Lewis Dot Electron Diagrams
Lewis Dot Electron Diagrams Illustrate how atoms are bonded and recognise bonding and non-bonding valence electrons 1. Count the valence electrons and ± any depending on molecule or ion 2. Assemble bonding framework with single bonds (the least electronegative element will be in the centre except for H) 3. Put 3 non-bonding pairs of electrons on the outer atoms except H 4. Assign the remaining valence electrons to the central atom 5. Minimise formal charges on all atoms (shared electrons count as 1 only)
For SO2 6e- + 6e+ 6e- = 18e-
Charge on sulphur = 6 valence e- - 4 Lewis e- = +2 Charge on oxygen’s = 6 valence e- - 7 Lewis e- = -1
Note: Period 3 elements can have ‘expanded octets’ which have 10 valence electrons (availability of 3d shells). Some molecules/ions have charge that are not all zero; negative charges should be on the most electronegative atoms Resonance
When completing step 5, there can be more than one way to minimise the formal charge This is called resonance, illustrated by NO3- ions below No single Lewis structure is sufficient
Electron Deficient Species and Radicals
The octet rule is not always obeyed e.g. beryllium in BeCl2 (electron deficient) Smaller atoms can have less e.g. Be, B, Al Larger atoms (period 3 and onwards) can have more than 8 valence electrons Radicals are stable with an odd number of electrons, like NO 2
Valence-Shell-Electron-Pair-Repulsion (VSEPR) Theory Based on the ideas that pairs of electrons (in bonds and in lone pairs) repel one-another 1. Determine the molecule’s Lewis structure 2. Count the sets of bonding and lone pairs around the central atom and determine the optimum geometry (makes no distinction between single, double and triple bonds) 3. Modify the geometry to suit the electron repulsions
No. electron pairs 2 3 4 5 6
Optimum geometry linear trigonal planar tetrahedral trigonal bi-pyramidal octahedral
Two pairs
Two electron pairs must be situated as furthest away as possible, leading to an 180 o linear shape
Three pairs
Sets are oriented on one plane at 120o angles When a lone-pair repulsion kicks in, the angle is slightly small e.g. NO2- has a 115o angle rather than a 120o
Four pairs
Tetrahedral geometry with 109.5o angles optimally
Five pairs
Trigonal by-pyramidal shape as it can be viewed as two pyramids sharing the same base
Six pairs
Octahedral geometry as it has 8 triangular faces
Note: When resonance ties in with VSEPR, the structure becomes an average of the resonance contributors
Notating Molecules e.g. AX2E A = central atom X = number of outer atoms E = number of lone pairs
For SO2 6e- + 6e+ 6e- = 18e-
Charge on sulphur = 6 valence e- - 4 Lewis e- = +2 Charge on oxygen’s = 6 valence e- - 7 Lewis e- = -1
Note: Period 3 elements can have ‘expanded octets’ which have 10 valence electrons (availability of 3d shells). Some molecules/ions have charge that are not all zero; negative charges should be on the most electronegative atoms Resonance
When completing step 5, there can be more than one way to minimise the formal charge This is called resonance, illustrated by NO3- ions below No single Lewis structure is sufficient
Electron Deficient Species and Radicals
The octet rule is not always obeyed e.g. beryllium in BeCl2 (electron deficient) Smaller atoms can have less e.g. Be, B, Al Larger atoms (period 3 and onwards) can have more than 8 valence electrons Radicals are stable with an odd number of electrons, like NO 2
Valence-Shell-Electron-Pair-Repulsion (VSEPR) Theory Based on the ideas that pairs of electrons (in bonds and in lone pairs) repel one-another 1. Determine the molecule’s Lewis structure 2. Count the sets of bonding and lone pairs around the central atom and determine the optimum geometry (makes no distinction between single, double and triple bonds) 3. Modify the geometry to suit the electron repulsions
No. electron pairs 2 3 4 5 6
Optimum geometry linear trigonal planar tetrahedral trigonal bi-pyramidal octahedral
Two pairs
Two electron pairs must be situated as furthest away as possible, leading to an 180 o linear shape
Three pairs
Sets are oriented on one plane at 120o angles When a lone-pair repulsion kicks in, the angle is slightly small e.g. NO2- has a 115o angle rather than a 120o
Four pairs
Tetrahedral geometry with 109.5o angles optimally
Five pairs
Trigonal by-pyramidal shape as it can be viewed as two pyramids sharing the same base
Six pairs
Octahedral geometry as it has 8 triangular faces
Note: When resonance ties in with VSEPR, the structure becomes an average of the resonance contributors
Notating Molecules e.g. AX2E A = central atom X = number of outer atoms E = number of lone pairs
