Part 5: VSEPR THEORY“Valence Shell Electron Pair Repulsion Theory”

Objectives• -Explain VSEPR Theory• -Use VSEPR to predict shapes of

molecules

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Determining Shape• Each molecule has a different Dot

Structure and shape which determines its properties

• Shape is dependent on how many electron pairs surround the central atom

• Some of the pairs are “bonding pairs” and some are “lone pairs”

VSEPR Theory• All pairs of electrons try to get as far

apart as possible (like charges repel)• This “spreading out” of electron pairs

creates certain bonding angles and shapes that differ from molecule to molecule

Domains• Each number of items a central

element has (lone pair or bonded atom) is referred to as a “domain”

• We are going to discuss these different molecules by the number of domains they have

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2 Domains• Ex.) BeCl2, CO2

–2 domains: 2 bonded atoms, 0 lone pairs

–Shape = “linear”• Be does this and does

not follow octet rule!

3 Domains• Ex.) BF3, BCl3

– 3 domains: 3 bonded atoms, 0 lone pairs

– Shape = “trigonal planar” or “triangular planar”

– 120 degree angles• Only Boron does this!• Central atom does not

follow octet rule

3 Domains• Ex.) SO2• 3 domains: 2

bonded atoms, 1 lone pair

• The 2 bonded atoms are pushed apart by 3rd pair (not seen)

• Shape = “bent” due to repulsion of lone pair

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4 Domains• CH4• 4 domains: 4 bonded atoms, 0 lone pairs• All angles are 109 degrees apart• Shape: “tetrahedral”

4 Domains

• NH3 , PCl3• 4 domains: 3

bonded atoms, 1 lone pair

• 109 degrees• Shape: “trigonal

pyramidal”

Tetrahedral vs. Trigonal pyramidal

Tetrahedral Molecular Shape

Trigonal Pyramidal Molecular Shape

4 Domains• Ex.) H2O• 4 domains: 2 bonded

atoms, 2 lone pairs• Shape: “bent”

4 Domain• Ex.) HF• Fluorine is

central• 4 domains: 1

bonded atom and 3 lone pairs

• Shape: “linear”

5 Domains• Ex.) PCl5• 5 domains: 5

bonded atoms, 0 lone pairs

• Shape: “trigonal bipyramidal”

5 Domains• Ex.) SF4• 5 domains: 4 bonded

atoms, 1 lone pair• Shape: “seesaw”

5 Domains • Ex.) ClF3, BrF3• 5 domains: 3

bonded atoms, 2 lone pairs

• Shape: “T-shaped”

5 Domains• Ex.) XeF2• 5 domains: 2

bonded atoms, 3 lone pairs

• Shape: “linear”

5 Domains

5 atoms, 0 lone pairs 4 atoms, 1 lone pair

3 atoms, 2 lone pairs 2 atoms, 3 lone pairs

6 Domains• Ex.) SF6• 6 domains: 6

bonded atoms, 0 lone pairs

• All angles 90 degree• Shape: “octahedral”

6 Domains• Ex.) BrF5• 6 domains: 5

bonded atoms, 1 lone pair

• Shape: “square pyramidal”

6 Domains• Ex.) XeF4• 6 domains: 4

bonded atoms, 2 lone pairs

• Shape: “square planar”

6 Domains

6 bonded atoms, 0 lone pairs 5 bonded atoms, 1 lone pair

4 bonded atoms, 2 lone pairs

Steps for using VSEPR:

1. Draw a Lewis Dot Structure2. Predict the geometry around the central atom3. Predict the molecular shape—how many domains are there?Use the chart and examples!

… also, we can try and predict the angles between atoms.

All e- pairs push each other as far apart as

possible• Shared (bonded) pairs are

“stretched” between two atoms that want them– “Longer & Thinner”

• Unshared (non-bonding) pairs are not “stretched.”– “Shorter & Thicker”

Electron Pair Repulsion• 2 lone pairs require the most space

& repel most, resulting in the greatest distance (angle)

• 1 lone pair (thick) & 1 bonding pair (thin) require less space

• 2 bonding pairs between atoms (thin lines) and repel each other the smallest distance (angle).

Objectives• -Explain VSEPR Theory• -Use VSEPR to predict shapes of

molecules

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