Tuesday, June 11, 2013

Hydrogen Bond

                      a hydrogen bond is a type of attractive ( dipole-dipole ) interaction between an electronegative atom and a hydrogen atom bonded to another electronegative atom. Hydrogen is bound to a highly electronegative  atom such as nitrogen ( N ), oxygen ( O ), or fluorine ( F ). Hydrogen bond can occur between molecules or within parts of a single molecule. The hydrogen bond is stronger than Vander Walls interaction but weaker than covalent or ionic bonds. This type of bond can occur in inorganic molecules such as and in organic molecules like DNA and proteins. Highly electronegativity of the atom bonded to hydrogen atom so that bond sufficiently polar.
Types Of Hydrogen Bonds :

  1. Intra molecular hydrogen bond
  2. Inter molecular hydrogen bond
Inter molecular hydrogen bond :
                This bond occur between separate molecule in a substance . They can occur between number of like or unlike molecules as long as hydrogen donors and acceptors are present in position, which they can interact.
  • An example of hydrogen bond donor is ethanol. which has a hydrogen bonded to oxygen.
  • An example of hydrogen bond acceptor , which does not have a hydrogen bonded to it is the oxygen atom on diethyl ether.  
For Example :
Inter molecular hydrogen bonds can occur between  NH3  molecules alone between  H2O  molecules alone or between NH3 and H2O molecules.

Intra molecular hydrogen bond :
          Intra molecular hydrogen bonds occurs within one single molecule. Both hydrogen donor and acceptor must be present within one molecule. This type of hydrogen bond is commonly called chelation and is more frequently found in organic compounds.
Example :

Strength of hydrogen bonds :
  • Its strength depends upon the electronegativity of atom to which H atom is covalently bonded.
  • Electronegativity of  F > O > N . The strength of hydrogen bond is in the order of H-F - - - H > H-O --- H > H-N --- H .
  • Hydrogen bonds are much weaker than covalent bonds.
  • The bond strength of different bonds is in the order. Ionic bond > Covalent bond > Hydrogen bond > dipole-dipole interaction, vander wall's ( London forces ).

Monday, June 10, 2013

Anti bonding

                               Anto bonding is a type of chemical bonding. Anti bonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei. The overlap of constituent atomic orbitals is said to be out of phase and the electrons present in each anti bonding orbitals are repulsive and act to destabilize the molecule as a whole.
Example :
        H2-    is a diatomic molecule containing 3 electrons. One of the electron found in anti bonding orbital.  Hydrogen atoms have a single 1s electron. The 1s orbital has 2 electrons. One is spin 'up' electron, another one is spin 'down' electron. If a hydrogen atom contains an extra electron, forming a H-  ions , the 1s orbital is filled.
       H atom and  H-   ions approach each other to form a sigma bond between the two atoms. Each atom contribute the electrons to fill the lower energy sigma bond. Extra electrons fill a higher energy state to avoid interaction with other 2 electrons. The higher energy orbital is called the anti bonding orbital. ( σ * ).

MO Theory ( Molecular Orbital Theory )

  • MO Theory was originally called Hund-Mulliken theory.
  • The word orbital was introduced by mulliken in 1932.
  • In a molecule electrons are present in new orbitals called MO.

  • MO are formed by linear combination of atomic orbitals ( LCAO ) of equal energies  ( homonuclear ) or comparable energies  ( hetero nuclear molecule ).    ( Details : The molecule  H2   is composed of two hydrogen atoms. Both H atom have 1s orbital, so when bonded together there are two MO.  
  • Two atomic orbitals can combine to form two molecular orbital.One has a lower energy ( bonding MO ) and other has higher energy ( anti bonding ).  (  Explain :  Bonding MO helps to stablize a system of atoms since less energy.  Anti bonding MO cause a system to be unstabilized. since more energy.                                                                      

  • Bonding MO are represented by  σ (sigma ),  π  ( pi ) ,  δ ( delta ).
  • Anti bonding MO are represented by   σ * , δ*,  π*  .
  • The MO are filled in the increasing order of their energies starting with orbitals of least energy.     (Aufbau Principle ).
  • MO can accommodate only two electrons and these two electrons must have opposite spin. ( Paul's Exclusion Principle )
  • Every orbital in a sub level is singly occupied before any orbital is doubly occupied. All electrons in singly occupied orbitals have the same spin. ( Hund's Rule ).

Paul's Exclusion Priciple

                    The principle was formulated by Austrian physicist Wolfgang Pauli in 1925. Molecular orbital can accommodate only two electrons must have opposite spin . ( no two electrons can have the identical quantum mechanical state in the same ). For example no two electrons in a single atom can have the same four quantum numbers if n, l, and      ml  ,      ms.

Hunds Rule

       This rule was formulated by German physicist Friedrich Hund around 1927. Every orbital in a sublevel is singly occupied before is doubly occupied . All electrons in singly occupied orbitals have the same spin. ( while filling molecular orbital of equal energy pairing of electrons does not takes place untill all such molecular orbitals are singly filled with electrons having the parallel spins.
Example :

Aufbau Rule

                             The molecular orbitals are filled in the increasing order of their energies starting with orbital of least energy. This principle is used to determine the electronic configuration of an atom, molecule, or ion. In German Aufbau meaning "building up, construction " . This is other wise called as aufbau rule Or building- up principle.
                            These orbitals are filled in the order of n+l rule. n+l rule also known as Madelung rule or klechkowski rule. Orbitals with lower n+l value are filled before those with higher n+l value.

                   n   ===  Principle quantum number.
                   l    ===  Azimuthal quantum number.
the value l=0,1,2,3. corresponds to the s,p,d and f labels respectively.

Example :
2p and 3s orbitals.
 In 2p orbital n=2 and l=2 .
In 3s orbital n=3 and l=1.
( n+l=4 ) in both cases.
But the 2p orbital has the lower energy or lower n value and will get filled before the 3s shell.
                                      The order of the orbital energies from lowest to highest.

  noe  ===  electronic configuration is represented .
n ---------- energy level
o ---------- orbital type ( s, p, d and f )
e ---------- no. of electrons in that orbital shell.

Example :
Oxygen has 8 protons and 8 electrons. First 2 electrons would fill the 1s orbitals. Next 2 would fill the 2s orbitals and the remaining 4 electrons to take spots in the 2p orbitals.
Electronic configuration of oxygen is  1s2 2s2 2p4

Friday, June 7, 2013

G P Thomson's Experiment

                       G P Thomson in 1928 performed experiments with thin foil of gold in place of nickel crystal. Thomson observed, beam of electrons after passing through the thin foil of gold is received on the photographic plate placed perpendicular to the direction of the beam, a diffraction pattern is observed before. This conforms the wave nature of electrons.