MOLECULAR MODEL

MOLECULAR MODELJONALYN M. SHENTON

Valence Bond TheoryLinus Pauling became the champion of the valence bond model, which is easier to visualize and use.

Valence Bond TheoryThe valence bond theory was proposed by Heitler and London to explain the formation of covalent bond quantitatively using quantum mechanics.Later on, Linus Pauling improved this theory by introducing the concept of hybridization.Valence bond (VB) theoryassumes that all bonds are localized bonds formed between two atoms by the donation of an electron from each atom.

Valence Bond TheoryValence Bond theory describes covalent bond formation as well as the electronic structure of molecules.

The theory assumes that electrons occupy atomic orbital's of individual atoms within a molecule, and that the electrons of one atom are attracted to the nucleus of another atom.

Valence Bond TheoryA covalent bond is formed by the overlapping of two half filled valence atomic orbital's of two different atoms.The electrons in the overlapping orbital's get paired and confined between the nuclei of two atoms.The electron density between two bonded atoms increases due to overlapping. This confers stability to the molecule.

Valence Bond TheoryGreater the extent of overlapping, stronger is the bond formed. The direction of the covalent bond is along the region of overlapping of the atomic orbital's i.e., covalent bond is directional.

Valence Bond TheoryBy: Linus Pauling

JONALYN M. SHENTONOverlap of Atomic OrbitalsThe sharing of electrons between atoms is viewed as an overlap of atomic orbitals of the bonding atoms.valence bond model says that an s-orbital on one atom overlaps with an s-orbital on the other to form a bond.

Valence Bond Theory-bondA sigma bond (symbol: ) is a covalent bondformed via linear overlap of two orbital's.

-bondA pi bond (symbol: ) is a covalent bondformed via parallel overlap of two orbital's.

There are two types of covalent bonds based on the pattern of overlapping as follows: bond

-bondThe covalent bond formed due to overlapping of atomic orbital along the inter nucleus axis is called -bond. It is a stronger bond and cylindrically symmetrical.Depending on the types of orbital's overlapping, the -bond is divided into following types:

(i): s-sbond, (ii): p-pbond, (iii): s-pbond:

Sigma and Pi Bonds

Sigma bond: A covalent bond resulting from the formation of a molecular orbital by the end-to-end overlap of atomic orbitals, denoted by the symbol .

For example, theC-Csigma bond in ethane is formed by the head-on overlap of twosp3orbitals. The C-H sigma bonds are formed bys-sp3overlap.A double bond contains one sigma bond and one pi bond (side-on overlap ofp orbitals).

-bonds-sbond:

-bonds-pbond:

-bondp-pbond:

Pi bond: A covalent bond resulting from the formation of a molecular orbital by side-to-side overlap of atomic orbitals along a plane perpendicular to a line connecting the nuclei of the atoms, denoted by the symbol .

-bondThe covalent bond formed by sidewise overlapping of atomic orbital's is called - bond. In this bond, the electron density is present above and below the inter nuclear axis. It is relatively a weaker bond since the electrons are not strongly attracted by the nuclei of bonding atoms.

Note: The 's' orbital's can only form -bonds, whereas the p, d & f orbital's can form both and-bonds.

In ethylene, theC=Cdouble bond consists of a sigma bond and a pi bond. The sigma bond is formed by the head-on overlap of twosp2orbitals. The pi bond is formed by the side-on overlap of two2porbitals.

Illustration

H2 moleculeThe electronic configuration of hydrogen atom in the ground state is 1s1.In the formation of hydrogen molecule, two half filled 1s orbital's of hydrogen atoms overlap along the inter-nuclear axis and thus by forming a s-sbond.

Cl2 moleculeThe electronic configuration of Cl atom in the ground state is [Ne]3s23px23py23pz1.The two half filled 3pzatomic orbital's of two chlorine atoms overlap along the inter-nuclear axis and thus by forming a p-pbond.

HCl moleculeIn the ground state, the electronic configuration of hydrogen atom is 1s1.And the ground state electronic configuration of Cl atom is [Ne]3s23px23py23pz1.The half filled 1s orbital of hydrogen overlap with the half filled 3pzatomic orbital of chlorine atom along the inter-nuclear axis to form a s-pbond.

O2 moleculeThe electronic configuration of O in the ground state is [He] 2s22px22py12pz1.The half filled 2pyorbital's of two oxygen atoms overlap along the inter-nuclear axis and form p-pbond.The remaining half filled 2pzorbital's overlap laterally to form ap-pbond.

O2 moleculeThus a double bond (one p-pand onep-p) is formed between two oxygen atoms.

N2 moleculeThe ground state electronic configuration of N is [He] 2s22px12py12pz1.

A p-pbond is formed between two nitrogen atoms due to overlapping of half filled 2pxatomic orbital's along the inter-nuclear axis.

N2 moleculeThe remaining half filled 2pyand 2pzorbital's form twop-pbonds due to lateral overlapping. Thus a triple bond (one and two) is formed between two nitrogen atoms.

Atomic orbital- An orbital that is associated with only one particular atom. This is in contrast to molecular orbitals, which are spread across a collection of atoms.

Similarly a2s orbitalisthe region where there is the greatest chance of finding the electron thats further from the nucleus - this is an orbital at the second energy level. So the nearer an electron is to the nucleus, the lower is its energy.s orbitals.The orbital occupied by the hydrogen electron is called a1s orbital.The"1"represents the fact that the orbital is in the energy level closest to the nucleus. The"s givesthe shape of the orbital. s orbitals are spherically symmetric around the nucleus.

The p orbitalsA p orbital is rather like 2 identical balloons tied together at the nucleus.Since p orbitals occupy space in this manner, they can be said to have certain directions as well. The two lobes can be pointing in any possible direction opposite to each other as the atom tumbles around in space. This means that at any one energy level it is also possible to have three absolutely equivalent p orbitals pointing at right angles to each other which can be in the x,y and z axis. Therefore p orbitals can have directions px, pyand pzas show in the illustration below.

SIGMA AND PI BOND

HybridizationThe intermixing of two or more pure atomic orbital's of an atom with almost same energy to give same number of identical and degenerate new type of orbital's is known as hybridization.The new orbital's formed are also known as hybrid orbital's.During hybridization, the atomic orbital's with different characteristics are mixed with each other.

Hybridization- The linear combination of atomic orbitals into hybrid orbitals that accommodate particular molecular geometries.Hybrid orbital- Orbitals formed from the combination of atomic orbitals that accommodate particular geometries.Molecular orbital- Orbitals that, in contrast to atomic orbitals, are distributed over an entire molecule instead of being localized to specific atoms.

Hybridization

In chemistry, hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridized orbitals are very useful in the explanation of the shape of molecules. It is an integral part of valence bond theory.

Types of Hybridizationspsp2sp3sp3dsp3d2sp3d3

sp HybridizationIntermixing of one 's' and one 'p' orbital's of almost equal energy to give two identical and degenerate hybrid orbital's is called 'sp' hybridization. These sp-hybrid orbital's are arranged linearly at by making 180 of angle. They possess 50% 's' and 50% 'p' character.

sp2 hybridizationIntermixing of one 's' and two 'p' orbital's of almost equal energy to give three identical and degenerate hybrid orbital's is known as sp2 hybridization. The three sp2 hybrid orbital's are oriented in trigonal planar symmetry at angles of 120 to each other. The sp2 hybrid orbital's have 33.3% 's' character and 66.6% 'p' character.

sp3 hybridizationIn sp3 hybridization, one 's' and three 'p' orbital's of almost equal energy intermix to give four identical and degenerate hybrid orbital's. These four sp3 hybrid orbital's are oriented in tetrahedral symmetry with 109 28' angle with each other. The sp3 hybrid orbital's have 25% s character and 75% 'p' character.

sp3d hybridization In sp3d hybridization, one 's', three 'p' and one 'd' orbital's of almost equal energy intermix to give five identical and degenerate hybrid orbital's, which are arranged in trigonal bipyramidal symmetry. Among them, three are arranged in trigonal plane and the remaining two orbital's are present above and below the trigonal plane at right angles.The sp3d hybrid orbital's have 20% 's', 60% 'p' and 20% 'd' characters.

sp3d2 hybridizationIntermixing of one 's', three 'p' and two 'd' orbital's of almost same energy by giving six identical and degenerate hybrid orbital's is called sp3d2 hybridization. These six sp3d2 orbital's are arranged in octahedral symmetry by making 90 angles to each other. This arrangement can be visualized as four orbital's arranged in a square plane and the remaining two are oriented above and below this plane perpendicularly.

sp3d3 hybridizationIn sp3d3 hybridization, one 's', three 'p' and three 'd' orbital's of almost same energy intermix to give seven sp3d3 hybrid orbital's, which are oriented in pentagonal bipyramidal symmetry. Five among the sp3d3 orbital's are arranged in a pentagonal plane by making 72 of angles. The remaining are arranged perpendicularly above and below this pentagonal plane.

e PairHybridizationShape2splinear3sp2trigonal planar4sp3tetrahedral, pyramidal, or bent5sp3dtrigonal bipyramidal, trigonal planar, or linear6sp3d2octahedral, square planar, or linear

ResonanceResonance is a mental exercise and method within theValence Bond Theoryof bonding that describes the delocalization of electrons within molecules. It compares and contrasts two or more possibleLewis structuresthat can represent a particular molecule. Resonance structuresare usedwhen oneLewis structure for a single moleculecannotfully describe thebonding that takes placebetweenneighboring atoms relative to the empirical data for the actual bond lengths between those atoms. Resonance forms illustrate areas of higher probabilities (electron densities).Resonance structures do not change the relative positions of the atoms like your arms in the metaphor. The skeleton of theLewis Structureremains the same, only the electron locations change.

"Pick the Correct Arrow for the Job"Most arrows in chemistry cannot be used interchangeably and care must be given to selecting the correct arrow for the job.: A double headed arrow on both ends of the arrow between Lewis structures is used to show their inter-connectivity:Double harpoons are used to designate equilibria: A single harpoon on one end indicate the movement ofoneelectron:A double headed arrow on one end is used to indicate the movement oftwoelectrons

Example 1: OzoneConsider ozone (O3)

DelocalizationandResonance Structures RulesIn resonance structures, the electrons are able to move to help stabilize the molecule. This movement of the electrons is calleddelocalization.Resonance structures should have the same number of electrons, do not add or subtract any electrons. (check the number of electrons by simply counting them).All resonance structures must follow the rules of writingLewis Structures.Thehybridizationof the structure must stay the same.The skeleton of the structure can not be changed (only the electrons move).Resonance structures must also have the same amount of lone pairs.

Example 2: Thiocyanate IonConsider the thiocyanate(CNSCNS) ion.SOLUTION1. Find the Lewis Structure of the molecule. (Remember the Lewis Structure rules.)

2. Resonance:All elements want an octet, and we can do that in multiple ways by moving the terminal atom's electrons around (bonds too).