Molecular geometry
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MOLECULAR GEOMETRY. Recap. Bonding types: Ionic Transfer of e- Attraction between ions Covalent (molecular) Sharing of e- Covalent network Metallic. Objectives. Be able to determine the Lewis Dot structure for a molecule Know what a coordinate covalent bond is

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Recap
Recap

  • Bonding types:

    • Ionic

      • Transfer of e-

      • Attraction between ions

    • Covalent (molecular)

      • Sharing of e-

    • Covalent network

    • Metallic


Objectives
Objectives

  • Be able to determine the Lewis Dot structure for a molecule

  • Know what a coordinate covalent bond is

  • Be able to identify a coordinate covalent bond within a molecule

  • Identify elements in a molecule that might violate the octet rule

  • Know what VSEPR is

  • Use VSEPR to explain geometry

  • Identify electron pair geometry

  • Identify molecular geometry


How do they share
How do they share

  • Satisfied octet

    • Octet rule: full s & p sub shell (usually 8 electrons but not always) is “stable”

  • Lewis dot structures

    • Shows how elements bond in covalent compounds while satisfying their octet


Two ways to lewis dot
Two ways to “Lewis Dot”

  • Atoms with Valence e-

    • Like a puzzle, assembled piece by piece

    • Pro: great for small molecules because it’s fast

    • Con: hard for large molecules or molecules with complex bonding

  • “Valence e- pool” method

    • Strip valence e- from atoms in compound, assemble compound and add valence e- back in as you go

    • Pro: Great for complex molecules

    • Con: Doesn’t explain why elements bond the way they do


Atoms w valence e
Atoms w/ valence e-

  • NCl3

  • SO

  • SO2

  • Coordinate covalent bond- one atom donates both electrons in the bond

  • This method makes it easier to see these

  • What to look for:

    • The number of e- not involved in bonding is = to its total valence e-


Rules for valence e pool method
Rules for “Valence e- pool” method

  • Determine the number of valence electrons in compound

    • if molecule has a charge determine how many e- it lost or gained and include this in the total # of valence e-

      Ex: SiS2 Si has 4 & S has 6 and there are two of them for a total of 16 valence e-

      Try these: FCl NH3 CH3OH SO2


Rules continued
Rules continued

  • Determine the central atom

    • The central atom is usually the atom that needs the MOST valence e- to be stable

      • doesn’t matter if only 2 atoms

      • if all atoms need the same # choose the atom with lower electronegativity (typically the atom that is first)

        Ex: SiS2 Si needs 4 S only needs 2, Si is central atom

        Try these: FCl NH3 CH3OH SO2


Rules continued1
Rules continued

  • Write the symbol of central atom with the symbols of other elements around it. Then connect atoms with single bonds (subtract bonding e- from the total valence e-)

    Ex: S Si S Total v e- 16

    bonding e- 4

    remaining e- 12

    Try these: FCl NH3 CH3OH SO2


Rules continued2
Rules continued

4. Place remaining valence e- as lone pairs.

  • Most electroneg. atom first, until satisfied

  • 2nd most electroneg. next, etc.

  • check total # of valence e- to make sure you used them all

    (S is most electronegative here)

    Ex: S Si S 12-12 = 0 v e- left Good!

    Try these: FCl NH3 CH3OH SO2


Rules continued3
Rules continued

  • Check each atom to see if it is satisfied.

    • Octet rule

    • If there’re atoms not satisfied, move lone pairs to form double or triple bonds.

    • If charged put it in brackets & put a charge on it

      Ex: S Si S

      Here we see that Si is not satisfied but we CANNOT add any more e-!!

      Move a lone pair from each S to make a bond.

      S = Si = S

      Try these: FCl NH3 CH3OH SO2


Octet rule violators
Octet rule violators

  • Some elements can violate the octet rule

  • Low side: these elements don’t need to have full s & p sub-shells

    • Boron most notable, can sometimes have just 6 (BH3)

  • High side: These elements can have as many as 12 valence e-

    • Any element in the 3rd period or below


Lewis dot practice
Lewis Dot Practice

  • Complete the first column of the worksheet


MOLECULAR GEOMETRY

Molecule adopts the shape that minimizes the electron pair repulsions.

VSEPR

  • Valence Shell Electron Pair Repulsion theory.

  • Most important factor in determining geometry is relative repulsion between electron pairs.


Electron pairs densities
Electron pairs (densities)

  • VSEPR is all about the repulsion of electrons

  • Focus is on areas of electron density instead of individual electrons

    • Lone pair = one area

    • Single bond = one area

    • Double bond = one area

    • Triple bond = one area

      Basically everything is only 1 area


Gumdrop molecules
Gumdrop molecules

  • Per group of two people

    • 7 gumdrops (atoms)

    • 6 toothpicks (electron densities)

  • CO2

  • CH4

    • NH3

    • H2O

  • SO3



What is meant by region of electron density
What is meant by region of electron density?

  • Could be a BONDED pair of electrons

  • Could be a LONE pair of electrons


Electron pair geometry
Electron Pair Geometry

  • Does NOT distinguish between atoms (bonding pairs) and lone pairs

  • Molecules will be one of 5 electron geometries

  • Draw the shape of the molecular geometry and write the Electron Pair Geometry in the square


2 regions 180 bond angle
2 regions180° bond angle



o

o



o

o

o



Structure determination by vsepr

••

H

H

N

H

Structure Determination by VSEPR

Ammonia, NH3

1. Draw electron dot structure

2. Count BP’s and LP’s = 4

3. The 4 electron pairs are at the corners of a tetrahedron.


Structure determination by vsepr1
Structure Determination by VSEPR

Ammonia, NH3

There are 4 electron pairs at the corners of a tetrahedron.

The ELECTRON PAIR GEOMETRY is tetrahedral.


Structure determination by vsepr2
Structure Determination by VSEPR

Ammonia, NH3

The electron pair geometry is tetrahedral.

The MOLECULAR GEOMETRY — the positions of the atoms — is TRIGONAL PYRAMIDAL.


Structure determination by vsepr3
Structure Determination by VSEPR

Water, H2O

1. Draw electron dot structure

2. Count BP’s and LP’s = 4

3. The 4 electron pairs are at the corners of a tetrahedron.

The electron pair geometry is TETRAHEDRAL.


Structure determination by vsepr4
Structure Determination by VSEPR

Water, H2O

The electron pair geometry is TETRAHEDRAL

The molecular geometry is BENT.


Final column
Final Column

  • Molecular Geometry


Geometries for Four Electron PairsWhy are the angles different?How do the repuslive forces of lone pairs differ from bonded pairs?


Structure determination by vsepr5

O

H

C

H

O

C

H

H

Structure Determination by VSEPR

Formaldehyde, CH2O

1. Draw electron dot structure

2. Count BP’s and LP’s at C

3. There are 3 areas of electron density around C at the corners of a planar triangle.

The electron pair geometry is PLANAR TRIGONAL with 120o bond angles.


Structure determination by vsepr6
Structure Determination by VSEPR

Formaldehyde, CH2O

The electron pair geometry is TRIGONAL planar

The molecular geometry is also trigonal planar.




Structures with central atoms with more than or less than 4 electron pairs
Structures with Central Atoms with More Than or Less Than 4 Electron Pairs

Often occurs with Group 3A elements and with those of 3rd period and higher.


Boron compounds
Boron Compounds Electron Pairs

Consider boron trifluoride, BF3

The B atom is surrounded by only 3 electron pairs.

Bond angles are 120o

Geometry described as planar trigonal


5 regions
5 regions Electron Pairs


Compounds with 5 pairs around the central atom
Compounds with 5 Pairs Around the Central Atom Electron Pairs

o

o

5 electron pairs



Sulfur tetrafluoride sf 4
Sulfur Tetrafluoride, SF Electron Pairs4

  • Number of valence electrons = 34

  • Central atom = S

  • Dot structure

o

Electron pair geometry

--> trigonal bipyramid(because there are 5 pairs around the S)

o


Sulfur tetrafluoride sf 41
Sulfur Tetrafluoride, SF Electron Pairs4

Lone pair is in the equator because it requires more room.

o

o



Compounds with 6 pairs around the central atom
Compounds with 6 Pairs Around the Central Atom Electron Pairs

o

o

6 electron pairs



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