Shapes The shape of a molecule can play a large role in how it reacts and how we as chemists can use it. POS Checklist:

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1 hem 20 Unit A Bonding Molecular Shapes and VSEPR Theory You will... PS hecklist: apply VSEPR theory to predict molecular shapes for, angular (V shaped, bent) tetrahedral, trigonal pyramidal and trigonal planar molecules. Aug 23 11:08 AM Aug 23 11:13 AM Review: Draw LDD for the following: a) peroxide, 2 2(l) b) perchlorate ion c) ammonia d) ammonium ion Sep 18 4:07 PM Aug 23 11:26 AM Shapes The shape of a molecule can play a large role in how it reacts and how we as chemists can use it. ex) A biological molecule, enzymes each have specific shapes to react with specific molecules in the body (each cell in your body has around 3000 enzymes). Q: But how do we know the shape a molecule will take? To answer this question, we look to the field of stereochemistry and the VSEPR theory. VSEPR Aug 23 11:28 AM Aug 23 11:32 AM 1

2 VSEPR hemistry Stereochemistry is the study of the 3 D arrangement of atoms in a molecule. The main tool we will use to determine this arrangement is VSEPR. This theory works on the basis that electron pairs repel one another because of their similar charge. Let's look at a demonstration: Balloon Demo Aug 23 11:36 AM Aug 23 11:46 AM ow lets look at the some of the shapes predicts. Remember the basic idea behind this theory: VSEPR All pairs of electrons repel each other and want to get as far apart as possible. In order to determine shape, we must first draw the LDDs. We will also use the AE system to keep track of atoms and lone pairs: central atom AE peripheral atom lone pairs Steps to Determining a VSEPR Structure: 1. Draw the LDD, including all bonding and lone pairs. 2. ount the number of pairs of electrons around the central atom. *Important note: double and triple bonds count as one pair of electrons for this system! Aug 23 11:55 AM Aug 23 1:33 PM 3. The number of pairs of electrons corresponds to a shape: 2 pairs 3 pairs trigonal planar/pyramidal 4 pairs tetrahedral 5 pairs trigonal bi pyramidal 6 pairs octahedral We only deal with these in hem 20. Molecule Shapes: 1. Linear general formula: A 2 # of electron pairs: 2 A 180 o A with only two pairs of electrons, the electrons move as far apart as possible: exactly 180 o apart on either side of the central atom. Sep 18 3:43 PM Aug 23 11:53 AM 2

3 ex) beryllium dihydride, Be 2(g) ex) carbon dioxide, 2(g) Be 2 A 2 *notice how this formula matches the general formula Be A = Be atom = atoms (2 of them) Be 2 A 2 *Remember: double bonds count as one pair of electrons. Aug 23 12:14 PM Sep 18 3:46 PM Molecule Shapes: 2. Trigonal Planar general formula: A 3 # of electron pairs: 3 A 120 o 120 o A 120 o ex) boron trihydride, B 3(g) B 3 A 3 B B with three pairs of electrons, the electrons move as far apart as possible: exactly 120 o apart on three sides of the central atom. *the name trigonal planar is given because there are 3 peripheral atoms all in the same plane (i.e. flat). Aug 23 1:02 PM Aug 23 1:14 PM ex) formaldehyde, 2 (l) ex) carbonate ion, 3 2 (aq) 2 Molecule Shapes: 3. Tetrahedral general formula: A 4 # of electron pairs: 4 = out of the page = into the page A A o 4 pairs of electrons distribute equally around the central atom. The structure is shown in 3 D space, with one peripheral going into and one coming out of the page. Aug 23 1:19 PM Aug 23 2:01 PM 3

4 ex) methane, 4(g) The Effect of Lone Pairs: ften, the central atom will have one or more lone pairs around it. These lone pairs act just like really big peripheral atoms. ex) ammonia, 3(l) general formula: A 3 E ex) ammonium, 4 + (aq) + the lone pair acts like an atom, pushing the other atoms apart. trigonal pyramid Aug 23 2:09 PM Aug 23 2:14 PM ote the similarities in structure: the lone pair takes the place of the atom o tetrahedral trigonal pyramid ex) water, (l) general formula: A 2 E o The two lone pairs occupy more space than two atoms, squeezing the remaining atoms together even closer than before. angular (bent) *ote: because the lone pair occupy more space than an atom, the new atom spacing is o, not o. Aug 23 2:26 PM Aug 23 2:20 PM tetrahedral ex) hydroiodic acid, I (aq) general formula: AE 3 I I otice the similarities to the tetrahedral structure. angular This forms another sort of structure. Aug 23 2:35 PM Aug 23 2:38 PM 4

5 Instructions for drawing Lewis Dot Diagrams of Molecules: 1. ount the number of valence electrons in the molecule. Add any charge (if it is a polyatomic ion). ex) nitrate, 3 = 1(5) = 5e = 3(6) = 18echarge = 1etotal = 24e 2. Draw the central atom and place the peripherals around it. Place one pair of bonding electrons between each peripheral and the central atom = 18e left Sep 20 10:57 AM Sep 20 11:01 AM 3. Place electrons around the peripheral atoms to complete their octets = 0e left 4. Place any remaining electrons around the central atom. *Since we have used up all 24 electrons, we can't do this step for this example. Sep 20 11:05 AM Sep 20 11:07 AM 5. If the central atom s octet is not complete, move in some lone pairs from a peripheral atom to form double bonds. Practice: Draw and name the structure for each molecule or ion. Include the general formula. a) P 3(g) P AS trigonal pyramid A 3 E b) hydrobromic acid Br AS A 3 c) phosgene (l 2(g)) l las Trigonal Planar A3 d) sulphite ion S AS A3E pyramidal Sep 20 11:10 AM Aug 23 1:54 PM 5

6 e) sulphate ion f) phosphorus trichloride P S l l l AS trigonal AS pyramid tetrahedral A 3 E A 4 Practice: Page #1 8 "Round" molecules block mosquitos sensory nerves better than flat, molecules, keeping them from sensing 2 DEET (g) and heat from humans. Aug 23 2:47 PM Aug 23 2:42 PM 6