9. LEWIS STRUCTURES Flashcards

1
Q
  1. What do Lewis Structures show?
A
  • they show the distribution of the valence electrons on the atoms that are being bonded together
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2
Q
  1. How do we know what the number of valence electrons of the element is?
A
  • it is the group number of the element
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3
Q
  1. List the process of drawing accurate Lewis Diagrams?
A
  1. count the total number of valence electrons
  2. join the atoms symmetrically using single bonds
    (one single bond is the joining of two electrons)
  3. subtract the electrons used from the total number of valence electrons
  4. complete the octet (8 electrons) on the exterior atoms
  5. subtract the electrons used up from the total number of valence electrons
  6. place any of the left over electrons on the central atom
    (as lone pairs)
  7. if there are not enough electrons to give an octet to the central atom, then you must try adding double or triple bonds to the Lewis Structure
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4
Q
  1. Which two elements can break the Octet rule?
A
  • B and Be
  • Boron and Beryllium
  • they are the exceptions to the rule
  • they have less than 8 electrons in their valence shells
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5
Q
  1. Which elements can have more than 8 valence electrons in their outermost shell?
A
  • Elements in the 3rd row/period or below (3<) can have violate the octet rule
  • they can have more than an octet
  • elements in Period 1 and 2 cannot violate this rule
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6
Q
  1. How many Resonance forms are there?
A
  • there are three different resonance forms
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7
Q
  1. Which bonds are the shortest?
    WHY?
A
  • triple bonds
  • they have more electrons pulling the two atoms together
  • the bond is stronger and shorter

NB: length and strength are inversely proportional

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8
Q
  1. Is the real structure a time average between the three Resonance forms?
A
  • NO
  • it is a resonance hybrid
  • it is like an average of the 3 resonance forms
  • it is a blend of the 3 resonance forms
  • this means that the electron cloud density will be equal for each point of the real structure
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9
Q
  1. What is Formal Charge?
A
  • it is the charge an atom would have in a molecule
    IF the bonding electrons were equally shared
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10
Q
  1. What is the equation to work out Formal charge?
A
  • Formal charge= Unbonded - Bonded
  • Unbonded: this is the number of valence electrons in the
    neutral atom
    (group number)
  • Bonded: this is all the shared electrons +
    one half of all the shared electrons
    (electrons in the lone pairs + the number of bonds)
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11
Q
  1. What is an easy way to work out the Formal charge?
A
  • Formal charge= Group number - (Dashes + Dots)
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12
Q
  1. Why is Formal Charge necessary to work out?
A
  • it is used to decide between different resonance forms
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13
Q
  1. How do you pick the most preferred Resonance form?
A
  • the resonance form with the most zeroes is preferred
  • it is the most stable
  • if there are negative charges present:
    • they should be on the most electronegative atom
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14
Q
  1. How do the percentage contributions towards the Resonance Hybrid work?
A
  • the preferred resonance form makes up the highest %
    (usually between 50%-60%)
  • the second best resonance form makes up the second
    highest %
    (usually between 30%-40%)
  • the least stable resonance form makes up the smallest
    percentage
    (usually around 10%)
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