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Formal Charges

Formal Charges. Alix Arungah Sarah Helman Michael Busam Michael Zuzak. Definition. Formal charge of any atom in a molecule is the charge the atom would have if all the atoms in the molecule had the same electronegativity

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Formal Charges

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  1. Formal Charges Alix Arungah Sarah Helman Michael Busam Michael Zuzak

  2. Definition • Formal charge of any atom in a molecule is the charge the atom would have if all the atoms in the molecule had the same electronegativity • So basically, that means: the charges of the atoms if the molecule was non polar • Why is it important? Because it helps us decide what Lewis structure should be preferred

  3. Process • Draw the Lewis structure(s) • Determine the valence electrons that each atom in the molecule has • Determine the number of electrons you assign to each atom • All the non-bonding electrons (lone pairs) + half of the shared electrons (so one electron per bond) • Subtract the assigned electrons from the valence electrons for each atom

  4.  FC = V - (L + 1/2 S)  • FC= formal charge • V= valence electrons • L= lone pairs • S= shared electrons • https://chemistry.twu.edu/tutorial/FormalChargeSum.html

  5. What it tells you • When you have multiple options for a Lewis structure, the one with the lowest formal charge numbers is more accurate • If there’s a situation in which the results are not all zero, the more electronegative atoms should have the more negative charges • Remember: in most cases, the octet rule trumps formal charge. • Finally, the sum of the charges will always be the overall charge of the molecule

  6. Example Problem Set Up Electron Table Pic: http://www.cem.msu.edu/~reusch/VirtualText/intro2.htm

  7. Example Problem Fill out Valence Electrons

  8. Example Problem Fill out the Assigned Electrons (each electron in the lone pairs plus one for each bond)

  9. Example Problem Subtract Assigned electrons from Valence Electrons to get Formal Charge Note: This showed process, not the differentiation between structures

  10. Example Problem Or you could use the formula FC = Val e- - (all lone pair e- + half of shared e-) Nitrogen: FC = 5 – (4 + 2) = -1 Nitrogen: FC = 5 – (0 + 4) = +1 Nitrogen: FC = 5 – (4 + 2) = -1

  11. Another Example (8.11)

  12. Another Example (8.11)

  13. Another Example (8.11)

  14. Another Example (8.11) Winner!!! Structure 2 has the least numbers, and the negative over the more electronegative atom.

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