Prediction of molecular geometry on the basis of VSEPR and hybridization.

To use this theory for predicting the shapes of molecules, the number of electron pairs (both, shared and lone pairs) is simply counted. This is illustrated by taking a typical molecule of the type AB_n. 'A' is the central atom, 'B' atoms are bonded to 'A' by single covalent bonds (single electron pair bonds), and 'n' is the number of 'B' atoms bonded to one atom of 'A'.

Shapes of the molecules having only the bond (shared) pairs of electrons molecules with two bond pairs

In a molecule having two bond pairs of electrons around its central atom, the bond pairs are located on the opposite sides (at an angle of 180^o) so that the repulsion between them is minimum. Such molecules are therefore linear. Some molecules, which show linear geometry are: BeF₂ (beryllium fluoride), BeCl₂ (beryllium chloride), BeH₂ (beryllium hydride), ZnCl₂ (zinc chloride), and HgCl₂(mercuric chloride).

Problem

9. Predict the shape of a molecule of beryllium chloride molecule following the VSEPR theory.

Solution

In a beryllium chloride molecule represented by the type AB₂, the central atom A has two electron pairs located on either side of it, the molecule AB₂ takes a linear geometry.

Molecules with three bond pairs

In a molecule having three bond pairs of electrons around its central atom, the electron pairs form an equilateral triangular arrangement around the central atom. These molecules have trigonal planar (or triangularplanar) shape and the three bond pairs are at 120°C with respect of each other.

In a molecule of the type AB₃, the three bond pairs of electrons are located around A in a triangular arrangement and the molecule AB₃, has a triangular planar geometry. Some molecules that show triangular planar geometry are BCl₃, BF₃, etc.

borontrifluoride is a trigonal planar molecule

Molecules with four bond pairs

Molecule having four bond pairs of electrons around the central atom, arrange their electrons tetrahedrally. These molecules have tetrahedral shapes and the four bond pairs are at an angle of 109�28' with respect to each other. Some molecules which show tetrahedral geometry are CH₄, CCl₄, NH₄⁺, SiH₄ etc.

Molecules with five bond pairs

Five bond pairs orient themselves around the central atom in a trigonal bipyramidal way. A molecule having five bond pairs around its central atom has a triangular bipyramidal shape. Three bond pairs are arranged in an equatorial triangular plane and are oriented at an angle of 120° with respect to each other. The other two bond pairs are opposite to each other, and at right angles to the triangular plane formed by the three bond pairs. Some other molecules, which show trigonal bipyramidal geometry are; PCl₅, PF₅, SbCl₅.

For example in a molecule of the type AB₅, the five bond pairs are distributed in a trigonal bipyramidal around the central atom 'A'. Therefore, the molecules of the type AB₅ are trigonal bipyramidal in shape.

AB_5, five bond pairs are oriented around A in a trigonal bipyramidal shape.Similarly,PCl₅ also has a trigonal bipyramidal shape.

Molecules with six bond pairs

Six bond pairs in a molecule are distributed octahedrally around the central atom. A molecule having six bond pairs around its central atom has an octahedral shape. In a molecule of the type AB₆, the six 'B' atoms are placed octahedrally around 'A'. Thus, the molecules of the type AB₆ are octahedral. The molecule SF₆ has an octahedral geometry.

AB₆ type molecules are octahedral in shape. SF₆ molecule has an

octahedral geometry.

Shapes of the molecules having bond pairs and lone pairs of electrons

The pairs of electrons in the valence shell of an atom, which are not involved in bonding are called lone pair of electrons. Well known instances of these types of molecules are: the oxygen atom in water molecule H₂O, has two lone pairs of electrons; the nitrogen atom in ammonia molecule NH₃ has one lone pair of electrons. Mentioned below are a few illustrative examples of these types of molecules.

Molecules having three bond pairs and one lone pair

A molecule having three bond pairs, and one lone pair of electrons, has these four pairs of electrons distributed tetrahedrally around the central atom. A molecule of AB₃ type has a triangular pyramidal shape and is called a trigonal pyramidal molecule. Typical molecules of this type are NH₃, NF₃, PCl₃, H₃0+ etc. Distribution of three bond pairs, and one lone pair in a molecule of the type PH₃ and GeF₂ are shown below:

Phosphorus atom has 5 electrons in its outermost orbit. 3H-atoms contribute one electron each to make in all 8 electrons around P-atom. Thus, 4 pairs of electrons would be distributed in a tetrahedral fashion around the central atom. Three pairs form three P-H bonds while the fourth pair remains unused. As there is a lone pair in a PH₃ molecule, the bond pairs are pushed slightly inwards due to greater lone pair- bond pair repulsions. Thus, the ideal bond angle of 109°28' for a tetrahedral arrangement in such molecules is lower, the actual HPH angle, being 104^o. Thus, PH₃ is a trigonal pyramidal molecule.

GeF₂, has a very peculiar tetrahedral structure. This is termed as j tetrahedral structure. In this Ge has a lone pair of electrons and is connected to 3F atoms. 2 atoms of F being shared by adjacent Ge atoms to give chains as shown below:

Molecules with two bond pairs and two lone pairs

The four electron pairs (two bond pairs + two lone pairs) are distributed tetrahedrally around the central atom as shown below. The two lone pairs on the central atom repel the bond pair slightly inwards due to greater lone pair-bond pair repulsion. As a result, the bond angle in such a molecule is less than the tetrahedral value of 109°28'. The presence of only two bonds in the molecules gives a bent (V-shaped) structure.

Examples include H₂S, H₂O, F₂O and SCl₂.

H₂S: Bent (V-shaped) structure. S has 6 electrons in its outermost shell. 2H-atoms contribute 2 electrons during bonding. Thus, there are 8 electrons or 4 electron pairs around S. This gives a tetrahedral distribution of electron pairs around S. The two corners of the tetrahedron are occupied by H-atoms and the other two by the lone-pairs of electrons. Thus, H₂S is a bent structure molecule.

Molecules with four bond pairs and two lone pairs

The four bond pairs are distributed in a planar distribution. The two lone pairs are in a direction at right angles to this plane. This gives a square planar shape to such molecules.

Examples include ICl^-₄, XeF₄ and [Ni(CN) ₄]^2-.

Problem

10. Predict the shapes of the following molecules using the valence shell electron pair repulsion (VSEPR) theory. AsF₅, HgBr₂,

Solution

According to the VSEPR theory, the electron pairs present is the valence-shell of the central atom/ion arrange themselves in the space around it so as to keep them as far as possible from each other, so as to minimize the electrostatic repulsions.

5">

AsF₅

As has five electrons in its outermost orbit. Due to sharing of 5 electrons from 5F-atoms, there are in all 5 electron pairs. These are distributed in space to form a trigonal bipyramid as shown here.

2">

HgBr₂

Hg has only two electrons in its outermost orbit and sharing these electrons with two Br gives 2 pairs of electrons around Hg. This gives a linear structure to HgBr₂ (electron pairs are positioned at 180� to each other).