resonance + formal charge(3.11) Flashcards
how do resonance structures relate to lewis structures?
resonance is the condition in which there is more than 1 correct Lewis structure for a molecule.
how is the actual electron structure of a molecule found/determined?
it’s an average of its resonance structures.
what does the arrow signify?
it implies that the actual electron structure is an average of its resonance structures
what is different in each resonance structure?
the placement of the electrons.
why do we need to understand resonance? what does it reveal about the localized electron model?
it points out a flawed assumption, which is that electrons are localized (they only stay in between a pair of atoms or on top of one) WHEN IN REALITY, electrons are delocalized (meaning they can move around the whole molecule.
what makes an equivalent lewis structure equal?
it has the same number of single bonds and multiple bonds. sketch lewis structures for O3 for reference.
how do you determine which lewis structure is close to the actual bonding structure of a molecule?
you calculate the formal charge of each atom.
what is the formula for the formal charge?
FC = # of valence electrons on the neutral/regular atom MINUS # of bonds (sticks) MINUS # of lone pairs (stones)
OR if you remember,
FC = # of valence electrons on the neutral/regular atom MINUS the # of assigned valence electrons in the molecule
how do you count the shared electrons/bonds when calculating formal charge?
you equally split the shared electron pairs with the other atom it shares it with. 1/2 the number of bonds around the specific atom
how do you count lone pairs when calculating formal charge?
it belongs to the atom that’s being calculating for formal charge so either COUNT IT AS PAIRS (2, 4, 6, etc.) or COUNT EACH INDIVIDUAL ELECTRON IN THE LONE PAIRS (1, 2, 3, etc.)
how do you calculate the number of valence electrons that are assigned to a specific atom in a molecule?
assigned valence electrons = # of lone pairs + 1/2 the # of bonding pairs.
ex: find assigned valence electrons of N in NH3.
assigned valence electrons of N = 2 (cuz it has 1 lone pair = 2 electrons in a pair) PLUS 1/2 of 6 (cuz there are 3 bonding pairs and each contains 2 electrons)
2+(0.5)x6 = 5 assigned valence electrons of N
what is the formal charge of an atom that has the same number of valence electrons assigned in a molecule as it does in its neutral state? (its # of valence electrons on PT)
it has a formal charge of 0, which means that the positive and negative balance out.
what can we assume about an atom with a negative formal charge?
it’s probably one of the most electronegative atoms. (don’t know if it’s general or for that specific molecule)
what are the two defining rules for picking the lewis structure that is closest to the molecule?
1) those with formal charges close to 0
2) any negative formal charges on the most electronegative atoms
is the formal charges a final and accurate depiction of an atom’s charge?
NO, it’s just an estimate of its charge
