Unit 10 Chemistry - Molecular Geometry

Chemistry Unit 10 - Molecular Geometry

Definition:

• a 3-D arrangement of the molecule's atoms in a space (the shape).

  • for covalent compounds

How many?

• 8, but we'll focus on 6 of them

What is molecular geometry?

The shape of the molecule depends on:

1. the number of atoms bonded to the central atom

2. the number of lone pairs on the central atom

3. VSEPR Theory (valence shell electron pair repulsion)

How do we determine the shapes?

This theory states that there is repulsion (a force) between lone pairs of electrons and bond pairs.

• This means they will want to get as far away from each other as possible.

  • To do that they will create an angle as they try to minimize that repulsion.

VSEPR Theory

The shape the molecule makes is based on the number of domains on the central atom.

• Domains are regions around the central atom

  • domains can be:

    • single bond = 1 domain

    • double bond = 1 domain

    • triple bond = 1 domain

    • a lone pair = 1 domain

VSEPR Theory Continued

Multiple Choice

How many domains does NH₃ have?

• 1

• 2

• 3

• 4

​n	VSEPR Formula​	Molecular Geometry (shape)​	Bond Angle​	Diagram​	Example​
​ 2	​ AX2	​ linear	​ 180o	​	CO2

​A X E

outer atoms

lone pairs_{(on central atom}

​central atom

​

​

​n	VSEPR Formula​	Molecular Geometry (shape)​	Bond Angle​	Diagram​	Example​
​ 2	​ AX2E	​ bent	​ < 120o	​	SO2

​A X E

outer atoms

lone pairs_{(on central atom}

​central atom

​

​n	VSEPR Formula​	Molecular Geometry (shape)​	Bond Angle​	Diagram​	Example​
​ 2	​ AX2E2	​ bent	​ < 109.5o	​	H2O

​A X E

outer atoms

lone pairs_{(on central atom}

​central atom

​

​n	VSEPR Formula​	Molecular Geometry (shape)​	Bond Angle​	Diagram​	Example​
​ 3	​ AX3	trigonal planar	​ 120o		BF3

​A X E

outer atoms

lone pairs_{(on central atom}

​central atom

​

​n	VSEPR Formula​	Molecular Geometry (shape)​	Bond Angle​	Diagram​	Example​
​ 3	​ AX3E	​ trigonal pyramidal	​ 109.5o	​	NH3

​A X E

outer atoms

lone pairs_{(on central atom}

​central atom

​

​n	VSEPR Formula​	Molecular Geometry (shape)​	Bond Angle​	Diagram​	Example​
4	​ AX4	​ tetrahedral	​ 109.5o	​	CF4

​A X E

outer atoms

lone pairs_{(on central atom}

​central atom

​

​

Multiple Choice

what type of shape will AlCl₃ have?

• linear

• bent

• trigonal planar

• trigonal pyramidal

• tetrahedral

Multiple Choice

what type of shape will CH₃Br have?

• linear

• bent

• trigonal planar

• trigonal pyramidal

• tetrahedral

Bond polarity is the measure of how equally or unequally electrons are shared in covalent bonds

• RECALL:

  • electronegativity is atoms tendency to attract electrons to itself when it forms a bond

    • we can use this to determine bond polarity

Bond Polarity vs Electronegativity

​Polar (P)	Nonpolar (NP) ​
​include nonmetals	​include nonmetals
​unequal e- sharing	​equal e- sharing
​electronegativity difference: 0.3 - 1.7	​electronegativity difference: 0 - 0.3
​Examples: H2O NH3	​Examples: H2 Br2

Polarity Examples

F₂

• Electronegativity difference = ____

• Type of Bond? ______

• Electrons are ____________ shared.

Polarity Examples

F₂

• Electronegativity difference = 0

• Type of Bond? nonpolar

• Electrons are equally shared.

​F

​F

​4.0

​4.0

​these arrow represent the magnitude of the electronegativity value. They pull in the opposite direction, but with the same strength. This means they will cancel out.

Polarity Examples

CF

• Electronegativity difference = ____

• Type of Bond? ______

• Electrons are ____________ shared.

Polarity Examples

CF

• Electronegativity difference = 1.5

• Type of Bond? Polar

• Electrons are unequally shared.

​C

​F

​4.0

​2.5

These dipoles pull in the opposite direction, but one will be stronger than the other. The atom with the higher electronegativity. This means the overall direction of the dipole is towards Fluorine.

​electrons will move to be closer to fluorine

Polar Molecules

• because the electrons in polar molecules are closer to one atom than the other it causes them to have a partial positive and partial negative end.

  • this is why it has a percentage of ionic character and some bonds between nonmetals can exhibit ionic behavior.