Chemical Bonding and VSEPR Theory Notes

Chemical Bonding: Chemical Formulas

Covalent compounds consist of separate molecules, each possessing particular shapes. The Valence Shell Electron Pair Repulsion (VSEPR) Theory explains these shapes, asserting that the shape of a molecule depends on the number of electron pairs around the central atom. Since electrons are negatively charged, these pairs repel each other and arrange themselves to maximize their separation.

Example 5.17: Beryllium Chloride (BeCl₂)

A beryllium chloride molecule (BeCl₂) has two electron pairs around the central beryllium atom. The VSEPR theory focuses only on the electrons around the central atom, ignoring lone pairs on the chlorine atoms. Experimentally, BeCl₂ is found to be a linear molecule with a bond angle of 180∘180^\circ. This linearity arises because the two electron pairs repel each other and position themselves as far apart as possible, minimizing repulsion. Any molecule with two electron pairs around the central atom adopts this linear shape.

Example 5.18: Boron Trichloride (BCl₃)

Boron trichloride (BCl₃) consists of three electron pairs around the central boron atom. Experimentally, the bond angle is 120∘120^\circ, indicating that the three electron pairs are equally spaced to minimize repulsion. All three pairs lie in the same plane, thus BCl₃ is described as triangular planar or trigonal planar. This shape is characteristic of molecules with three electron pairs around the central atom.

Example 5.19: Methane (CH₄)

A methane molecule (CH₄) has four electron pairs around the central carbon atom. The experimentally determined bond angle is 109.5∘109.5^\circ. This arrangement forms a tetrahedral shape, minimizing repulsion between the four electron pairs. Representing a tetrahedron on paper involves using wedges to indicate bonds coming out of the plane and dashed lines for bonds going behind the plane.

Lone Pairs and VSEPR Theory

The VSEPR theory is further refined to account for the effects of lone pairs (non-bonding pairs of electrons in the outer energy level) on molecular shapes. The repulsion hierarchy is: lone pair/lone pair > lone pair/bond pair > bond pair/bond pair.

Lone pairs, being closer to the nucleus, exert greater repulsive force than bond pairs. This difference in repulsion leads to slight deviations in bond angles from expected values.

Example 5.20: Ammonia (NH₃)

To deduce the shape of ammonia (NH₃), use VSEPR theory:

  1. Nitrogen is in Group V, thus it has five valence electrons.
  2. The three hydrogen atoms contribute three electrons (one each).
  3. Total number of electrons = 5 (from N) + 3 (from 3H) = 8 electrons, which equals four electron pairs.

Based on four electron pairs, a tetrahedral shape is predicted. However, the experimentally determined bond angle is 107∘107^\circ, less than the regular tetrahedral angle of 109.5∘109.5^\circ. This reduction is due to the presence of one lone pair and three bond pairs. The increased repulsion from the lone pair reduces the bond angle by approximately 2.5∘2.5^\circ, resulting in a pyramidal shape.

Example 5.21: Water (H₂O)

To deduce the shape of water (H₂O) using the VSEPR theory:

  • Oxygen has 6 valence electrons.
  • The two hydrogen atoms contribute 2 electrons.
  • Total = 8 electrons = four electron pairs, suggesting a tetrahedral shape.

The water molecule is V-shaped. The bond angle is 104.5∘104.5^\circ.