Structure of Atom

Flashcard maker : Steven Colyer

J. J. Thompson model

  • 1898
  • atom possesses a spherical shape (r = 10-10 m = 1 Å) – +ve charge uniformly distributed – e embedded into it – to give most stable electrostatic arrangement
  • Also called apple pie, plum pudding, raisin pudding, watermelon.
  • can be visualized as a watermelon of positive charge w/ seeds (e-) into it.
  • Imp feature: mass of atom uniformly distributed
  • Advantages: able to explain the neutrality of atom
  • Disadvan: not consistent w/ results of later exp.




Rutherford’s Nuclear Model of Atom

;Rutherford ; his students – bombarded thin gold ; ; foil – w/ ;-particles.



  • Stream of high energy ;-particles from radioactive ; source – directed at thin foil (~100 nm) – of Au
  • foil had circular fluoroscent zinc sulphide screen ; ; ; around it – to tell which ;-particles hit it – tiny flash of light produced when ;-particles struck it.







;;Expected results:

  • accn. to Thompson’s model – mass of atom spread evenly – particles had enough energy to pass through the mass
  • particles – slow down – deflect by small angle – while passing

>Actual results:

  •  most α-particles passed w/o deflection
  • small fraction deflected by small angles
  • ~1 in 20,000 deflected by large angle (~180°)

>Conclusions drawn by Rutherford:

  •  most space in atom – empty –  as most particles went undelfected
  • few positively charged α-particles deflected – due to enormous repulsive force – showing – positive charge of atom – not spread over whole of atom (first disproval of Thompson’s model) – positive charge – concentrated in very small volume
  • Calculations by R. – vol. occupied by nucleus – negligibly small : atom. radius of atom – 10-10 m, while nucleus – 10-15 m – appreciable difference.
  • if crick ball nucleus – 5 km – radius of atom.

;R.’s nuclear model:

  •  positive charge – most mass – densely concen. in extremely small region – very small region called NUCLEUS
  • nucleus – surrounded by e  – move around nucleus w/ high speed – circular paths (second disproval of T’s model) called orbits. R’s model resembles mini solar system.
  • e & nucleus – held together – electrostatic forces of attraction



Atomic No. & Mass No.

  • presence of +ve charge – due to nucleus (p+)
  • charge on the p+ = – (opp.) charge on e
  • atomic no. (Z) = no. of p+ = no. of e (in a neutral atom)
  • +ve charge of nucleus due to p+ – mass due to p+ & n
  • p+ & n together called nucleons
  • total no. of  nucleons = mass no. (A) = no. of p+(Z) + no. of n


  • no. of p+ in Na = 11 = no. of e 
  • thus, Z = 11
  • no. of n = 12
  • thus, A = 11 + 12 = 23

Isobars & Isotopes

>composition of any atom – rep. by 
  AZX, where A = mass no., Z = atomic no.
  • atoms w/ same mass no. but diff. atomic no. (they are essentially different elements)
  • eg: Carbon (A=14) & Nitrogen (A = 14),                 but C (Z) = 6 & N (Z) = 7.


  • atoms w/ same atomic no. but diff. mass nos. (they are essentially the same elements)
  • difference in isotopes is due to difference in no. of neutrons.
  • eg: Hydrogen (Z) = 1, exists in three forms -Protium (found 99.985%) has A = 1 (0 n) Deuterium (found 0.015%) has A = 2 (1 n) Tritium (traces on earth) has A = 3 (2 n)
>imp. point:
  • chem properties – controlled by no. of electrons = no. of protons = Z
  • no. of neutrons have very little effect on chem prop.
  • thus, all isotopes of any element show same chem behavior. 

Lim. of R’s model

  • R. model – small scale solar system – nucleus (sun) – electrons (planets)
  • coulomb force (kq1q2/r2, where;q1 ; q2 are charges, r is dist. of separation, k is propor. constant) betn e and nucleus – mathematically similar – Grav. force (G.m1m2/r2)
  • classical mech. when applied to solar system – planets describe well-defined orbits
  • theory can also calculate precisely planetary orbits similarity betn them – electrons should also move in well-defined orbits
  • but, when body – moving in orbit – it undergoes acceleration (changing direction)
  • so, e in orbit – under acceleration
  • accn. to Maxwell’s electromagnetic theory – charged particles, when accelerating – emit electromagnetic radiation
  • for an electron, energy carried by radiation comes from electronic motion
  • orbits should thus shrink
  • accn to calculations – e should take 10-8 s – to fall into nucleus – thus, atom – unstable & destroyed
  • but this does not happen
  • R’s model – couldn’t explain stability of atom
  • if accn to classical phy. and elec.mag. theory, atoms cannot be stable, why not consider e to be stationary?
  • ans: if e were sta. – electrostatic forces of attraction – betn nucleus and e – pull e towards nucleus – form mini T’s model
  • another serious drawback of R’s model – says nothing about electronic structure of atoms (how e are distributed around nucleus & what are the energies of the e)

Developments leading to Bohr’s Model

Two developments – major role – formation of Bohr’s atomic model:
  1.  Dual character of electromagnetic radiation – radiations possess both wave like & particle like prop.
  2. Exp. results – atomic spectra – explained only by assuming quantized electronic energy levels in atoms 

Wave Nature Of Electromagnetic Radiation


Concept of Electromagnetism 

James Maxwell – 1st to give comprehensive expln about interaction between charged bodies & behavior of elec. & mag. fields on microscopic level.
when charged particles accelerates – alternate elec. & mag. fields are produced & transmitted
these fields – trans. in forms of waves called electromagnetic waves / electromagnetic radiation.

Wave Nature Of Electromagnetic Radiation


Light – as ele.mag. waves 

Light – form of radiation – known from early days
speculation about nature – dates to ancient times
Newtonian times – light – made of particles (corpuscules)
only in 19th century – wave nature established
Maxwell – first to reveal – light waves – associated w/ oscillating elec. & mag. character

Wave Nature Of Electromagnetic Radiation


Properties of Elec.mag. Wave Motion 

  1. Oscillating elec. & mag. fields produced by oscillating chegrd particles – perpendicular to each other – & to direction of propagation[image]
  2. Unlike sound & water waves – elec.mag. waves – need no medium – can travel in vacuum.
  3. Many types of elec.mag. radiations – differ in wavelength (λ) & frequency (ν)
  • constitute Electromagnetic Spectrum – diff regions identified by diff names


   4. Different kinds of units – used to rep elec.mag.        radiation

Wave Nature Of Electromagnetic Radiation


Frequency, Wavelength, Speed of Light, Wavenumber 

>Radiations – characterized by prop:

  1.  Frequency:
  • Defn: no. of waves – pass a given point in 1 s
  • S. I. unit – Hertz (Hz / s-1)
  • Symbol: λ 

  2.  Wavelength:

  • Defn: length betn one crest & another
  • S. I. unit – Metre (m), often smaller units used
  • Symbol: ν 

  3.  Speed of Light:

  • Defn: In vacuum – all types of elec.mag. rad. (regardless of wavelength) – travel at same speed – this speed – called speed of light – 3 × 108 m/s.
  • Unit: m/s
  • Symbol: c 
  4.  Wavenumber:
  • Defn: no. of wavelengths per unit length
  • S. I. Unit: reciprocal of wavelength, i.e. m-1, mostly cm-1 is used
  • Symbol: ν


>Relation betn ν, λ and c:
  c = νλ


Wave Nature Of Electromagnetic Radiation


Particle Nature of Elec.mag. Rad.


Phenomenon explained by Particle Nature 

  • Diffraction & Interference – some phenomenon – explained by wave nature
  • Foll. phenomenon couldn’t be explained by wave nature – but w/ particle:
  1. nature of emission of radiation from hot bodies (black body radiation)
  2. ejection of e from metal surface when radiation strikes (photoelectric effect)
  3. variation of heat capacity of solids as fn of temp.
  4. line spectra of atoms w/ sp. ref. to H

Particle Nature of Electromagnetic Radiation


Black Body Radiation;

Max Planck – gave 1st concrete expl. for bbr. (black body radiation)
as follows:
solids heated ; emit radiation over wide range of wavelengths (eg : Fe rod – emits dull red, then more red, then white, blue, w/ increase in temp).
rad. emitted goes from lower frequency to higher freq. as temp. increases.
The ideal body – emits and absorbs all frequencies – called black body – radiation emitted by such a body – called bbr.
Exact frequency distribution of emitted rad. (intensity vs. frequency curve of radiation) depends only on temp.;
at a given temp., intensity of radiation emitted – increases w/ decrease in wavelength, reaches a max value at given wavelength, then decreases w/ further decrease in wavelength.;

Particle Nature of Electromagnetic Radiation


Max Planck’s Quantum Theory 

Planck suggested – atoms & mole. – emit or absorb energy – in discrete quantities – not in continuous manner
quantum – a name given by planck – smallest quantity of energy that can be emitted or absorbed.
energy (E) of a quantum of radiation – proportional to its frequency – expressed by:
E = hv,
where h = Planck’s constant = 6.626 ; 10-34 J s.
Planck – able to explain – distribution of intensity in radiation – from black body – as fn of freq. or wave. at diff. temp.;

Particle Nature of Electromagnetic Radiation


Photoelectric Effect


H. Hertz’s Experiment 

H. Hertz
experiment where – electrons (or current) – ejected when certain metals (K, Rb, Cs, etc) – exposed to beam of light.


Phenomenon – called – photoelectric effect.
Results of exp.:
  1. electrons ejected from metal – as soon as light strikes surface – no time lag betn striking of light & ejection from metal surface
  2. no. of electrons ejected – proportional – intensity or brightness of light
  3. each metal – specific minimum frequency, ν0 – also called threshold frequency.
  • Below this, photoelectric effect – not observed.
  • When frequency, vv0electrons come out w/ kinetic energy.
  • These kinetic energies – increase – with increase in frequency of light used.

Particle Nature of Electromagnetic Radiation


Photoelectric Effect


Unexplainability by Classical Physics 

The results of photoelectric exp. – not explained by classical physics – cuz accn to c.p., energy content of beam of light depends on intensity of light.
i.e., no. of electrons ejected, and their kinetic energy would depend on intensity of light.
but, as it happens, only no. of electrons ejected depends on i.o.l., not their kinetic energy. 

Particle Nature of Electromagnetic Radiation


Photoelectric Effect


Einstein’s explaination;

Shining beam of light onto metal – is – shooting beam of particles (photons).
Photon of sufficient energy strikes an ein atom of metal – transfers its energy instantaneously to eduring collision – eejected w/o time lag.
greater energy of photon – greater transfer of energy to e – greater kinetic energy possessed by e.
kinetic energy of ejected e – proportional – frequency of elec.mag. rad.
striking photon’s energy = hv,
minimum energy req. to eject e (also called work fn) = hv0,
difference in energy = transferred as kinetic energy of photoelectron = hv – hv0
accn to conservation of energy, kinetic energy  of ejected e :
hv = hv0 + ½ mev2
where me = mass of e, v = velocity associated w/ the ejected e
A more intense beam of light – larger no. of photons – larger no. of ejected e 

Particle Nature of Electromagnetic  Radiation


Dual Behavior of Electromagnetic Radiation 

Particle nature of light – dilemma for scientists:
One on hand – it explained bbr & photoelectric effect
On other hand – was not consistent w/ known wave behavior – which explained interference & diffraction.
Only way to resolve dilemma – accept existence of both particle- & wave-like properties (i.e. dual behavior)
Light can behave as any, depending on experiment
Rad. exhibits:
Particle like prop. – interacts w/ matter
Wave like prop. – propagates
Scientists took long time to be convinced – due to their old habit – ‘old habits die hard’
Microscopic particles (e) also display wave-particle prop.

Evidence for the Quantized Electronic Energy Levels


Atomic Spectra 

speed of light – depends on – nature of medium of propagation
thus, light – deviated – when it passes from one medium to another
when ray of white light passes through prism – wave w/ shorter wavelength bends more than one w/ longer wavelength.
ordinary white light – made up of all wavelengths in the visible range – ray of white light – spread into series of colored bands – spectrum.
Continuous spectrum – spectrum where one frequency merges into another – continuous – no break.
visible light – small portion of elec.mag. radiation
when elec.mag. radiation – interacts w/ matter – atoms & mole. may absorb energy – reach to higher evergy state.
higher energy – unstable state – excited state
while returning to ground state – e emit radiations in various regions of elec.mag. spectrum. 

Evidence for the Quantized Electronic Energy Levels


Emission & Absorption Spectra 

Spectrum of radiation – by a substance – w/ absorbed energy – called emission spectrum.
partciles w/ absorbed radiation – called ‘excited’
To produce emission spectrum – body – excited – heating / irradiating – wavelength / frequency of radiation – recorded.
Absorption spectrum –  

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