Lewis Structures and Molecular Shapes

Transcription

1 Lewis Structures and Molecular Shapes

2 Drawing Lewis Structures Determine from formula if ionic or covalent Count the electrons If ionic : add valence # to charge if (-), subtract if (+) - = 7+1 electrons; Na + = 1-1 electrons Total= 8 electrons If molecular or covalent, add valence numbers S 3 : S=6 e =6 e x 3 Total= 24 electrons

3 Draw the Skeleton Structure Place the least electronegative atom in the middle (halogens, hydrogen or oxygen usually go outside) Draw a dash between each atom (e - pairs) Distribute electrons around the structure until all atoms (except ) have eight electrons

4 Satisfy ctet Rule Place remaining valence electrons around the rest of the atoms (halogens, N, P etc.). The middle atom usually has none. Electronegative atoms usually have lone pair electrons if any are available. or odd electron species, electronegative elements are better radicals, so they get the odd electron

5 Determine electronic character C -2 3 : non-metals, must be covalent Electronegativity of C: 2.5 Electronegativity of : 3.5 C must be central

6 Count electrons Carbon: groupiv, has 4 valence electrons xygen: group VI, has 6 valence electrons as a charge: add 2 electrons Place a double bond between atoms where the octet rule is not satisfied. This may require removing lone pairs from an electronegative atom connected to the deficient atom

7 Write the Lewis structure of the carbonate ion (C 3 2- ). Step 1 C is less electronegative than, put C in center Step 2 Count valence electrons C - 4 (2s 2 2p 2 ) and - 6 (2s 2 2p 4 ) -2 charge 2e (3 x 6) + 2 = 24 valence electrons Step 3 Draw single bonds between C and atoms and complete octet on C and atoms. Step 4 - Check, are # of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons Step 5 - Too many electrons, form double bond and re-check # of e - C 2 single bonds (2x2) = 4 1 double bond = 4 8 lone pairs (8x2) = 16 Total = 24

8 What are the resonance structures of the carbonate (C 32 -) ion? - - C C - - C - - Resonance describes the shortend bond length observed In C 3 2-.

9 Structural electron pairs There are four structural pairs on the in 2 There are three structural pairs on the N in N 2 N

10 Practice ow many structural pairs on the central atoms? C S S Xe

11 Practice ow many structural pairs on the central atoms? C 3 S 5 S 6 Xe 5

12 Two structural pairs--linear 180

13 Three structural pairs--trigonal planar

14 our structural pairs--tetrahedral

15 Practice ind the structural-pair geometry, molecular geometry and bond angle(s) for this: C

16 Practice C structural pairs on central atom 3 structural pair geometry molecular geometry trigonal planar trigonal planar bond angle(s) 120

17 Sigma and pi bonds 1 σ + 1 π 1 σ + 2 π N N 1 σ + ½ π

18 Practice ow many sigma bonds and pi bonds connect each pair of atoms in the molecule and polyatomic ion shown below? C C C N C C C

19 Practice 1 σ 1 σ C C C N 1 σ + 1 π 1 σ + 2 π C C C all C- bonds in C 3 2 are 1 σ + 1 / 3 π

20 Cl Be Cl 0 lone pairs on central atom 2 atoms bonded to central atom 10.1

21 10.1

22 10.1

23 Effect of Lone Pair on Central Atom Total 3 ED Class # of atoms bonded to central atom # lone Pairs/fr on central atom AB AB 2 E 2 1 Arrangement of electron pairs trigonal planar trigonal planar Molecular Geometry trigonal planar bent

24 Effect of Lone Pair on Central Atom Total 4 ED Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB tetrahedral tetrahedral AB 3 E 3 1 tetrahedral trigonal pyramidal AB 2 E tetrahedral bent

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26 N 2 vs C 2 Class Molecular Structure N 2 AB 2 E Bent, Angular Bond Angle < 120 C N C 2 AB 2 Linear 180

27 B 3 vs N 3 Class Molecular Structure B 3 AB 3 Trigonal Planar Bond Angle 120 B N 3 AB 3 E Trigonal Pyramidal <109.5 N

28 Molecular Structure Depends on # Atoms and # ED Linear, 180 Regions of ED around Central Atom Angular (Bent), <120 3 Trigonal Planar, 120 Angular (Bent), <109.5 Trigonal Pyramidal, Tetrahedral, 109.5

29 Dipole Moments and Polar Molecules electron poor region electron rich region δ+ δ 10.2

30 10.2

31 Determining Polarity of Molecules 1) Draw Lewis Dot Structure 2) Determine Molecular Structure (3-D Shape) 3) Look for Dipoles Within Molecule 4) Determine if Dipoles Reinforce or Cancel Reinforce => Polar Molecule Cancel => Nonpolar Molecule

32 Which of the following molecules have a dipole moment? 2, C 2, S 2, and C 4 S dipole moment polar molecule dipole moment polar molecule C C no dipole moment nonpolar molecule no dipole moment nonpolar molecule 10.2