IONIC OR ELECTROVALENT BOND
The bond formed, as a result of the electrostatic attraction between the positive and negative ions was termed as the electrovalent bond. The formation of ionic compounds primarily depends on:
- The ease of formation of the positive and negative ions from the respective neutral atoms.
- The arrangement of the positive and negative ions in the solid, that is, the lattice of the crystalline compound.
Ionic bonds are formed more easily between elements with comparatively low ionization enthalpies and elements with comparatively high negative value of electron gain enthalpy.
Lattice enthalpy: The lattice enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent.
- Bond length: It is the equilibrium distance between the nuclei of two bonded atoms in a molecule.
- Bond angle: It is the angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion.
- Bond enthalpy: It is the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state.
- Bond order: It is given by the number of bonds between the two atoms in a molecule.
- Resonance structures: These are the structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons are taken as the canonical structures.
THE VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY
The main postulates of the VSEPR theory are:
- The shape of a molecule depends upon the number of valence shell electron pairs (bonded or non-bonded) around the central atom.
- Pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged.
- These pairs of electrons tend to occupy such positions in space that minimize repulsion and thus maximize distance between them.
- The valence shell is taken as a sphere with the electron pairs localizing on the spherical surface at maximum distance from one another.
- A multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.
- When two or more resonance structures can represent a molecule, the VSEPR model is applicable to any such structure.
VALENCE BOND THEORY
VBT discusses bond formation in terms of overlap of orbitals. Because of orbital overlap the electron density between the nuclei increases which helps in bringing them closer. However, the actual bond enthalpy and bond length values are not obtained by overlap alone and other variables have to be taken into account.
Molecular Orbital Theory
The MO theory describes bonding in terms of the combination and arrangement of atomic orbitals to form molecular orbitals that are associated with the molecule as a whole. The number of molecular orbitals is always equal to the number of atomic orbitals from which they are formed. Bonding molecular orbitals increase electron density between the nuclei and are lower in energy than the individual atomic orbitals. Anti-bonding molecular orbitals have a region of zero electron density between the nuclei and have more energy than the individual atomic orbitals.