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Unit 7 Lewis Structures and VSEPR

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Lewis Structures Show the valence electrons and bonds for a compound. 1. Draw the electron dot for each element. 2. Determine if the bond will be ionic or covalent. 3. If Ionic: Draw arrows showing the electrons transferring. 4. If Covalent: Draw a line showing the bond (shared electrons).

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Lets make some Lewis Structures! Draw a Lewis Dot for Na & O.

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Lewis Structures – Page 8 1. Write the symbols for the elements in the correct order. - The central atom is the 1 st element in the formula. - It will never be hydrogen. Carbon is always the central atom. 2. Calculate the total number of valence electrons. 3. Place one pair of electrons between each pair of bonded atoms. 4. Beginning with the outside atoms, place the remaining electrons around each atom until there are 8 around each or all electrons have been used. - If there are extra, place them on the central atom.

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More Lewis Structures 5. If not enough electrons are available to give all atoms an octet, move unshared pairs to form double or triple bonds. 6. Examine your Lewis structure to determine if there is resonance. 7. Check your answer: - Correct number of atoms? - Correct number of electrons? - Octet rule for all atoms?

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Lets make some Lewis Structures! Draw a Lewis Dot for CH 4.

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Step 5 – Multiple Bonds H 2 CS CO

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Formal Charge – Page 5 A way of determining the validity of a Lewis structure. In a neutral atom, want all atoms to have a Formal Charge of zero. In a polyatomic ion, formal charges must add up to the ion’s charge. Formal charge = [# of valence e-] – [e- in lone pairs + 1/2 the number of bonding e-]

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Let’s Try CO

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What about formaldehyde?

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Resonance Structures – Page 6 Let’s look at the homework.

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Resonance Structures

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- - - H He Li VSEPR Theory new theory: Valence Shell Electron Pair Repulsion “push” electron pairs as far away from each other as possible electrons repel each other like Thomson model of atom

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Count groups of electrons any collection of valence electrons localized around central atom each electron group repelled by every other e - group can be comprised of unpaired electrons bonding electrons unpaired 1 group = a single electron e.g. free radical 1 group = 1 lone pair bonding 1 group = single bond 1 group = double bond 1 group = triple bond

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VSEPR “rules” for predicting geometry: 1. start with valid Lewis dot structure 2. count number of electron groups around central atom 3. determine number of bonding vs. unbonded electron groups 4. describe molecular geometry learn chart see VSEPR Theory Chart

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total # e- groups # bond # lone pairs molecular geometry structure 220linear 180 o 330trigonal planar 120 o 321 trigonal planar bent 120 o possible geometries:

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440tetrahedral 109.5 o total # e- groups # bond # lone pairs molecular geometry structure

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431tetrahedral trigonal pyramidal <109.5 o 422 tetrahedral bent <109.5 o 440tetrahedral 109.5 o total # e- groups # bond # lone pairs molecular geometry structure

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550trigonal bipyramidal PCl 5 twisted stick- figure 90 o 120 o total # e- groups # bond # lone pairs molecular geometry structuree.g.

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5 41trigonal bipyramidal see-saw SCl 4 see-saw <90 o <120 o 532 trigonal bipyramidal T-shaped ClF 3 “ T ” 90 o 523 trigonal bipyramidal linear XeF 2 line 180 o 550trigonal bipyramidal PCl 5 twisted stick- figure 90 o 120 o total # e- groups # bond # lone pairs molecular geometry structuree.g.

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660 octahedral SF 6 “ jax ” 90 o total # e- groups # bond # lone pairs molecular geometry structuree.g.

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651octahedral square pyramidal BrCl 5 <90 o 642 octahedral square planar XeF 4 square 90 o 660 octahedral SF 6 “ jax ” 90 o total # e- groups # bond # lone pairs molecular geometry structuree.g.

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VSEPR Questions Know chart on Pages 16-17 or Page 19. Let’s make flashcards! (Cut them in half!) Use Pages 20-24 as a guide. FRONTBACK Lewis StructureVSEPR Name VSEPR Drawing (w/ Angles) AXE Formula

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Questions on Pages 25 & 26

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