Question

Provide the correct molecular geometry for (a), given the number of lone pairs and bonding groups on the central atom. 

trigonal planar  octahedral  trigonal bipyramidal  trigonal pyramidal  square pyramidal  tetrahedral  seesaw  bent  linear

Provide the correct molecular geometry for (b), given the number of lone pairs and bonding groups on the central atom. 

 trigonal bipyramidal  square pyramidal  bent  tetrahedral  trigonal planar  seesaw  trigonal pyramidal  octahedral  linear 

Provide the correct molecular geometry for (c), given the number of lone pairs and bonding groups on the central atom.

 linear  seesaw  tetrahedral  square pyramidal  trigonal bipyramidal  trigonal planar  octahedral  bent  trigonal pyramidal 

Answer 1

Concepts and reason

The concept used here is VSEPR theory. The theory is used to predict the geometry of most polyatomic molecules or ions by considering the number of electron pairs around the central atom. Each group around the central atom is designated as a bond pair or lone pair. The repulsion between the valence electron pairs on the central atom determines the geometry of the molecule.

Fundamentals

For predicting the geometry of molecules, the number of electron pairs (bonding and lone pairs) is counted. Lone pairs repel more than the bonding pairs. The number of lone pairs determines the correct molecular geometry of a molecule.

Part A

Number of valence electrons pairs (bond pairs + lone pairs) on the central atom is equal to 4. The number of bond pairs is 2 and number of lone pairs is also 2. The number of valence electrons pairs is 4. Thus, geometry of this structure cannot be octahedral, trigonal bipyramidal, trigonal pyramidal, square pyramidal, seesaw or linear.

Part A

The four electron pairs are spread tetrahedrally around the central atom. But, the presence of two bond pairs and two lone pairs causes bond pair-lone pair repulsion, which results in bent structure.

Part B

Number of valence electrons pairs (bond pairs + lone pairs) on the central atom is 5. The number of bond pair and lone pair are 2 and 3 respectively. The number of valence electrons are 5. Thus, geometry cannot be square pyramidal, bent, tetrahedral, seesaw, octahedral , trigonal planar or trigonal pyramidal.

Part B

The five electron pairs are distributed in trigonal bipyramidal arrangement. To minimize the repulsion, the three lone pairs are arranged in equatorial position. This results in linear molecular geometry.

Part C

Number of valence electrons pairs (bond pairs + lone pairs) on the central atom is 6. The number of bonding pair and lone pairs are 5 and 1 respectively. Due to 6 valence electrons, geometry cannot be bent, tetrahedral, seesaw, linear, trigonal planar or trigonal pyramidal.

The six electron pairs give octahedral structure. Presence of one lone pair results in square pyramidal geometry.

Ans: Part A

The correct molecular geometry for (a) is bent.

Part B

The correct molecular geometry for (b) is linear.

Part C

The correct molecular geometry for (c) is square pyramidal.