ATOM :







The smallest particle possible in the  substance or element which does not exit in free state , but take part in chemical reactions and all characteries of the substance or element are present in it is called the atom.
              The world atom in greek means indivisible . The founder of atomic theory John Dalton postulated his theory as.

                                           


  • Matter is composed of atoms which cannot be subdivide.

  •  Atom is the smallest particle of matter that takes part in a chemical reaction,

  • Atoms of the same element have similar properties like mass, volume and chemical reactivity.
  • Atoms combine in definite proportion to from molecules.


Main Fundamental particles of the atom

  • Electron :- This fundamental particle was invented by J.J. Thomsam in 1897 and it is a negatively charged particle which rotates in various orbits around the nucleus. The mass of an electron = 9.1 *10 -31  kg  and the charge of an electron =  - 1.6 * 10 -19 coulomb . 

  • Proton : - This fundamental particle was invented by Golds ten in 1919 and it is a positively charged stable particle . The mass of a proton = 1.6 * 10 -27 kg and the charge of a proton = 1.6 * 10-19 coulomb. 

  • Neutron :- This fundamental particle was invented by Chadwick in 1932 which is a neutral unstable particle . The masses of a proton and neutron are nearly equal. mass of a neutron = 1.6 * 10 -22 kg .


  • Mass Number :- The sum of the neutrons and protons is known as mass number . it is denoted by a . 

                                Mass Number  =  Number of neutron + number of proton 
                                       A =  N + P

Atomic Number : 

 
The number of unit positive charge carried by the nucleus of an atom is termed atomic number. 
It is denoted by symbol   Z

       
                        Atomic Number Z = Number of unit positive charge the nucleus = Number of proton is the uncles P = Number of electrons revolving in orbits E 

                                Z = P = E 



Atomic number is written on the lower side of the symbol and the mass number is written on the upper side 



where 
        A=Mass Number 
Z = Atomic Number
X = symbol of the element 

   ISOTOPES :-

      Atoms of the same element having same  atomic number but different mass Number are called isotopes .

For example : - There are three  isotopes of  hydrogen having mass number 1 . 2. 3 respectively 
and each of them having atomic  number equal to 1 . they are represented as  1H , 2 H , 3 H 
and named protium , deuterium ,tritium .
  • Isotopes  of chlorine are written as  17 cl  and 17  cl 

  • Isobars :-Such atoms of different elements charging different atomic number but same mass numbers are called isobars . They have different properties . 

                              for example :  18 Ar 40         19  k  40        20  ca 40 

  • Isotones :- Such atoms of different elementary which contain the  same number of neutrons are called isotones . They have different properties .

                              for example :   6 c 14      7 n 15           8 o 16 

  • Isoelectronic :-  The species atom or ions containing the same number  of electrons are called  isoelectronic . 

                              for example :   O -2  ,  F -    , Na + ,  Mg  +2  , Al +3 

Bohrs Model of an Atom ;


Neil's  Bohrs  1913 combined some postulates of classical physics and some postulates of the planks quantum theory . 

The important postulates of Bohr's models of an atoms

  • Electrons are continuously revolving around the nucleus in a certain circular path know  as orbit shells / energy levels. These orbit are stationary these shells are denoted as 1,2,3,4,......Electrons move in these orbits of fired energy called stationary state.

  • The coulombic force of attraction between the nucleus and the electron holds the atom together .

  • As long as electron remains in a particular orbit, it does not lose or gain energy , it means if energy remains constant . so these orbits are stationary state . 

  • Only those orbits are permitted in which the angular momentum of an electron is an integral multiple of  h/ 2 . Where is the plank's constant this is known as principle of quantization of energy according to which . 


  • The energy is absorbed or emitted only when the electrons jump from one energy level to another . this is given by the expression.


successes of Bohrs model




  • Bohrs model could explain the stability of an atom :-   According to Borhs model an electron revolving in a particular orbit cannot lose energy . The electron can  lose energy  only if  it  jumps to some lower energy level . if no lower energy level  is vacant then electron will  keep in reveling  in the same orbit without losing  energy . 

  • Borhs  theory helped in calculating  energy  of an electron in a particular  orbit of hydrogen :-  on the basis of various postulates of Bohrs model  . it is  possible to derive a mathematical relation for energy of an electron  in nth orbit  of hydrogen . 

Borhs model could  explain the atomic  spectrum of  hydrogen :-  if  E1 & E2 are  the energies of lower and higher energy levels  respectively , them frequency of the radiation emitted is given by the following relation. 

Quantum Number :-


A set of four numbers used to specify energy , size, shape , orientation of the electron are called quantum number. 

Quantum Numbers are of four types

  • Principal Quantum Number  N 
  • Azimuthal Quantum Number L
  • Magnetic Quantum Number  M 
  • Spin Quantum Number  S 

Principle Quantum Number:- 

It is denoted by N , It have whole number values starting from 1 to 23......... , The value of n denotes the shell in which an electron is present. the shell with n=1 is called first shell and is denoted by K , Similarly , We have other shell like L,M, N, etc..
N also determines the distance of the electron in a shell when n=1 , electron is closed to the nucleus as the value of n increase , electron gets father away from the nucleus and thus size increase . 
N also determines the energy of the electron in given orbit. 
N also given maximum number of electron that a shell can accommodate . A shell with principle Quantum number N can contain 2n2 electron. 
maximum number of electron in k shell 2 .
maximum number of electron in L shell =8. 
maximum number of electron in M shell 18


Azimuthal Quantum Number :-

It is denoted by L
It is related to sub shell s , p , d & f .
I can have all positive whole number values from zero to etc...
L value enable us to  calculate the total number of subs shell a main shell has given below .


shell             principal  q n         values of L   subshell         total number of sub shell

K               1                              0                       S                    1 

L                2                             0 ,1                  S , P                  2

M                3                            0, 1 , 2             S , P , D             3

N               4                            0 , 1 , 2 , 3         S , P , D , F        4 



Thus this provides information regarding the shape of orbital .


Magnetic  Quantum Number :-

It is denoted by  M.
It given idea about the orientation of electron under the influence of an applied magnetic field. 


The value of  L determines  the allowed of M m can have values -L  to +L  though O. In other words given a total of 2l +1 possible values  .


Spin Quantum number :-

It is denoted by S .
It can only two values +1/2 ,  -1/2 . Instead of giving numbers for  s= 
+1/2 ,  -1/2 usually  designated arrow pointing ^.