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Andy's Atom Structure
follows the history of the discovery of the atom
7th Grade

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Atom Structure

a) Describe the Thomson Atom Model? ---

b) What is it based on?




a) J.J. Thomson's Atom Model: Plum Pudding Model

1) electron (small negatively charged particle)

2) surrounded by positively charged mass (proton)

filling the whole volume of the atom.

b) J.J. Thomson's Atom Model is based on this discovery (1897)

using the Hydrogen atom:

1) negatively charged particle (electron) can be ejected from atom.

2) mass of electron is small

relative to remaining positively charged mass (proton)

(mass of electron to mass of proton = 1 to 1840)



 Atom structure



a) Describe the Rutherford Atom Model?

b) What is it based on?




a) Rutherford's conclusion about the structure of the Atom in 1911

1) a very small nucleus contains all protons (p+) and neutrons (n0)

    (that means: the nucleus has a positve charge  and contains nearly all the mass)

2) the atom is mostly empty  space.

3)  electrons (e-) move around the nucleus on an orbit.

    (radius of  a  p+: 0.9*10 -1 5m; radius of  a Hydogen atom: 5*10 -11 m)

4) the number of protons (p+) is equal the number of electrons (e-)

    since an atom is electrically neutral. (#p+ = #e- in neutral atom)

 b) Rutherford's discovery through the Gold Foil Experiment:

”1 in 20,000 α-particles were turned through an average angle of 900 degrees in passing through a layer of gold-foil about 0.00004 cm thick.” (99.995% α-particles not deflected) 


Atom Structure


a) Describe the Bohr Atom Model?

b) What is it based on?




a)  Bohr's Atom Model (for Hydrogen: 1 proton + 1 electron)


      1) several orbits with different radius for the electron of Hydrogen.


      2) no photon absorbed: electron in smallest orbit (ground state)


      3) a)  photon with a specific wavelength  absorbed:      (visible light:  410,434,486,656 nm)


                  and  electron is lifted into a larger orbit  (excited state)       


                  the higher the energy of the proton the larger the orbit:  λ↑→ E↓ of photon


           b)  photon with same wavelength  emitted     (emission spectrum)    


                  and electron falls back into its previous orbit.


b)  Discoveries Niels Bohr incorporated into the Rutherford Model 

   1) Spectroscopy (measures the characteristic wavelength of light  emitted by electrons of energized atoms) 

        wavelenght of emission spectrum of Hydrogen for visible light: 410,434,486,656 nm                                                          

        Rydberg Formula: 1/λ=R*(1/n12–1/n22) [m-1]   (includes also some invisible light)

                                R=1.097*107; n1=1,2,… ; n2= n1+1, n2= n1+2,…

   2) wave-particle duality ( light particle=photon)      Planck constant 6.626*10-34 Js

       (a) each photon of light contains an energy quantum:

             Planck relation E=hν=hc/λ [J]           λ↑→ E↓ of photon

       (b) 1 electron + 1 photon [quantum 1 plus quantum 2]   Einstein's photoelectric effect  

   3) Rutherford Model: a) proton (+) in nucleus b) electron (-) in orbit

                                                  c) atom mostly empty   d) #p=#e (neutral)







a) What is the wave-particle duality of light?

b) When should you use light's

1) wave properties

2) particle properties?


a) Wave-particle duality of light:

(duality means an object has two different characteristics at the same time).


Light has at the same time

1) properties of a wave

continous: at different locations at the same time

characterized by wavelength λ and amplitude a (wave heights)

2) properties of a particle

discrete: 1, 2, 3 (at only 1 location at the same time)

b) use of light's

1) wave: most commonly observed phenomenons of light, e.g. optics

interference, polarization, diffraction

2) particle:

 photoelectric effect: light's interaction with electrons





a) How do you describe the wave of light?

b) What is the speed of light?



a) Description of the wave of light

     1)  its wavelength λ  (lamda, greek for l =el)  = type of light   (e.g. 656 nm, some red)

     2)  its wave heights (amplitude  a)            = intensity of light
        unit for both: m (meter) [1 nano meter = 1 nm = 10-9m]


b) speed of light

      c= 300,000 km/s = 3*105 km/s = 3*108 m/s


c) frequency ν ("nue" greek for n): cycles per second

     ν = c/λ    follows from a) and b)

       unit  Hz  = Hertz = s-1

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