Wednesday, January 30, 2013


              Hapticity is a term describing how a group of contiguous atoms are bonded to a central atom of a coordination compound. Each atom of the contiguous group is bound only once to the central atom. The Hapticity of a ligand is indicated by the Greak character ' eta ' . 'eta' denotes the number of contiguous atoms of the ligand that are bound to the metal.
Example :
             Ferrocene is a molecule with two cyclopentadienyl rings bound to a central iron m olecule. Five contiguous carbon atoms are bound to the central iron atom, the hapticity is five.

Pi Back Bonding

           Pi back bonding also called Pi back donation. In this bond electrons move from an atomic orbital on one atom to Pi * antibonding orbital on another atom or ligand. Mostly common in organo-metallic chemistry of transition metals with multi-atomic ligand such as carbon monoxide , ethylene or the nitrosonium cations. Electrons from the metals are used to bond to the ligand, in the process relieving the excess negative charge.

Dipolar Bond

         A dipolar bond is covalent bond between the two atoms where one of the atom provides both electrons that form the bond. A dipolar bond also known as dative covalent bond or coordinate bond is a kind of 2c - 2e bond.
        When Lewis base donates a pairs of electrons to a Lewis acid that time dipolar bonds are formed. This description of bonding is a characteristic of valence bond theory.It has no place in molecular orbital theory or ligand field theory. 

Tuesday, January 29, 2013

Bond Order

          Bond order is the number of chemical bonds between a pairs of atom. In a covalent bond between two atoms, an single bond has a bond order of one, a double bond has a bond order of two, a triple bond has a bond order of  3 and so on.
Example :

  • Determine the bond order of hydrogen gas ; H2
  • H - H   Bond order is one.

VSEPR Theory ( Valance Shell Electron Pair Repulsion )

  1. The electron pair in the valence shell around the central atom of a molecule repel each other and tend to orient in space so as to minimize the repulsion and maximize the distance between them.
  2. There are two types of valence shell electron pairs .
  • Bond pairs
  • Lone pairs
Bond pairs are shared 2 atoms and are attracted by 2 nuclei. Hence they occupy less space and cause less repulsion.
Lone pairs are not involved in bond formation and are in attraction with only one nucleus. hence they occupy more space. As a result the lone pair causes more repulsion.
The order of repulsion between the different types of electron pairs is as follows:
Lone pairs - Lone pairs > Lone pair - Bond pair > Bond pair - Bond pair.

Note :
     Bond pairs are usually represented by a solid line, where as the lone pairs are represented by a lobe with 2 electrons.

  1. In VSEPR theory the multiple bonds are treated as if they where single bond. The electron pairs in multiple bonds are treated collectively as a super pair. The repulsion caused by bonds increases with increase in the number of bonded pairs between two atoms. ie : The triple bond causes more repulsion than the double bond which turn causes more repulsion than single bond. 
  2. The shape of molecules can be predicted from the number and types of valence shell electron pairs around the central atom. When the valence shells of central atom contains only bond pairs. The molecules assumes symmetrical geometry due to even repulsion between them. However the symmetry disorted when there are also lone pairs along with bond pairs due to uneven repulsion forces.
  3. Primary and secondary effects on bond angle and shape :
  •  The bond angle decreases due to the presence of lone pairs, which cause more repulsion on the bond pairs and as a result the bond pairs tend to come closer.
  • The repulsion between electron pairs increase with increase in electro negativity of central atom and hence the bond angle is increases. The bond pairs are closer and thus by shortening the distance between them, which turn to increase the repulsion.Hence the bond tends to move away from each other.
The bond pairs tend to move away from each other since the distance between them is shortened as they are more localized on more electronegativity central atom.
However the bond angle decreases when the electronegativity of the ligand atoms are more than that of central atom. There is increase in distance between bond pairs since they are closer to ligand atoms. Due to this they tend to move closer resulting in the decrease in bond angle.
The bond pairs tend to come closer since the distance between them is increases as they are more localized on more electronegative ligand atoms.
The bond angle decrease with increase in the size of central atom.
On smaller central atoms the bond pairs are closer and hence tend to move away from each other so as to minimize repulsion. Hence bond angle will be more.
On bigger atoms, the bond pairs are distant from each other and hence there is less repulsion. They tend to move closer and thus by decrease the bond angle. However the bond angle in the size of the ligand atoms, which surrounded the central atom.
There is less repulsion between smaller ligand atoms and they can move closer to each other and decrease the bond angle.
There is more repulsion between bigger ligand atoms and hence they tend to move away from each other and the bond angle is increases.
The bond angles are also changed when multiple bonds are present.It is due to uneven repulsion.
  1. when there are two or more resonance structures, the VSEPR theory is applicable to any of such contributing structure. 

Periodic Table Trends

Moving Left To Right :

  • Atomic radius decreases.
  • Ionization energy increases.
  • Electronegativity decreases.
Moving Top To Bottom :
  • Atomic radius Increases
  • Ionization energy decreases
  • Electronegativity decreases.


                 Electronegativity is the measure of the attraction of an atom from the electron in a chemical bond. The higher electronegativity of an atom , the greater attraction for bonding electron. Electronegativity is related to ionization energy.

  • Electrons with low ionization energy have low electronegativity . Because their nuclei do not exert a strong attractive force on electrons.
  • Electrons with high ionization energies have high electronegativity due to the strong pill exerted on electrons by the nucleus.
                 In a group the electronegativity decreases as atomic number increases. As a result increase distance between the valence electron and nucleus. ( greater atomic radius )

Example :
  • Electropositive (ie: low electronegativity) element is cesium.
  • High elecgtronegativity element is fluorine.


                 Electron affinity reflects the ability of an atom to accept an electron. It is the energy change that occurs when an electron is added to a gaseous atom. Atoms with stronger effective nuclear charge have greater electron affinity.
  •  Group II a elements the alkaline earth have low electron affinity value. These elements are relatively stable because they have filled  S subshells. 
  • Group VII A elements the halogens have high electron affinity because the addition of an electron to an atom results in a completely filled shell.
  • Group VIII A elements of noble gases have electron affinities near zero. Because each atom posses a stable octet and will not accept an electron readily. Elements of other groups have low electron affinity.

Ionization Energy

                  Ionization energy or ionization potential is the energy required to completely remove an electron from gases atom or ion. The closer and more tightly bound an electron is to the nucleus, the more difficult will be to remove. the higher its ionization energy will be.

  • The first ionization energy is the energy required to remove one electron from the parent atom.
  • Second ionization energy is the energy required to remove second valence electron from the divalent ion and so on.Successive ionization energy increases.
                      Second ionization energy is always greater than the first ionization energy. Ionization energy increases moving left to right across a periods. ( decreases atomic radii ). Ionization energy decreases moving down the group ( increases atomic radii ).

                       Group one elements have low ionization energy because the loss of an electron from a stable octet. 

Monday, January 28, 2013

Atomic Radius

                             Atomic radius of an element is half of the distance between the centers of two atoms of that element that are just touching each other.

  • Atomic radius decreases across a period from left to right and increases down the group.
  • The atoms with the largest atomic radii are located in group I and at the bottom of groups.
                             Moving left to right across periods, electrons are added one at a time to the outer energy shell. Electrons within a shell cannot shield each other from the attraction to protons. Since the number of protons also increases the effective nuclear charge increases across a periods. So the atomic radius increases.
                             Moving down the group in periodic table no. of electrons and filled electrons shells increases, but the number of valence electrons remains the same. The outer most electrons in the group are exposed to the same effective nuclear charge but electrons are found farther from the nucleus as the number of filled energy shell increases. So atomic radii increases.                   

Periodic Properties

                Elements tends to gain or lose valence electrons to achieve stable octet formation. ( Inert gases or noble gases of group VII of the periodic table ).


  • Electron added one at a time moving left to right across a periods. As this happens the electrons of the outermost shell experience increases strong nuclear attraction, so electron becomes closer to the nucleus and more tightly bound to it.
  • Moving down a column in the periodic table the outer most electrons become less tightly bounds to the nucleus. This happens because the number of fitted principle energy level increases downward within each group.
  • These trends explain the periodicity observed in the elemental properties of atomic radius, ionization energy, electron affinity and electronegativity.

Acid Rain Definition

               Acid rain is a form of precipitation usually acidic in nature. That means it contain low pH value. [ liquid with pH less than 7 are acidic . pH grater than 7 are alkaline ]. So the unpolluted rain has acidic pH.     ( low pH ). Acid rain is caused by the emission of sulfur dioxide and nitrogen oxide. These two oxides react with water molecules in the atmosphere to produce acids. [ Sulfuric acid & Nitric acid ].

  • In the presence of water sulfur trioxide is converted to sulfuric acid.
  • Nitrogen dioxide react with OH to form nitric acid.
Sulfur trioxide  and nitrogen dioxides are released from power plants and other sources. Nitrogen oxide naturally produced by lightning strikes. Sulfur dioxide is naturally produced by volcanic eruption. In areas where is wet acid rain can fall to earth in the form of rain, snow, fog, and cloud water. Where the weather is dry it is in the form of gases ( smoke ) and acidifying particles ( dust ).

        Acid rain can damage the environment . Human health and property. Acid rain can causes paint to peel, corrosion of steel ( iron , copper ) structure. Erosion of stone statues ( limestone & marble ).     

Wednesday, January 23, 2013

Absorption Coefficient definition

                   The absorption coefficient is defined as how easily energy or matter ( light, sound, particles ) passes through a medium or material. Absorption coefficient is measured by the units of reciprocal length. Absorption coefficient is also called as attenuation coefficient, linear attenuation coefficient. Absorption coefficient are different for different frequencies. Small absorption coefficient means medium is relatively transparent to the beam. Large absorption coefficient means beam is quickly passes through the medium. See also Beer- Lambert Law.

Bond Angle Definition

The geometry of a molecule depends on the arrangement of the orbital with respect to one another around the central atom, i.e., on the angles which these orbital make with one another in space around the central atom. These angles are known as bond angles.

Hybridisation Definition

The phenomenon of redistribution of energy in different orbital belonging to the outermost energy level of an atom to give new orbital of equivalent energy is called hybridization. The new orbital thus formed are called hybrid orbital. The bond formed by such orbital are called hybrid bond. The compounds which result from these bonds are known as hybrids.

Sigma Bond Definition

A bond is formed by overlapping of the orbital along the same axis, it is known as a sigma bond.

Ionic Compounds Properties

• Ionic solids do not conduct electricity in solid state.
• They conduct electricity in the molten state.
• They have high melting points.
• They have low volatility and high boiling points.
• They are freely soluble in water but only slightly soluble in organic solvents

Cation & Anion Definition

The smaller the atom, the greater will be its tendency to accept extra electron and to change into an anion.

The larger size of an atom, the greater is the ease with the valence electrons can be eliminated from it.

Bond Energy Definition

The energy required to break one mole (i.e., Avogadro’s number) of bonds is called bond energy.

Kelvin Scale Definition ( Absolute scale )

                                            Kelvin scale was developed by Lord kelvin ( William Thomson ) in 1848. Lord Kelvin developed this scale with the help of Carnot engine. Kelvin is a unit of measurement of temperature. It is the one of the seven base units in the International system of units ( SI ). The Kelvin symbol K is the SI unit of temperature. It is defined as the fraction 1/273.16 of the thermodynamic ( absolute ) temperature of the triple point of water.The zero point of this scale is equivalent to Celsius scale. This zero point is considered the lowest possible temperature of anything in the universe. Therefore, the kelvin scale is also known as "absolute scale".

  • Freezing point of water, kelvin scale reads to 273 K.
  • Boiling point of water, it reads to 373 K.

Absolute Zero Definition

                            Absolute zero is the coldest possible temperature. ( absolute zero is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance ) . It is the temperature at which entropy reaches its minimum value. A system at absolute zero still posses quantum mechanical zero point energy, the energy of its ground state. The kinetic energy of the ground state cannot be removed. The zero point of any thermodynamic temperature scale, such as Kelvin or Rankine scale is set as absolute zero.

Absolute zero is defined as,
O K on the kelvin scale
-273.12 degree on the Celsius scale
O  R on the Rankine scale.

               In classical kinetic theory there should be no movement of individual molecules at absolute zero, but experimental evidences shows this is not in the case.