Engineeringdev

BOND ENERGY Definition : ·       The energy required to break a bond between the two bonded atoms is called bond energy. Or ·       The energy required to convert one mole of a diatomic molecular substance in the gaseous state into its free atoms is known as bond energy.  ·       As the bond energy increases, the stability of the bond also increases. ·       sigma - bonds are stronger than pi bonds ·       Number of bonds between atoms increases the overall bond strength increases -C-C- < -C=C- < -CC- BOND LENGTH AND ENGERY BOND LENGTH Definition: The distance between the centres of the nuclei of two covalently bonded atoms is known as bond length. ·       Bond lengths are measured in angstroms or nanometers.   METALLIC BOND ·       Properties of Metals: 1.       Metals are good electrical and thermal conductors 2.       They are opaque and have high refracting power 3.       They have high melting and boiling points 4.       They crystallise with high coordination number

Defects of Lewis electron-dot model : It can not explain a) Stability of molecules which do not obey octet rule b) Shape of molecules c) Bond characters  Valence bond theory (VBT)  1)It (VBT) explains the directional nature and strength of covalent bond. 2)Covalent bond is formed by the overlapping of half filled orbitals of two bonding atoms. 3)The electrons involved in the bond formation must have opposite spin.  4)Strength of covalent bond is directly proportional to the extent of overlapping of orbitals. 5)Covalent bond is formed in the direction in which the bonding orbital is concentrated to a maximum extent. TYPES OF COVALENT BOND ·       Overlapping of orbitals are of two types. a) Sigma bond   ·       A bond formed by the axial or end-on over lapping ·       Electron density of -bond is concentrated along the inter nuclear axis b) Pi-Bond ·       A bond formed by the lateral or side wise overlapping ·       Electron density of pi bond is concentrated above and below the inte

Covalent Bonding  Definition : A chemical bond formed by mutual sharing of electrons between two atoms is called covalent bond Covalent bond formed between two atoms: ·       If the electronegativity value difference is less than 1.7 ·       Each atom contributes electrons equally for sharing. ·       The sharing takes in pairs ·       The shared electron pair belongs to both the bonded atoms. The sharing can involve in 1.     One pair of electrons, Single bond (-) formed 2.     Two pairs of electrons, double bond (=) 3.     Three pairs of electrons, Triple bond (≡)  Lewis dot model or Electron dot model : ·       Electrons present in the outer shell only are used for bonding ·       The sharing of outer electrons results the covalent bond ·       The bonded atoms get octet configuration to get stability ·       Each valence electron of bonded atom is regarded as a point charge indicated as ‘dot’ Example 1: Hydrogen molecule (H2) ·       Hydrogen atom has a single electron in 1st orbi

CHEMICAL BONDING  Definition : The attractive force present between the atoms that holds the atoms together as a molecule is known as “ Chemical Bond” ·       1) Why do atoms combine to form a molecule? ·       2) Why do atoms combine in a fixed ratio? ·       3) How can the properties of compounds be understood in term of attractive forces? CAUSE OF CHEMICAL COMBINATION ·       Tendency to acquire inert gas electronic configuration (ns2np6). Atoms of elements acquire eight electrons in their valence shell by involving in chemical combination is known as “ Octet rule” ·       Tendency to acquire minimum energy 1) The atoms are unstable and possess high energy 2) During bond formation energy is released. This energy is known as bond energy. 3) Bond formation between atoms leads to stability with lesser energy. OCTET RULE ·       Noble gas elements, except He(1s2), contain 8 electrons in the valence shell. The stability of these elements is due to its octet configuration. ·       Ot