+1 Chem: 2. Structure of Atom

• What was the first atomic theory?

  Dalton's Atomic Theory
• What were the postulates of Dalton's Atomic Theory?

  1. Elements are made of tiny particles called atoms. 2. Atoms of the same element are identical. 3. Atoms of different elements can combine to form compounds. 4. Chemical reactions involve the rearrangement of atoms.
• 1. Electron was discovered first by?

  J.J Thompson, by the Discharge table experiment (particles were attracted to positive anode at low pressure 10^-4)
• Characteristics of Cathode rays?

  1. Consist of negatively charged particles
  2. They travel in straight lines
  3. Are attracted to the anode
  4. Produces glow when striking fluorescent material (ZnS)
  5. Deflected by electrical & magnetic field
  6. Rotate the paddle when kept on their path
• Electron charge: Mass of electron -- Ratio?

  -1.602 x 10^-19 Coulombs : 9.11 x 10^-31 kg = 1.76 x 10^11 C/kg
• Mass of electron?

  9.11 x 10^-31 kg (electron charge / electron charge over mass of electron)
• Proton was discovered second by...

  Goldstein. Used Perforated CATHODE instead of anode. Canal Rays (anode rays) were the particles going from anode to cathode.

  The mass of proton is... 1.672 x 10^-27 kg
• Neutron was discovered third by...

  James Chadwick. Found that a neutron was emitted when doing a Fusion reaction (Be + He -> C + Neutron)

  Mass of neutron is 1.674 x 10^-27 kg
• Comparisons..

  Electron: Absolute charge is -1.6 x 10^-19, relative charge is -1

  Proton: Absolute charge is +1.6 x 10^-19, relative charge is +1

  Neutron: Absolute charge is 0, relative charge is 0
• What was Rutherford's Alpha Scattering Experiment?

  Gold foil experiment (finding nucleus)

  Radioactive source of alpha particles were allowed to pass thru a lead plate to go thru a thin gold foil

  Experiment setup done by Geiger & Mavsden (students)
• What is Coulomb's Law?

  The mathematical relationship between the electric force between two charged objects and the distance between them.

  |q1| represents proton x charge of electron

  |q2| represents the # of electrons

  
• Atom = 10^-10 m

  Radius of nucleus = 10^-15 m

  The radius of atom = 10^5 x radius of the nucleus
• Isobars: different elements with same mass number
• Isotones: Atoms of diff elements but same number of neutrons
• Drawbacks of the Rutherford Model...

  1. Failed to explain the electronic structure of an atom
  2. Could not explain the stability of an atom
• First Theory of Light!

  Newton's Corpuscular Theory of Light!!

  • Light is a stream of minute perfectly elastic particles (corpuscles), so light has a particle nature, and it bounces back
  • It could explain reflection and refraction
  • but failed to explain wave phenomenon of light (such as interference, diffraction, and polarization
• Second Theory of Light!

  Huygens Wave Theory of Light!

  • Light travels in the form of transverse waves (like sine waves)
  • Showed the dual nature of light: light possess particles as well as wave nature
• Relation btw time period & frequency?

  Frequency (mu): $1/time period$

  Time period (T) : $1/frequency$
• Relation btw frequency, wavelength, and wave velocity?

  Frequency, wavelength, and wave velocity are related by the equation: wave velocity = frequency x wavelength.

  and velocity of an electron.. = 3 x 10^8 m/s
• reciprocal of wavelength gives wave number per unit distance
• THIRD THEORY OF LIGHT?

  James Maxwell's Electromagnetic Theory of Light: a body can emit of absorb energy continuously in the form of electromagnetic radiation (light)

  1. These waves consist of 2 fields: magnetic and electric

  2. light or radiation can travel in a vacuum

  3. This theory could identify diff parts of radiation that are having diff wavelengths & frequencies

  4. Could arrange them in the ascending order of their wavelengths.

  • Called electromagnetic waves: waves that are produced from oscillating charges and consist of electric & magnetic field which are perpendicular to each other

  
• Failures of the Electromagnetic Wave Theory?

  FAILURES: does not explain black bodies: an ideal body that is a perfect emitter & absorber

  2. Also failed to explain Hydrogen line spectrum

  3. Failed to explain the photolytic effect
• FOURTH THEORY OF LIGHT

  Planck's Quantum Theory: Light energy is emitted by a body in the form of small packets of energy called quanta (contains energy)

  • Light energy packets - photons
• Energy of each quantum = h x frequency

  h = 6.626 x 10^-34 Js
• Different forms of Planck's equation

  6 forms... (h = 6.626 x 10^-34 Js)

  1. For normal: E = h x frequency

  • Or E = hc/wavelength (bc frequency = c/wavelength
  • Or E = hc x wave number (bc 1/wavelength = wave #)

  1. For number: E = n x h x frequency

  • E = nhc/wavelength
  • E = nhc x wave number

  1. For mole: E = NA x h x frequency

  • E= NA x hc/wavelength
  • E= NA x hc x wave #
• 1 electron volt = 1.6 x 10^-19 J

  So energy in electron volt: E(in joules)/1.6 x 10^-19
• PHOTOELECTRIC EFFECT?

  A detector is kept and connected to the positively charged terminal of the battery. Light of some energy falls on the metal (alkaline) surface

  Light of some energy falls on the metal (alkaline) surface

  • frequency = incident frequency

  when the circuit is completed & when light radiation of frequency falls on the metal surface, some electrons are ejected from metal surface.
• Features of the photoelectric effect?

  1. It is an instantaneous phenomenon
  2. Electrons are ejected w maximum speed so that is possess max KE (so KE max only depends on the incident frequency
  3. The number of photoelectrons ejected per second is directly proportional to the intensity of light.
• Einsteins' Equation

  E = Wo (work potential) + KEmax

  E = h x frequency + Wo = hUo

  • or E= h (incident frequency - threshold frequency)