9. LEWIS STRUCTURES Flashcards
1
Q
- What do Lewis Structures show?
A
- they show the distribution of the valence electrons on the atoms that are being bonded together
2
Q
- How do we know what the number of valence electrons of the element is?
A
- it is the group number of the element
3
Q
- List the process of drawing accurate Lewis Diagrams?
A
- count the total number of valence electrons
- join the atoms symmetrically using single bonds
(one single bond is the joining of two electrons) - subtract the electrons used from the total number of valence electrons
- complete the octet (8 electrons) on the exterior atoms
- subtract the electrons used up from the total number of valence electrons
- place any of the left over electrons on the central atom
(as lone pairs) - 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
4
Q
- 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
5
Q
- 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
6
Q
- How many Resonance forms are there?
A
- there are three different resonance forms
7
Q
- 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
8
Q
- 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
9
Q
- What is Formal Charge?
A
- it is the charge an atom would have in a molecule
IF the bonding electrons were equally shared
10
Q
- 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)
11
Q
- What is an easy way to work out the Formal charge?
A
- Formal charge= Group number - (Dashes + Dots)
12
Q
- Why is Formal Charge necessary to work out?
A
- it is used to decide between different resonance forms
13
Q
- 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
14
Q
- 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%)