CHAPTER NO 7

                                            CHEMICAL BOND                                                       

CHEMICAL BOND
Chemical bond can be defined as :    " the force of attraction between two atoms or ions that hold them together in a unit is called    Chemical Bond".    Actually chemical bond is the main factor that makes molecules and compounds. By the interaction of    outer electrons, great forces of attraction are developed between two atoms. This force of attraction is    called chemical bond.
TYPES OF CHEMICAL BOND
There are three types of chemical bonds:    Ionic bond or Electrovalent bond   Covalent bond   Coordinate Covalent bond
IONIC BOND OR ELECTROVALENT BOND
Chemical bond formed between two atoms due to transfer of electron(s) from one atom to the other    atom is called "Ionic bond" or "electrovalent bond".
EXPLANATION
In ionic bond formation one atom looses electron(s) and the other picks it up.The atom that looses the    electron acquires positive charge and the other atom which gains the electron becomes a negatively    charged particle. Due to opposite charge an electrostatic force of attraction is setup between them. This   force hold these atoms together in a unit. This force of attraction is referred to as "CHEMICAL BOND".
FORMATION OF IONIC BOND BETWEEN Na & Cl
Electronic Configuration of Na is K = 2 , L = 8, M = 1
Due to low ionization potential value, Na-atom loses its valance electron
Electronic configuration after losing an electron is K = 2, L = 8 , M = 0
Na è Na+ + e-
Due to loss of one electron Na becomes Na+ -ion.
Electronic configuration of Cl is K = 2, L = 8, M = 7
Electron lost by Na atom picks up by Cl atom.
Electronic configuration after acquiring an electron: K = 2, L = 8, M = 8
Cl + e- è Cl-
Chlorine atom is now converted into Cl- ion
We know that positive and negative ions attract each other, therefore an electrostatic force of    attraction is set up between Na+ & Cl- ions. This force unites these ions in a unit. In this way ionic    bond is formed between Na and Cl atoms which results the formation of sodium chloride. STATE     Due to strong forces of attraction, all the ionic compounds are exist in solid state. THERMAL STABILITY     Ionic compounds have high melting points and boiling points. CONDUCTANCE     They are strong electrolytes.In solid state they do not conduct electricity.But in molten state and in   aqueous solution they conduct electricity. SOLUBILITY     Generally ionic compounds are soluble in water and in many polar solvents.   Ionic compounds are insoluble in the organic compounds. HARDNESS     They are very hard. REACTIVITY     Generally ionic compounds are very reactive .    NON-MOLECULAR FORM     Ionic compounds do not exist in the forms of molecules. Their formula only indicates the number of atoms   present in the compound COVALENT BOND Chemical bond formed between two atoms by the mutual sharing of electrons is known as Covalent bond.   In covalent bond formation each atom provides equal number of electrons for sharing but no transfer of   electrons takes place. Each electron pair is attracted by both the nucleus. REPRESENTATION     Covalent bond between two atoms is represented by a short line () COVALENT BOND BETWEEN H-ATOM AND Cl-ATOM      Consider the example of HCl molecule.    H atom has only one electron in its valance shell.    It requires one electron to complete its duplet or achieve inert gas configuration.    Electronic configuration of Cl is K = 2 , L = 8 and M = 7.    Cl atom requires 1 electron to complete its valence shell or octate.    Due to small difference between the values of E.N electron transfer from H or Cl is not possible.    H and Cl share one electron.    In this way without any transfer of electron both H & Cl complete their outer shell and a stable HCl        molecule is formed.                                            TYPES OF COVALENT BOND                                         There are three types of covalent bond depending upon the number of shared electron pairs.   SINGLE COVALENT BOND  DOUBLE COVALENT BOND  TRIPLE COVALENT BOND SINGLE COVALENT BOND     A covalent bond formed by the mutual sharing of one electron pair between two atoms is called a "Single   Covalent bond."   It is denoted by single short line()   Examples:   In single bond formation each atom provide one electron. DOUBLE COVALENT BOND     A covalent bond formed between two atoms by the mutual sharing of two electron pairs is called a "double   covalent bond". It is denoted by double short line ()   Examples: TRIPLE COVALENT BOND     A covalent bond formed by the mutual sharing of three electron pairs is called a "Triple covalent bond". It is   denoted by triple short line ( ).   Examples: POLAR COVALENT BOND     A covalent bond formed between two different atoms is known as Polar covalent bond.   For example when a Covalent bond is formed between H and Cl , it is polar in nature because Cl is more   electronegative than H atom . Therefore, electron cloud is shifted towards Cl atom. Due to this reason a   partial -ve charge appeared on Cl atom and an equal +ve charge on H atom   Examples: NON-POLAR BOND     A covalent bond formed between two like atoms is known as Non-polar bond. Since difference of   electro negativity is zero therefore, both atoms attract electron pair equally and no charge appears on   any atom and the whole molecule becomes neutral.   Examples: H - H Cl - Cl F - F                       PROPERTIES OF COVALENT COMPOUNDS                   PROPERTIES OF COVALENT COMPOUNDS Covalent compounds are in the form of stable molecules .  Covalent molecules are held together by weak inter molecular force of attraction.  Covalent compounds are either gases or liquids. Some covalent molecules are solids.  NH3 , CO2(gas)  H2O ,CCl4(liquid)  Iodine (solid)  They are volatile.  They have low melting points and boiling points.  They are soluble in the organic compounds.  Non-polar covalent compounds do not conduct electricity.  Polar covalent compounds conduct small amount of electricity. COORDINATE COVALENT BOND   The type of chemical bond in which one atom provides shared pair of electron for bond formation is called "Coordinate Covalent Bond".In the formation of coordinate bond other atom does not provide electron for sharing. It is one sided sharing. Formation of coordinate covalent bond is the property of atoms that have lone pair of electrons. The atom that provides electron pair is called "Donor".The other which takes it is called "Acceptor". Symbol:is represented by an arrow (è ), pointing from donor atom to the acceptor. Examples: Ammonium ion , Hydronium ion COORDINATION COMPOUND   Complex compounds in which a fixed number of molecules or ions or ligands are bounded to a transition metal through coordinate covalent bond are known as "Coordination compounds." Examples: K4[Fe(CN)6] , [Cu(NH3)4(OH)2] COORDINATION NUMBER   Number of ligands attached to the control transition metal in a coordination compound is called coordination number.