Overview
This lecture reviews how to calculate formal charges in molecules, emphasizing easy methods for quick determination, especially important for organic chemistry exams.
Formal Charges vs. Oxidation States
- Formal charges and oxidation states are not the same; formal charge looks at electron sharing in Lewis structures.
- Atoms making their "normal" number of bonds typically have a formal charge of zero.
Calculating Formal Charges
- Formal charge formula: (valence electrons) - (½ bonding electrons + non-bonding electrons).
- Alternate method: Formal charge = (valence electrons) - (number of dots and lines around the atom).
- "Dots" are lone pair electrons; "lines" are bonds (each bond counts as one line).
Example Applications
- Carbon, typically with four bonds, usually has a formal charge of zero.
- Oxygen typically forms two bonds; deviation often results in a formal charge.
- Nitrogen usually makes three bonds; if it has four, its formal charge is +1.
- Chlorine usually makes one bond and has seven valence electrons, resulting in a formal charge of zero when following the norm.
- Aluminum and boron sometimes break the octet rule; abnormal bonding generally leads to non-zero formal charges.
Key Points for Quick Assessment
- If an atom forms an atypical number of bonds, check for a formal charge.
- Use the "valence minus dots and lines" shortcut for speed and simplicity.
Key Terms & Definitions
- Formal Charge — the charge assigned to an atom in a molecule, calculated by comparing valence electrons with those assigned from bonds and lone pairs.
- Valence Electrons — electrons in the outermost shell of an atom used for bonding.
- Lewis Structure — diagram showing bonds and lone pairs in a molecule.
- Octet Rule — atoms tend to have eight electrons in their valence shell (exceptions: boron, aluminum).
Action Items / Next Steps
- Practice assigning formal charges quickly for sample Lewis structures.
- Review and memorize common valence electron counts for key elements (e.g., C, N, O, Cl, Al).
- Prepare for exam questions on formal charges in first organic chemistry test.