BOND LENGTH, RELATIONSHIP BETWEEN BOND ENERGY, BOND LENGTH AND REACTIVITY OF MOLECULES, METALLIC BONDING, Model of metallic lattice, PHYSICAL PROPERTIES OF METALS

 BOND LENGTH:

             The distance between the two nuclei which are covalently bonded in a molecular orbital is called bond length. Shorter covalent bonds are morestronger because in shorter bonds atomic orbitals are more closely lying and have better overlapping which makes shorter covalent bond more stronger.

 

                        Bond                           Bond Energy J.J mole-1                    Bond length nm

                        C – C                                        347                                                     0.154

                        C = C                                       612                                                     0.134

                        C C                                        805                                                     0.120

 RELATIONSHIP BETWEEN BOND ENERGY, BOND LENGTH AND REACTIVITY OF MOLECULES:

             The strength of a covalent bond depends upon the number of shared pair of electrons between covalently bonded atoms. Hence more number of shared pair of electrons, more overlapping and covalent bond is shorter and stronger but the reactivity depends on the availability of electrons. In sigma (  covalent bond electrons are hell strongly between the nuclei of two bonded atoms whereas in pi (  covalent bond electrons are available above and below the joining nuclei. Hence double and triple covalent bonds are more reactive than single covalent bond. In comparison of single covalent bonds, the strength of the covalent bond decreases as inter nuclear distance increases.

            Hence, with the increase in bond length single covalent bond become weak and it could easily be attacked hence, longer and weaker covalent bonds are more reactive.

 METALLIC BONDING:

             The electrostatic forces of attraction between the metal cations and the delocalized free electrons are called metallic bonding

 Model of metallic lattice:

In a metallic lattice the metal atoms lose their outer shell electrons and changed into metal cations. The lost electrons more freely through the lattice. The electrons are said to be delocalized or ‘sea’ of mobile free electrons. The metal lattice is held together by the strong attractive forces between the mobile free electrons and the positive ions (cations). Metallic bonding decreases with the increasing ionic radius. This is why melting points decreases down the groups for metals. On the other hand metallic bonding become more strong with the increase in the charge density of the cation. This is the reason that melting points of metals increases along the period i.e; from left to right e.g; Sodium magnesium aluminum melting point (K) 371, 922, 933.

 

            AS well as number of electrons contributed by each atom to the sea of free electron cloud increases from sodium to aluminum i.e; one electron in sodium to aluminum i.e;  one electron in sodium and three electrons in aluminum. Thus, the metallic bond is stronger in aluminum and weaker in sodium. In metal atoms the valence electrons are not held strongly by the nuclei. Thus it is relatively easy to remove, resulting in the formation of metal cations and delocalized sea of free electrons. Due to stronger electrostatic forces between metal cations and sea of free electrons metallic bonding exist in metallic lattice.

 

PHYSICAL PROPERTIES OF METALS:

             Physical properties of metals can be explained in terms of metallic bonding.

 

(1)       Metals are good conductor of electricity: because in metallic lattice sea of mobile free electrons are available which can move throughout the lattice.

 

(2)       Metals are good conductor of heat: Conduction of heat occurs by vibration of the positive ions as well as through the free mobile electrons.

 

(3)       Metals are both ductile and malleable: because the bonding in the metallic lattice is not broken when they are physical deformed. The metal ions slide over each other to attain new lattice position.

 

(1)              Metals are both haroler and high tensile strength transition metals are even more harder and stronger than other metals: s Hardness and high tensile strength are also due to the strong attractive forces between the metal ions and the mobile free electrons in the lattice. As Transition metals contribute more electrons per atom in the sea of free electrons that is why they have higher melting points and are more harder and stronger.

 

(2)              In appearance metals areusually shiny:  Because cut end of a metal have uniformly arranged positive and negative charge and due to proper reflection of light they looks shiny.

 

(3)              Mixture of metals is used as analloy: e.g; Bronze – an alloy of tin and copper.  Solid an alloy of lead and tin.