Main group section 1 - VSEPR

Question 1

Describe VSEPR theory

Answer 1

The shapes of molecules containing a central p-block element tend to be controlled by the number of electrons in the valence shell. The theory is used to predict the shapes of molecules

Question 2

Describe the rules for VSEPR theory, for molecule EXn with central atom E

Answer 2

• Electron-electron repulsions decrease in order: lp-lp>lp-bp>bp-bp - Each valence shell electron pair containing E-X single bonds is stereo-chemically significant and repulsions between them determine the molecular shape. - When the central atom E is involved in multiple bond formations to atom X, electron-electron repulsions decrease; triple bond-single bond>double bond-single bond>single bond-single bond - Note that this model does not take into account the relative size of the substituents (ie steric factors)

Question 3

Describe the nature of axial ligands

Answer 3

We may expect axial ligands to be longer than equatorial ligands

Question 4

State the shape of a molecule with two electron pairs

Answer 4

Linear

Question 5

State the possible shapes of a molecule with three electron pairs

Answer 5

Planar triangular/ trigonal planar or bent/non-linear

Question 6

State the possible shapes of a molecule with four electron pairs

Answer 6

Tetrahedral, trigonal pyramidal or bent/non-linear

Question 7

State the possible shapes of a molecule with five electron pairs

Answer 7

Trigonal bipyramidal, seesaw or distorted T

Question 8

State the possible shapes of a molecule with six electron pairs

Answer 8

Octahedral, square pyramidal or square planar

Question 9

Describe the limitations of VSEPR

Answer 9

• If the presence of the lone pair has no geometric consequence it is termed stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect.in this case, VSEPR will fail. - Stereo-chemical inactive lone pairs are usually observed for the heaviest members of a periodic group.

Question 10

State the name and bond angle of a molecule with two electron pairs around the central atom

Answer 10

Linear. Bond angle is 180 degrees

Question 11

State the name and bond angle of a molecule with two bond pairs and one lone pair around the central atom

Answer 11

Bent or non-linear. Bond angle is 119 degrees

Question 12

State the name and bond angle of a molecule with three bond pairs around the central atom

Answer 12

Trigonal planar/planar triangular. Bond angle is 120 degrees

Question 13

State the name and bond angle of a molecule with four bond pairs around the central atom

Answer 13

Tetrahedral. Bond angle is 109.5 degrees

Question 14

State the name and bond angle of a molecule with two bond pairs and two lone pairs around the central atom

Answer 14

Bent or non-linear. Bond angle is 104.5 degrees

Question 15

State the name and bond angle of a molecule with three bond pairs and one lone pair around the central atom

Answer 15

Trigonal pyramidal. Bond angle is 107 degrees

Question 16

State the name and bond angle of a molecule with five bond pairs around the central atom

Answer 16

Trigonal bipyramidal. Bond angles are 90 and 120 degrees

Question 17

State the name and bond angle of a molecule with four bond pairs and one lone pair around the central atom

Answer 17

Seesaw. Bond angles are 87 and 102 degrees

Question 18

State the name and bond angle of a molecule with three bond pairs and two lone pairs around the central atom

Answer 18

Distorted T. bond angle is 87.5 degrees

Question 19

State the name and bond angle of a molecule with six bond pairs around the central atom

Answer 19

Octahedral. Bond angle is 90 degrees

Question 20

State the name and bond angle of a molecule with five bond pairs and one lone pair around the central atom

Answer 20

Square pyramidal. Bond angles are 81.9 and 90 degrees

Question 21

State the name and bond angle of a molecule with four bond pairs and two lone pairs around the central atom

Answer 21

Square planar. Bond angle is 90 degrees