Lewis Dot Structures, VSEPR & Polarity Thursday, December 7th, 2017

Dot Diagrams vs. Structures Lewis Dot Diagrams show the number of valence of electrons electrons (in the form of dots) for a single element Lewis Structures are diagrams that show the bonding between atoms of a molecule and the lone pairs that may exist in the molecule

Drawing Lewis Structures Review Sum the # of valence electrons from all atoms Anions: add e– (CO32- : add 2 e– ) Cation: subtract e– (NH4+: minus 1 e– ) Predict the arrangement of the atoms Usually the first element is in the center (often C, never H) Make a single bond (2 e–) between each pair of atoms Arrange remaining e– to satisfy octets (8 e– around each) Place electrons in pairs (lone pairs) Too few? Form multiple bonds between atoms: double bond (4 e-) and triple bond (6 e-) Check your structure! All electrons have been used All atoms have 8e- Exceptions: Remember that H only needs 2e– !

Lewis Structure “Useful Hints” C group: Forms a combo of 4 bonds and no LP (Lone Pairs) N group: Forms a combo of 3 bonds and 1 LP O group: Forms a combo of 2 bonds and 2 LP F group (halogens): Forms 1 bond and 3 LP Note that these are NOT always true!

Lewis Structures Practice Practice drawing the following compounds in your notebook. CO2 NH3 HCl CH4

Lewis Structures Practice - ANSWERS CO2 NH3 HCl CH4

How do you determine if a structure is polar or nonpolar? A structure will be nonpolar if is symmetrical around the central atom. A structure will be polar if it is asymmetrical around the central atom.

Identify the following structures as polar or nonpolar.

Identify the following structures as polar or nonpolar.

WHATTTTTT?! That is NOT symmetrical! NONPOLAR

https://www.youtube.com/watch?v=keHS-CASZfc&feature=related

VSEPR REFERENCE CHART You may use this handout on all quizzes/tests/the midterm in this class. DO NOT LOSE IT.

Molecular Shape VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs.

VSEPR Theory Based on Electron Dot (Lewis structures) VSEPR Theory predicts shapes of compounds VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves) Electrons around central nucleus repel each other. So, structures have atoms maximally spread out

VSEPR overview tetrahedral trigonal pyramidal Bent Linear Each shape has a name (you should know these 5) Names of Shapes: tetrahedral trigonal pyramidal Bent Linear trigonal planar

Triangular Planar Bent Tetrahedral Linear Trigonal pyramidal

Example 1: Methane (CH4) Example 2: Ammonia (NH3) Example 3: Water(H2O)

methane, CH4 Tetrahedral 109.5° Bonds are all evenly spaced electrons

.. ammonia NH3 .. .. .. Trigonal Pyramidal Less repulsion between the bonding pairs of electrons

.. .. water, H2O 109.5° (109.5°) 109.5° (107°) 109.5° (104.5°)

.. ..

Bent or V 2 unshared pairs of e’s at top of O repel bonds and force them to bend

Steps to Success Draw the Lewis Structure for each atom - Count up the valence electrons, determine central atom (put a star), bond terminal (outside) atoms, recount, add lone electron pairs, check & shift to share if necessary Determine VSEPR Structure -- # atoms bound on central atom -- # unpaired electrons on central atom Redraw as VSEPR Structure

Example 4: Carbon Dioxide (CO2)

Example 4: Carbon Dioxide (CO2)