An atom contains three fundamental particles; electrons, protons and neutrons. These particles are arranged in such a way that the atom becomes stable. Out of these fundamental particles, electrons are negatively charged and protons are positively charged particles. Neutrons are neutral in nature with no charge, but they contribute to the mass of atom. Protons and neutrons are placed at the center of the atom that is called as a nucleus. The term nucleus was purposed by Erenst Rutherford.
Electrons are arranged in certain atomic orbitals which have fixed energy. Electrons can transfer from one atomic orbital to another by the absorption or emission of energy in the form of photon. The atomic orbitals involve in the bond formed with other elements to form chemical bonds in molecules. The chemical bonding in molecules can be explained with the help of hybridisation which is a concept of valence bond theory. Valence bond theory is based on the concept overlapping of orbitals and hybridisation. The overlapping of orbital results the formation of chemical bonds between atoms.
Hybridisation is a phenomenon of mixing of atomic orbitals which forms orbitals of the same energy and shape. These same energy orbitals are called as degenerate orbitals and hybrid orbitals. Let’s discuss the formation of different type hybrid orbitals with their shape and geometry.
Hybrid orbitals are formed by the mixing of existing orbitals (2p, 3d, 4s....etc) to form new ones that are more directional and therefore able to overlap more effectively with the orbitals of another atom. The number of hybrid orbitals that an atom can form is dictated by two criteria.
- The number of valence level orbitals that are available for the atom.
- "Law of conservation of orbitals" that is the number of hybrid orbitals formed must equal the number of orbitals that are used in the formation of the hybrid orbitals.
Hybridization does not change the number of orbitals but their orientation and distribution are changed. The hybrid orbitals tend to form stronger sigma bonds with other atoms are used for lone pair electrons but empty hybrid orbitals do not exist as such.
Equal participation of different atomic orbitals gives equivalent hybrid orbitals; and if the contribution from each components is not the same, non-equivalent hybrid orbitals are formed. The lists of characteristics of some common hybrid orbitals are shown below.
Ozone O3 is a simple triatomic molecule with equal oxygen-oxygen bond lengths. Equal X-O bond lengths are also observed in other molecules and ions such as SO2 and NO2. Valence bond theory introduced resonance to rationalize the equivalent bonding to the oxygen atoms in these structures, but MO theory provides another view.
To understand the bonding in ozone assume that all three oxygen atoms are sp2
hybridized. The central atom uses its sp2
hybrid orbitals to form two sigma bonds and to accommodate a lone pair. The terminal atoms use their sp2
hybrid orbitals to form one sigma bond and to accommodate two lone pairs. In total the lone pairs and bonding pairs in the sigma framework of O3
account for seven of the nine valence electron pairs in O3
The pi bond in ozone arises from the two remaining pairs because each oxygen atom in ozone is sp2 hybridized, an un-hybridized p orbital perpendicular to the O3 plane remains on each on the three oxygen atoms. The orbitals are in the correct orientation to form pi bonds.
For effective hybridization the following conditions should be satisfied.
- The hybridized orbitals should have equal or nearly same energy. Hence a 2s orbital cannot be hybridized with 3s orbital as the energy difference is more.
- The hybridized orbitals should have proper symmetry so that there is maximum extent of overlap of orbitals.
- The number of atomic orbitals combined should be equal to number of hybridized orbitals formed. But is not mandatory to take all orbitals with single/paired electron into hybridization.
For example in the formation of ethylene, although carbon atom is having one electron in 2s orbital and three electrons in 2p orbitals only one 2s orbital and 2 2p orbital are selected to give sp2 hybridization leaving one electron in 2p untouched.
The Drawbacks of Hybridization has given Below:-
- For successful hybridization the orbitals should have nearly same energy. But it is not true in the case of hybridization involving d orbitals. As d orbitals have much more higher energy than p orbitals they are also involved in hybridization in some cases. But this can be explained by a huge positive charge developed on central atom which shrinks the d orbital and reduce the energy gap.
- For example in SF6 the sulfur uses d orbitals. This is because after sharing all the six electrons there is a partial positive charge over sulfur atom which shrinks the d orbitals and reduce the energy difference.
- The difference between inner orbital hybridization and outer orbital hybridization cannot be explained by hybridization.
The simplest hybrid orbitals are sp hybrid orbitals formed by the
combination of one s orbital and one p orbital; these atomic orbitals
combine to form two sp hybrid orbitals.
one s atomic orbital + one p atomic orbital → Two sp Hybrid orbitals
sp hybrid orbitals always occur in sets of two. The unhybridized 2p orbitals are on the y- and z-axis. Each sp orbital has 50% s-character and 50% p-character because those are the percentages of the orbitals combined when constructing them.
The bond angles is approximately 180o.
The mathematical combination of one 2s atomic orbital and two 2p atomic orbital forms three equivalent sp2 hybrid orbital. They are derived from three atomic orbitals, sp2 hybrid orbitals always occur in sets of three.
one s atomic orbital + Two p atomic orbital → Three sp2 Hybrid orbitals
The axis of the three sp2
hybrid orbitals lie in a plane and are directed toward the corners of an equilateral triangle; the angles between sp2
hybrid orbitals is 120o
If an s orbital and the three p orbital on a central atom are hybridized, four hybrid orbitals called sp3 hybrid orbitals are formed. They are derived form four atomic orbitals. sp3 hybrid orbitals always occur in sets of four.
one s atomic orbital + Three p atomic orbital → Four sp3 Hybrid orbitals
The four sp3
hybrid orbitals are equivalent and directed to the corners of a tetrahedron. The angle between the four sp3
hybrid orbitals is 109.5o