• 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 & his students - bombarded thin gold     foil - w/ α-particles.

>Experiment:

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

• 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

eg:

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

• 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.

>Isotopes:

• 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)

• 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. 

• 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^-)

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 

>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 

• Defⁿ: no. of wavelengths per unit length
• S. I. Unit: reciprocal of wavelength, i.e. m^-1, mostly cm^-1 is used
• Symbol: ν

• 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

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, v > v₀, electrons come out w/ kinetic energy.
• These kinetic energies - increase - with increase in frequency of light used.