9. LEWIS STRUCTURES

Question 1

1. What do Lewis Structures show?

Answer 1

• they show the distribution of the valence electrons on the atoms that are being bonded together

Question 2

2. How do we know what the number of valence electrons of the element is?

Answer 2

• it is the group number of the element

Question 3

3. List the process of drawing accurate Lewis Diagrams?

Answer 3

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

Question 4

4. Which two elements can break the Octet rule?

Answer 4

• B and Be
• Boron and Beryllium
• they are the exceptions to the rule
• they have less than 8 electrons in their valence shells

Question 5

5. Which elements can have more than 8 valence electrons in their outermost shell?

Answer 5

• 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

Question 6

6. How many Resonance forms are there?

Answer 6

• there are three different resonance forms

Question 7

7. Which bonds are the shortest?
   WHY?

Answer 7

• triple bonds
• they have more electrons pulling the two atoms together
• the bond is stronger and shorter

NB: length and strength are inversely proportional

Question 8

8. Is the real structure a time average between the three Resonance forms?

Answer 8

• 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

Question 9

9. What is Formal Charge?

Answer 9

• it is the charge an atom would have in a molecule
  IF the bonding electrons were equally shared

Question 10

10. What is the equation to work out Formal charge?

Answer 10

• 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)

Question 11

11. What is an easy way to work out the Formal charge?

Answer 11

• Formal charge= Group number - (Dashes + Dots)

Question 12

12. Why is Formal Charge necessary to work out?

Answer 12

• it is used to decide between different resonance forms

Question 13

13. How do you pick the most preferred Resonance form?

Answer 13

• 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

Question 14

14. How do the percentage contributions towards the Resonance Hybrid work?

Answer 14

• 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%)