other atoms. CO 2 O = C = O ¨ ¨ ¨ ¨

12 Drawing Lewis Structures 6)Some handy bond rules to remember for molecules: –Hydrogen and the halogens (F, Cl, Br, I) form a single bond. –The family oxygen (O, S, Se) forms 2 single bonds (or 1 double bond) –The family nitrogen (N, P, As) forms 3 single bonds, or 1 single bond and a double bond or a triple bond. So does boron. –The family carbon (C, Si) forms 4 single bonds, or 2 double bonds, or….. A good thing to do is to bond all the atoms together by single bonds, and then add the multiple bonds until the rules above are followed.

13 Drawing Lewis Structures Draw the Lewis structure for CHCl 3. # of Valence electrons = x(7) = 26 Draw all atoms, connected by single dash (single bond, 2 electrons). Place electrons around atoms bonded to central atoms for octet. C = central atom Cl HCCl Cl