Lewis symbol and rule of Lewis dot structure -why does atom combine with other atom?

 Chemical bond- 

·       A force that acts between two or more atoms to hold them together as a stable molecule. 

·       This process accompanied by decrease in energy.  

·       Decrease in potential energy (P.E.) is proportional to Strength of the bond.  

·       Therefore, molecules are more stable than atoms. 

Cause of Chemical Combination  

(A) Tendency to acquire minimum energy  

(a) When two atoms approach to each other. Nucleus of one atom attracts the electron of another atom.  

(b) If net result is attraction, the total energy of the molecule decreases and a chemical bond form. 

 (d) So, Attraction is inversely proportional 1/energy and proportional to Stability.  

(e) Bond formation is an exothermic process during formation of a molecule by combining of molecules 

(B) Tendency to acquire noble gas configuration:  

(a) Atom combines to acquire noble gas configuration.  

(b) Only outermost electrons or valence electrons i.e. ns, np and (n-1) d shells electrons participate in bond formation or chemical reaction. 

(c) Inert gas elements do not participate in bond formation, since they have stable electronic configuration as 1s² or ns²np⁶) 

Lewis Symbols  

A Lewis symbol is a convenient way to represent the valence electrons, which are shown as dots placed on the sides, top, or bottom of the symbol for the element.  

Na

Lewis Structures  

·       The Lewis structure is a representation of a molecule that shows how the valence  electrons are arranged among the atoms in the molecule. 

·       These representations are named after G. N. Lewis, who conceived the idea while lecturing to a class of general chemistry students in 1902. 

·       Lewis structures are based on achieving a noble gas electron configuration by atoms 

·       Writing Lewis structures is a trial-and-error process. 

·       The following steps are followed in constructing dot formulae for molecules and polyatomic ions:  

      (i) Write a symmetrical 'skeleton' for the molecules and poly atomic ions. 

                   a. The least electronegative element is usually taken as the central                                                                   element except Hydrogen. 

                   b. Oxygen atoms do not bond to each other except in O₂, 0 ₃, peroxide and superoxides. 

    c.       Hydrogen actually bonds to an oxygen atom and not to the central atom in ternary acids (oxyacids) ex-Nitrous acid HN02, has the skeleton H-O-N=O 

(ii) Calculate the number of electrons available in the valency shell of all the atoms (sum of the valence electrons for a molecule.) this is expressed by A  

         For negatively charged ions add to the total number of electrons equal to the charge on the anion and for positively charged ions, subtract the number of electrons equal to the charge on the cations 

Example – 

1). Sum of valence electrons for PO₄^3-  = 1 x 5 (for P atom) + 4 x 6 (for 0 atoms) + 3 (for charge)   

                      = 5 + 24 + 3  

                      = 32 electrons.

  

  2). Sum of valence electrons for NH₄⁺ ion  = 1 x 5 (for nitrogen atom) + 4 x 1 (for H                                      atoms) 1 (for positive charge)   

                                                                      = 5+4-1 

                                                                      =  8 electrons. 

  iii). Calculate the total number of electrons needed by all atoms to achieve noble gas                                       configuration.     

                                      This number is represented by N. 

                                         For example: N for  H2S04 

                                          N =2x2+8xl+8x4 =4+8+32 = 44 electrons. 

  iv).  Calculate the total number of electrons shared. This is represented by S which is equal to

                 N - A. For example: S for H2SO4 .

                                                S = N-A = 44- 32 =12 electrons. 

   v) Place the shared pair of electrons into the skeleton, using double and triple bonds only when                   necessary. 

  vi) Place the additional unshared (lone) pairs of electrons to fill the octet to every atom except                        hydrogen which can have only 2 electrons as the total comes equal to A