Lesson 5 Lewis Dot Structure of Molecular Covalent Compounds.pptx

The Lewis dot structure (or Lewis structure) is a diagram that represents the valence electrons of atoms within a molecule. It helps visualize how atoms bond and share electrons to form molecules.

1. Objective Draw the Lewis dot structure of molecular covalent compounds 

2.   Lewis Structure Assumptions  Only valence electrons are involved in bonding. Atoms in molecules need eight valance electrons (octet rule) except for hydrogen which needs two electrons (duet rule).

3. In covalent compounds atoms share electrons to form bonds in order to achieve stable noble gas electron configurations. In ionic compounds electrons are transfer red from one atom to another to achieve stable noble gas electron configurations

4. . Types of bond b ased on the Number of Shared Electron Pairs Single bond = 2 electrons to each atom Double bond = 4 electrons to each atom Triple Bond = 6 electrons to each atom Quadruple Bond = 8 electrons to each atom

5. Quadruple bonds only exist between transition metals , such as rhenium, molybdenum, chromium, and tungsten , due to the availability of d-orbitals that enable unique bonding interactions

6. Rules for Drawing   Lewis Structures Step 1: Count the total number of valance electrons. Step 2: Identify the central atom (the first atom written unless that atom is hydrogen). Place all terminal atoms around that atom. Hydrogen atoms NEVER have more than one bond.

7. Step 3: Complete the octet for all atoms in the Lewis structure with lone pairs of electrons (except hydrogen). Step 4: Check your structure by counting the number of valance electrons used (they will match step 1 if the structure is correct). If your valance electrons don’t match you will need to tweak your structure.

8. Example 1: CH 4 Step 1   Count the number of valence electrons C = 4 valence electrons H = 1 valence electron Step 2 Identify the central atom. This is usually the first atom written. Exception: Hydrogen . If that is the first atom written, then use the second atom. Example: C H 4 Central atom: C

9. Step 3: Complete the octet Draw each component as a (Lewis) dot diagram.

10. Step 4: Check your structure Check your structure by counting the number of valance electrons used Remember:Hydrogen is the exception to the octet rule. It only needs 2 electrons to be happy. Make sure that each atom in your new Lewis dot structure “feels” like it has eight electrons around it. Remember : One bond “feels” like two electrons to EACH element it borders.

11. If here are atoms whose octet rules are not satisfied, you may need to increase the number of bonds between atoms. 2 2 2 2 8

12. Example 2: Water (H₂O) Valence electrons: Oxygen: 6, Hydrogen: 1 each → Total = 8 Central atom: Oxygen (less electronegative than hydrogen) Single bonds: Connect H to O with single bonds Distribute remaining electrons: Place 4 more electrons as lone pairs on oxygen

13. Final structure:

14. Example 3: Carbon Dioxide (CO₂) Valence electrons: Carbon: 4, Oxygen: 6 each → Total = 16 Central atom: Carbon (least electronegative) Single bonds: Connect C to each O with single bonds Distribute electrons: Each oxygen gets 6 remaining electrons (3 lone pairs), but carbon has only 4 electrons. To satisfy the octet rule, change single bonds to double bonds .

15. Lewis Structures of CO 2 (carbon dioxide)

16. Final structure:

17. Lewis Structures CO  ( carbon monoxide)

18. Rules for Molecules   W ith an Overall Charge When figuring out the number of electrons available, make sure to add or subtract as indicated by the charge. Create the Lewis structure the same as always. Put square brackets [ ] around the structure. Write the charge in a superscript. Called a “Coordinate Covalent Bond” Lewis Structures –  Charged Species A species that has a - charge has a shortage of bonds over the normal number If a species has received electrons from elsewhere, it does not have to share as many electrons Therefore less bonds have to be made Lewis Structures –  Charged Species A species that has a + charge has an excess of bonds over the normal number. if a species has given up some electrons, it has to involve more of the electrons it has kept Therefore more bonds have to be made Lewis Structures –  Charged Species The size of the - or + charge tells you the shortage or excess of bonds +2 = 2 extra bonds; -3 = 3 bond shortage Coordinate Covalent Bond  ammonium ion Coordinate Covalent Bond  hydronium ion

19. “ Exceptions ” to  the Octet If you only have four or six valance electrons initially you can’t possibly fill the octet rule (usually BeH 2 or BH 3 ).(Incomplete Octet) Just place hydrogens around central atoms and call it done. Exceptions to the   Octet Rule (That are not H) There are two other exceptions to the Octet Rule (that are not Hydrogen) (Expanded Octet) Examples: PF 5 SF 6 Exceptions usually involve F

20. PF 5

21. SF 6

22.   “ Exceptions” to   the Octet Odd number of electrons: One atom will have to have less than eight electrons. Draw the Lewis structure as if it had one more valance electrons than it actually does. Then subtract one electron from the least electronegative element (often the central atom). “ Exceptions” to  the Octet Exceeding the Octet Rule: When you must break the octet rule draw the structure as you would in steps 1-4 and the place the extra electrons around the central atom. Even when breaking the octet rule no atom will ever have more than 12 electrons. Exceptions to Octet  Rule PF 5 expanded octet Exceptions to Octet  Rule SF 6 Expanded octet

23. Diatomic Molecules In nature, the following elements are always found in a paired molecule. They are never found solo. I 2 Br 2 Cl 2 F 2 O 2 N 2 H 2 I Br ing Cl ay F or O ur N ew H ouse

24. Summary: Steps to Draw a Lewis Structure: Count valence electrons 2. Determine the central atom The first element written and the least electronegative atom(except hydrogen) is usually the central atom. 3. Connect atoms with single bonds Draw single bonds (each bond = 2 electrons) between atoms.

25. 4. Distribute remaining electrons Complete octet (8 electrons) for outer atoms first, then for the central atom. 5. Check octet rule and adjust if needed If the central atom lacks an octet, form double or triple bonds. 6. Check the total number of valence electrons Make sure the number of electrons used matches the total count.