KI2051 KIMIA ORGANIK

TUGAS

ATOMIC ORBITAL & MOLECULAR ORBITAL BONDING

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oleh

Wardatun Malatsih

13013055

INSTITUT TEKNOLOGI BANDUNG

FAKULTAS TEKNOLOGI INDUSTRI

TEKNIK KIMIA 2015

ATOMIC ORBITAL

A wave function for an electron in an atom is called an atomic orbital; this atomic orbital

describes a region of space in which there is a high probability of finding the electron. Energy

changes within an atom are the result of an electron changing from a wave pattern with one

energy to a wave pattern with a different energy (usually accompanied by the absorption or

emission of a photon of light).

Hybrid Atomic Orbitals

Hybrid orbitals are formed by mixing pure s, p, and d orbitals. Hybrid orbitals overlap better

with other orbitals than the pure atomic orbitals from which they are formed, so bonds formed

by hybrid orbitals or stronger than those formed by ordinary atomic orbitals.

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 .

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 .

Quantum mechanical approaches by combining the

wave functions to give new wavefunctions are called

hybridization of atomic orbitals. Hybridization has a

sound mathematical fundation, but it is a little too

complicated to show the details here. Leaving out the

jargons, we can say that an imaginary mixing process

converts a set of atomic orbitals to a new set of hybrid

atomic orbitals or hybrid orbitals.

The geometries of the five different sets of hybrid atomic

orbitals (sp, sp2, sp3, sp3d and sp3d2).

Example: sp Hybridization in Magnesium Hydride

Figure 2 Formation of a pi bond

Figure 1 Formation of a sigma bond

Figure 3 Hybrid atomic orbitals

In magnesium hydride, the 3s orbital and one of the 3p orbitals from magnesium hybridize to

form two sp orbitals. The two frontal lobes of the sp orbitals face away from each other forming

a straight line leading to a linear structure. These two sp orbitals bond with the two 1s orbitals

of the two hydrogen atoms through sp-s orbital overlap.

Figure 4 sp Hybridization in Magnesium Hydride

MOLECULAR ORBITAL BONDING (MO)

Molecular orbital theory takes the view that molecules and atoms are alike. Both have energy

levels that correspond to various orbitals that can be populated by electrons. They are treated

as collections of nuclei and electrons, with the electrons of the molecule distributed among

molecular orbitals of different energies.

Figure 5 Combining 1s atomic orbitals to produce bonding and antibonding molecular orbitals

Molecular orbitals can spread over two or more nuclei and can be considered to be formed by

the constructive inteference of the overlapping electron waves corresponding to the atomic

orbitals of the atoms in the molecule (Fugure 1).

One common approximation that allows us to generate molecular orbital diagrams for some

small diatomic molecules is called the Linear Combination of Atomic Orbitals (LCAO) approach.

The following assumptions lie at the core of this model.

1. Molecular orbitals are formed from the overlap of atomic orbitals.

2. Only atomic orbitals of about the same energy interact to a significant degree.

3. When two atomic orbitals overlap, they interact in two extreme ways to form two

molecular orbitals, a bonding molecular orbital and an antibonding molecular orbital.

Bonding MOS concentrate electron density between nuclei; antibonding Mos remove

electron density from between nuclei. Nonbonding Mos do not effect the energy of the

molecule.

The stability of the molecule can be predicted using bond order.

bond order = 1/2 (#e- in bonding MO's - #e- in antibonding MO's)

1. If the bond order for a molecule is equal to zero, the molecule is unstable.

2. A bond order of greater than zero suggests a stable molecule.

3. The higher the bond order is, the more stable the bond.

The ability of MO theory to describe delocalized orbitals avoids the need for resonance theory.

Delocalization of bonds leads to a lowering of the energy by an amoun called the delocalization

energy and produce more stable molecular structure.

REFERENCES

Jespersen, Neil D., Brady, James E., & Hyslop, A. (2012). Chemistry: The Molecular Nature of Matter

(5th ed). United States of America: John Wiley & Sons, Inc, p. 445-453.

Martin S. Silberberg. (2000). Chemistry: The Molecular Nature of Matter and Change (2nd

ed). Boston: McGraw-Hill, p. 277-284, 293-307.

http://www.mpcfaculty.net/mark_bishop/molecular_orbital_theory.htm (Accessed 2015-02-09 at

00:33 a.m.)

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/hybrid.html (Accessed 2015-02-09

at 00:40 a.m.)

https://sites.google.com/site/ed350201003/Task (Accessed 2015-02-08) (Accessed 2015-02-09 at

00:51 a.m.)

http://chemwiki.ucdavis.edu/Organic_Chemistry/Fundamentals/Hybrid_Orbitals (Accessed 2015-02-

09 at 01:17 a.m.)

Figure 6 Approimate relative energies of molecular orbitals in Period 2 diatomic molecules. (a) Li2 through N2. (b) O2 throgh Ne2