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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