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Lewis Structures & VSEPR

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Presentation on theme: "Lewis Structures & VSEPR"— Presentation transcript:

1 Lewis Structures & VSEPR

2 Lewis Structure Lewis Structures – shows how the _______________ are arranged among the atoms of a molecule There are rules for Lewis Structures that are based on the formation of a _______________ Atoms want to achieve a _______________ configuration

3 Octet & Duet Rules Octet Rule – atoms want to have _______________ valence electrons Duet Rule – H is the exception. It wants to be like He & is stable with only _______________ valence electrons

4 Steps for drawing Lewis Structures
Sketch a simple structure with a central atom and all attached atoms Add up all of the valence electrons for each individual atom If you are drawing a Lewis structure for a negative ion add that many electrons to create the charge If you are drawing a Lewis structure for a positive ion subtract that many electrons to create the charge

5 Steps for drawing Lewis Structures
Subtract 2 electrons for each bond drawn Complete the octet on the central atom & subtract those electrons Complete the octet on the surrounding atoms & subtract those electrons Get your final number If 0  you are done! If +  add that many electrons to the central atom If -  need to form multiple bonds to take away that many electrons

6 Bond Types ____________ bonds () – single covalent bond
____________ bonds () – occur when multiple bonds are formed Single bond – sigma Double bond – 1 sigma & 1 pi Triple bond – 1 sigma & 2 pi

7 Bond length & Strength As the number of bonds increases, the bond length ____________ The shorter the bond, the ____________ the bond

8 Examples CCl4

9 Examples HF

10 Examples NH3

11 Examples NO+

12 Exceptions to the octet rule
Sometimes the central atom violates the octet rule and has more or less than 8 valence electrons Keep using the same rules to draw Lewis Structures

13 Examples SF4

14 Examples ICl3

15 Examples XeF4

16 Examples ICl-

17 Resonance When more than one Lewis Structure can be written for a particular molecule _______________ – all possible Lewis structures that could be formed The actual structure is the _______________ of all of the structures You MUST show all structures!

18 Examples SO3

19 Examples NO2-

20 Examples NO3-

21 Formal Charge Formal charges can be used in 1 of 2 ways…
Suggest where the __________ are Help select the most __________ structure from a set of resonance structures

22 1 - Suggest where the charges are
Formal charge =

23 Example Calculate the formal charge on each element in the carbonate ion CO3 2-

24 2 - Help select the most plausible structure from a set of resonance structures
When choosing the most likely resonance structure Most likely – All formal charges are __________ Next likely – All formal charges add up to __________ Next likely – Formal charges are closest to __________ Next likely – __________ charge is on most electronegative atom

25 Example Which of the following resonance structures is most likely for CH2O and why?

26 Another Example Which is the most likely structure for N2O?

27 VSEPR Theory VSEPR Theory ________________________
The structure around a given atom is determined by minimizing the _______________ The electrons and elements bonded to the central atom want to be _______________ as possible

28 VSEPR Steps Draw the Lewis structure for the molecule
Count the total number of things that are around the central atom to determine the electron pair geometry Imagine that the lone pairs of electrons are invisible and describe the molecular shape

29 2 Electron Pairs If there are 2 things attached to the central atom, the shape is linear Bond angle = 180°

30 3 Electron Pairs If there are 3 electron pairs the shape will be trigonal planar Bond angle = 120°

31 3 electron pairs Now imagine that you have 3 electron pairs, but one is just a lone pair (invisible) what would it look like then?

32 4 electron pairs If there are 4 electron pairs, the shape will be tetrahedral Bond angle = 109.5°

33 4 electron pairs What if 1 of the electron pairs is a lone pair (invisible)? What would it look like then? Trigonal Pyramidal

34 4 electron pairs What if there are 2 lone pairs (invisible)? What would it look like then? bent

35 5 electron pairs If there are 5 electron pairs the shape will be Trigonal Bipyramidal Bond angles = 90º & 120º

36 5 electron pairs What is there is 1 lone pair (invisible) Seesaw

37 5 electron pairs What is there are 2 lone pairs (invisible) T-shaped

38 6 electron pairs If there are 6 electron pairs the shape will be octahedral Bond angle = 90°

39 6 electron pairs What if there is 1 lone pair (invisible)?
Square pyramidal

40 6 electron pairs What if there are 2 lone pairs (invisible)
Square planar

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