1. Stereochemistry is determined by valence shell electron pairs arranging themselves to minimise repulsions 2. Relative strength of main interactions are: lp-lp > lp-bp > bp-bp 3. Geometry is determined only from the number of σ-pairs (bonding ) + lone pairs. 4. Bonding pair sizes: A≡B > A=B > A-B 5. The size of a bonding pair decreases with increasing electronegativity of the ligand

Topic 5: Predicting Molecular Shapes with VSEPR Flashcards by Lucy Scott

Lewis Theory

A covalent bond is formed when 2 neighbouring atoms share an electron pair

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Octet Rule

Atoms share electron pairs until they have acquired an octet of valence electrons

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VSEPR Theory

Valence Shell Electron Pair Repulsion Theory
Predicts molecular geometry + deviations
Predicts bond lengths and angles

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Regions of enhanced electron density …

stay as far as way as possible from eachother (lone pairs, bonding pairs, unpaired electrons)

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5 VSEPR Theory rules

Stereochemistry is determined by valence shell electron pairs arranging themselves to minimise repulsions
Relative strength of main interactions are: lp-lp > lp-bp > bp-bp
Geometry is determined only from the number of σ-pairs (bonding ) + lone pairs.
Bonding pair sizes: A≡B > A=B > A-B
The size of a bonding pair decreases with increasing electronegativity of the ligand

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How to use VSEPR Theory

Determine central atom (usually least electronegative) + how many valence hell electrons it has
Add total number of electrons donated by ligands
Adjust for any charge
Divide by 2 = number of pairs
Decide how to distribute lone + bonding pairs

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2-coordinate shape + draw (2)

linear (0/3 LP)
bent (1/2 LP)

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3-coordinate shape + draw (3)

trigonal planar (0 LP)
t-shapes (2 lone pairs at equatorial positions)
trigonal pyramidal (1 LP)

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4-coordinate shape + draw (3)

tetrahedral (0 LP)
square planar (2 LP)
saw horse (1 LP)

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5-coordinate shape + draw (2)

trigonal bipyramidal (0 LP)
square pyramidal (1 LP)

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6-coordinate shape + draw (2)

octahedral (0 LP)
trigonal prismatic (0 LP)

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Linear
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

2
sp
180

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Trigonal planar
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

3
sp2
120

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Tetrahedral
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

4
sp3
109.5

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Trigonal bipyramidal
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

5
sp3d
90,120

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Octahedral
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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6
sp3d2
90

Pentagonal bipyramidal
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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7
sp3d3
72,90

Bent
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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3/4
sp2/sp3
119/104.5

T-shaped
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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5
sp3d
90/180

Trigonal pyramidal
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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4
sp3
107

Saw horse
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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5
sp3d
90/120/180

Square pyramidal
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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6
sp3d2
90/180

Octahedral
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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6
sp3d2
90/180

Trigonal prismatic
Number of sigma-bonding & lone pairs
Hybridisation
Bond angles

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6
sp3d2
60/120

1st bond is ... 2nd and 3rd bond is ...

sigma pi

Lone pair has less effect on ... bonds than ... bonds

pi sigma

As the difference of electronegativity between the ligand and the central atom increases, ...

the electron density moves away from the central atom so the repulsion of that bond to other pairs decreases

A radical is ... It's treated as a ... in VSEPR and the electron pairs will be ...

a species with an unpaired electron in its valence shell small lone pair n.5

A ... is more repulsive than a radical

double bond

Deviations in bond angles in CCl2F2

F-C-F < 109.5 < Cl-C-Cl F more electronegative so pulls pairs of electrons further way from C freeing up space and making C-F less repulsive

Topic 5: Predicting Molecular Shapes with VSEPR Flashcards

(30 cards)