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Lewis Dot Structures & VSEPR Theory

Lewis Dot Structures & VSEPR Theory. Chemistry Ms. Piela. Lewis Dot. Shows the number of valence electrons for a given atom Rules Each atom is represented by its symbol No more than 2 electrons on each side of the element’s symbol Only the valence electrons appear in a Lewis Dot.

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Lewis Dot Structures & VSEPR Theory

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  1. Lewis Dot Structures & VSEPR Theory Chemistry Ms. Piela

  2. Lewis Dot • Shows the number of valence electrons for a given atom • Rules • Each atom is represented by its symbol • No more than 2 electrons on each side of the element’s symbol • Only the valence electrons appear in a Lewis Dot

  3. Examples • H • F

  4. Practice • C • Br • O • B • Ne

  5. Lewis Dot Structures • General Strategy • Count up the total number of valence electrons • Draw at least one bond to the central atom (each bond counts as 2 electrons) • Complete the octets of atoms attached to the central atom (remember: electrons only work in pairs) • If there are not enough electrons to satisfy octets consider using double/triple bonds

  6. Tips for Lewis Dot Greatness • Double/Triple bonds mostly form between C, N, O, and S • H, Be, and B are special exception to the octet rule. They do not follow it! • The central atom is the atom that forms the most bonds, or is usually written first • Elements beyond the 2nd period may have more than 8 valence electrons (not really seen here)

  7. Examples • Cl2 • NH3 • CH4

  8. Practice • CF4 • HF • H2O • BF3

  9. Complex Lewis Dot Structures • More than one single bond is possible • Double bond – two bonds between atoms (4 electrons total) • Triple bond – three bonds between atoms (6 electrons total) • Bond length decreases as you increase the number of bonds • Bond strength increases as you increase the number of bonds

  10. Examples • CO2 • CH2O

  11. Practice • SO3 • N2 • HCN

  12. Determining Molecular Shapes • VSEPR (Valence Shell Electron Pair Repulsion) Theory – A model that accounts for geometrical arrangements of shared and unshared electron pairs around a central atom • Shapes are determined by repulsive forces between shared and unshared electrons

  13. VSEPR Theory • To determine molecular geometry (shape), count the electron pairs surrounding the central atom • Electron pairs = (number of lone pairs on central atom) + (number of atoms bonded to central atom)

  14. Examples • BH3 • NH3 • CH4

  15. Examples • H2O • SO2

  16. Practice • SiF4 • HCN • BF3 • SO42-

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