Structure of Atom Class 11 Chemistry Notes

Structure of Atom Class 11 Chemistry Notes Chapter 2

Atomic  Theory of matter :-

                                            According to this theory, atom is the ultimate particle of matter. This atomic theory of matter is also known as Dalton’s  Atomic theory (1808).

Structure of Atom Class 11

Dear learner, if your basics are not clear about structure of atome then please read our post on basics under title Structure of atom class 9 notes. Otherwise continue reading these Structure of atom class 11 notes. If you want to learn in more details, then read our post on Structure of atom BSc chemistry notes

Cathode ray discharge tube experiments:
  • Cathode rays start from cathode and move toward anode.
  • These rays are not visible but there behavior can be observed with fluorescent or phosphorescent material.
  • In the absence of magnetic or electric field these rays travels in straight lines. 
  • In the presence of magnetic or electric field, the behavior of cathode rays is similar to Negatively  charged  particles. Which suggest that these rays contain negatively charge particles called electron.
  • Cathode rays (or electrons) do not depend on the material of the electrode and nature of the gas in the tube so electrons are basic constituent of all atoms.

Charge (e) to mass (me) ratio of electron 

  • Measured by  J. J. Thomson (1897).
  • By using cathode ray tube.
  • By applying electrical & magnetic field perpendicular to each other & also perpendicular to path of electrons.
  • He proposed  deviation of particles from their path in presence of magnetic or electrical field depend upon the following-

1.     Magnitude of  – ve  charge on particle
i.e. if magnitude of charge on particles is greater than interaction with magnetic or electric field is greater so deflection is also grater.
2.     Mass of particles
i.e. particle is lighter then deflection is greater.
                 3.  Strength of magnetic or electric field.
i.e. if strength of magnetic field or voltage, at electron is increases then deflection of e-  also increases.
        =>  value of e/me = 1.758820× 1011 C kg-1

Charge of electron

Determine by  R. A. Millikan
By oil drop experiment (1906-1914)
Charge on e- =  -1.6× 10-19 C
Present accepted value , e- = -1.6022× 10-19 C

Mass of electron

 From charge on e- & e/me
 We get,                 
             Me = 9.1094 ´ 10-31 kg

Discovery of protons

Discovered by E. Goldstein.
In modified cathode ray tube gives +ve charge carrying particles known as canal rays.
Lithest & smallest +ve ion obtained from Hydrogen called proton.


1)    Depend upon, nature of gas present in cathode ray tube.
2)    Charge to mass ratio of particles depends on gas from which these originate.
3)    Some of +ve charged particles carry a multiple unit of electrical charge.
4)    Behavior of protons in magnetic or electric field is opposite to that of electron behavior.

Discovery of neutrons

  Discovered by Chadwick (1932).
  By bombarding a thin sheet of beryllium by alpha particles.
  Electrically neutral particles were emitted known as neutrons.

Thomson model of atom

  Given by J.J Thomson (1898)
  According to J.J. Thomson, atoms posses a spherical shape, 
with radius about 10-10 m, in which + ve charge is uniformly distributed.
  Electrons are embedded in such a manner to give most stable electrostatic arrangement.
  Other names of this model plum pudding, raisin pudding, watermelon model.
  Mass is assumed to be uniformly distributed in atom.

Rutherford’s nuclear model of atom

Given by Rutherford & his students Ernest Marsden and Hans Geiger.
By α- particles scattering experiment-
Rutherford nuclear model of atom

Rutherford’s nuclear model of atom

When beam of high energy α- particles was directed at gold foil, then tiny flash of light observed at photographic plate.

          Rutherford observed that-

  • 1)    Most of the α-  practical  passed  through gold foil un-deflected.
  • 2)    A small fraction of α- particles was deflected by small angles.
  • 3)    A very few α- particles (about 1 in 20,000) bounced back means deflected by nearly 1800

                  From above observations he concludes the structure of atom.

1)    Most of space in atom is empty because most of α- particles passed un-deflected.
2)    Few +ve charged α- particles were deflected.
Because + ve charge of the atom present in center in very small volume that repelled & deflected the +ve charged α- particles.
3)    Volume of nucleus is negligible as compared to total volume of atom
i.e.  radius of atom = 10-10m  (approx)
radius of nucleus =  10-15m  (approx)

       On the basis of observation & conclusion Rutherford proposed model of atom as-

1)    +ve charge & most of mass present in the center of atom known as nucleus.
2)    Electrons moves around nucleus with very high speed in circular paths known as orbits.
3)    Electrons and nucleus (protons) are held together by electrostatic force of attraction .

Atomic number (Z) = no of protons in the nucleus of an atom
                                       = no of electrons in a neutral atom
Mass number (A) = number of protons (z) + number of Neutron (n)

Isobars :- 

              These are atoms with same mass number but different atomic number.

Isotopes: - 
               These are atoms with same atomic number but different atomic mass no.

Wave nature of electromagnetic radiations: -

First explanation given by James Maxwell (1870).
1)    Oscillating magnetic & electric fields produced by the oscillating charged particles are perpendicular to each other and also perpendicular to the wave direction of propagation.
2)    These waves do not require medium i.e. electromagnetic wave can travel in vacuum.
3)    Electromagnetic radiation differs from one another in frequency or wavelength gives electromagnetic spectrum.
4)    Different units are used to represent electromagnetic radiation.

                                 n = frequency, 
                                l = wavelength.

Particle nature of electromagnetic radiation :-   

                                                                        Also known as Planck’s  Quantum theory

Planck suggested that the atoms and molecules can absorb or emit energy in discrete quantities not in continuous manner. Planck gives it name as quantum. Energy (E) of  quantum of  radiation is directly proportional to its frequency(n)
 i.e.            E=hn
Where,      h = planks constant = 6.626× 10-34 js

Photo electric effect:-

given by H. Hertz (1887)
When a beam of light strike a metal surface then electrons were ejected. This phenomena is known as photo electric effect.
1.     Electrons ejected from metal surface, when beam of  light strike the metal surface.
2.     Number of electron ejected is directly proportional to intensity (or brightness) of light.
3.     There is characteristic minimum frequency (n0 threshold frequency) below which photoelectric effect is not observed.
4.     If n > n0 then electrons comes out with kinetic energy which increases with increase in frequency of light.

Kinetic energy of ejected electrons is given by-
h n = h n0+ ½(meV2)


                      study of  absorption or emission spectra is called spectroscopy .

Bohr’s model for hydrogen atom:-

Explain by Nails Bohr (1913).
Postulates for Bohr’s modal are,
    1.     Electron in hydrogen atom move around nucleus in circular path of fixed radius and energy. these paths are called orbits.
    2.     Energy of electron does not change with time.
However, when electron move from lower to higher stationary state. It absorbed some amount of energy and energy release when it comes back.
    3.     Frequency of radiations emitted or absorbed when transition of electron occur is given by
bohr model
Where, E1 & E2 is lower & higher energy state.
     4.     Angular momentum of n electron in given stationary state is given by-
bohr model
[Where n =1,2,3.....]

Limitation of Bohr’s model:-

1.     Bohr model fail to explain fine detail of hydrogen atom spectrum observed by spectroscopic, techniques.
2.     It fails to explain spectrum of other atom except hydrogen atom.
3.     It fails to explain splitting of the spectral lines in presence of electric field (stark effect) or magnetic field ( Zeeman effect ).
4.     Fail to explain formation of molecules from atoms by chemical bonding.

Dual behaviour of matter :-

Explain by de-Broglie (1924)
He explain that matter also behave like radiation and exhibit dual behavior means both like particle and wave like properties .
dual behavior of matter
   Where  l =  wavelength.
             m = mass of particle ,
             v = velocity of particle,
              p = momentum

Heisenberg’s uncertainty principle:-

 Given by Werner Heisenberg (1927)
He explain that it is impossible to determine simultaneously the exact position and exact momentum (or velocity) of an electron.
Mathematical explanation
Heisenberg uncertainty principle
     Where,  Dx= uncertainty  in position
                   DVx  = uncertainty in velocity or momentum

Quantum mechanical model of atom:-

  • Branches of science which explain duel behavior of Metter is called quantum mechanics. 
  • Quantum mechanics independently developed by Werner Heisenberg and Erwin Schrodinger (1926).
  • Fundamental equation developed by Schrodinger. (He won Nobel Prize in 1933).
  • Equation for a system (atom or molecules was, energy does not change with time)

Principle quantum number ‘n’ :-

·        It is a positive Integer with value of n = 1,2,3......
·        It determine size and energy of orbital.
·        It also identifies the shell with increase in number of allowed orbital. 
And given by n2
N      =1,    2,    3,    4........
Shell = k,    l,    m,    l......
·        Size of orbital increase with increase in n.

             Azimuthal quantum no.‘l’  :-

·        It is also known as orbital angular momentum or subsidiary quantum no.
·        It defined 3D shape of orbital.
·        For given value of n possible value of  
        L= 0,1,2,3,4,5,----------(n-1) ,
                    Ex :- if   n=1   then   l=0
                             if   n=2   then   l=0,1
                             if   n=5   then   l=0,2,3,4

·        Each shell consists of one or more sub-shells.
·        No of sub-shells = value of  n
If  n= 1  then  1 sub-shell =  (l=0)
If  n= 2  then  2 sub-shell =  (l=0,1)
If  n= 3  then  3 sub-shell =  (l=0,1,2)
·                      Value of    l =  0,  1,  2,  3,  4,  5  ----------
     Notation for sub-shell= s, p, d,   f,   g, h--------------
·        Sub-shell notation

Sub-shell   notation

 Magnetic orbital quantum no ‘mi’ :-   

·        This quantum no (mi) gives information about orientation of  the  orbital .
·        mi = (2l+1) i.e. if value of   l  is  1 then value of  mi = 2×1+1=3=(-1,0,1)

Value of p
Sub-shell notation
No of orbital’s

Electron spin quantum (ms)  :- 

·        Proposed by G. Uhlen Beck & S. Goodsmit (1925).
·        Electrons spins around its own axis.
·        Ms have two value +1/2 & -1/2.
·        Ms gives information about orientation of the spin of the electron.

Aufbau principle   :- 

                              According to this principle, in the ground state of the atoms the orbital’s are filled in order of their increasing energies. Means electrons enter higher energy orbital’s,  so order in which orbital’s are filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

Pauli exclusion principle  :-

·         Given by W. Pauli (1926).
·        Two electrons in an atoms can’t have same set of 4-quantum no.
·        Only two electrons may exist in same orbital and these electrons must have opposite spin.

Hund’s  rule of maximum multiplicity :-.

·        According to this rule, pairing of  electron in the orbital’s belonging to the same sub-shell (p, d or f) does not take place until each orbital belonging to that sub-shell has got one electron each i.e. it is singly occupied.


These Chemistry notes are published by Chemistry Notes Info website 

Structure of Atom Class 11 MCQ

1. Who discovered anode rays :

 J. Stanley
 J. J. Thomson

2. Neutron was discovered by :


3. Radioactive isotope of hydrogen has ________ number of neutrons :


4. Cathode rays are deflected by :

 A magnetic field only
 An electric field only
 By Both
 By None


Buy Structure of Atom Books


  1. Chemistry Apps
    Chemistry Android Apps for 9th, 10th, 11th, 12th, BSc, MSc and Spectroscopy

Previous Post Next Post