Rutherford’s atomic model

  1. \to Alpha ray

    • Mechanical wave

    • 2 +ve charge

    • 2 protons + 2 neutrons

    \to Radioactive substance (Radium)

    \to Lead cavity

    \to Gold foil ( Thickness = 0.00004cm)

    \to ZnS Screen ( Alpha particle shines in ZnS screen)

    \to Nucleus ( As a Result)

  2. \textbf{Findings}

    i) Outer hollow part – orbital or shell

    ii) Central dense part – Nucleus

    • 1 Fermi = 10^-15 m

  3. \textbf{Defects}

    i) Does not explain continuous spectrum or H- spectrum

    ii) The moving charge particle electron does not loss the energy (Does not follow law of electro dynamics)

Bohr’s atomic model

  1. \textbf{ Postulates}

    \to Stationary State: Energy neither loss nor gain when electron revolves round the nucleus is called stationary state.

    \to Quantization of angular momentum:

    (Angular Momentum) mvr=\dfrac{nh}{2π }

    n=Principle quantum no. = 1,2,3,4... = K,L,M,N…



    \to Formation of spectra: When we give energy externally, it jumps to the upper energy level and when it jumps to lower energy level, it emits energy in the form of spectra.

    E=hf (where h= Plank’s constant and f= frequency)

    Similarly, ∆E=E_2-E_1

  2. \textbf{Hydrogen Spectrum ( Emission spectrum ) }

    n_2 \to n_1 or E_2 \to E_1

    i) Lyman Series:

    • UV rays

    • n_1=1 and n_2=2,3,4….

    ii) Balmer Series:

    • Visible region = observe photochemical Reaction

    = 3500-8000 \dot{A}


    • n_1=2 and n_2=3,4,5….

    iii) Paschen Series:

    • IR

    • n_1=3 and n_2=4,5,6….

    iv) Brackett Series:

    • IR

    • n_1=4 and n_2=5,6,7….

    v) Pfund Series:

    • IR

    • n_1=5 and n_2=6,7,8….

  3. \textbf{Notes}: (Worth to remember)

    \to Total no of spectral lines = \dfrac{n(n-1)}{2}

    \to No of spectral lines in between two given series =\dfrac{∆n (∆n+1)}{2}

    \dfrac{1}{λ}=R(\dfrac{1}{n_1^2} -\dfrac{1}{n_2^2} ) = Balmer’s Formula

    Where R = Rydberg’s constant

  4. Defects of Bohr’s atomic model:

    i) It does not explain the micro or fine spectrum

    ii) It does not explain the spectrum of multi electron system but only explain the spectrum of mono electron system like HHe^+Li^{++}, etc.

    iii) It does not explain the Zeeman effect (splitting of lines due to magnetic field), Stark effect ( Splitting of lines due to electric field) and Shielding effect.

    iv) It does not explain the duel nature of electron

Quantum Number

  1. The state and nature of electron in orbit.

    Orbit NumberOrbit DesignationPrincipal Quantum Number(n)Max. Number of Electrons in the orbit(2n^2)
  2. \textbf{Principal Quantum Number}

    \to Main shell or main orbit or main axis

    \to Represented by n

    n=1, 2, 3, 4…. =K, L, M, N…. \to It determines the size of atom

    \to Total no. of electron in each orbit = 2n^2

    \to Total no. of orbital in each orbit = n^2

    Eg : L = 2s (s) , 2p ( px, py, pz)

    = 2^2

    = 4

    \to Also, radius ∝ n^2

    Energy ∝ 1/n^2

  3. \textbf{Azimuthal quantum number (l)}

    \to Sub shell or subsidiary or auxillary quantum number

    \toRepresented by ‘l’

    l = 0,1,2,3…(n-1) {1 less than principle quantum no.}

    = s,p,d,f…..

    \to Eg: For K shell


    l=0 …. (n-1)

    = 0…..(1-1)

    = 0 => s-sub shell

    \toFor M shell


    l=0 …. (n-1)

    = 0…..(3-1)