Slide 1 : What is an Atom? Thought to be the smallest indivisible particle of matter which was proved incorrect later. How were these discovered? How are they arranged in the atom?? Atom can be further subdivided into electrons, protons and neutrons. Atomic Structure
Slide 2 : Crooke’s Discharge tube
Slide 3 : Discovery of Cathode rays (a beam of electrons) What did you observe? Rays were emitted by the cathode. They produced a bright spot when they struck the zinc sulphide screen.
Slide 4 : The wheel started rotating. What is your conclusion?? Cathode rays Cathode rays possess kinetic energy and hence possess mass.
Slide 5 : When an opaque object is placed in the path of the cathode rays, a shadow is cast on the screen. What is your conclusion?? Cathode rays Cathode rays travel in straight lines.
Slide 6 : Cathode rays 3-Dimensional experimental setup
Slide 7 : When cathode rays were allowed to pass through electric field, they deflected towards the positive terminal of the electric field. Cathode rays What is your conclusion? Cathode rays are negatively charged. Schematic diagram
Slide 8 : Cathode rays 3-Dimensional experimental setup
Slide 9 : Cathode rays are deflected perpendicular to the magnetic field irrespective of the nature of the gas and the cathode used. Cathode rays What is your conclusion?? Cathode rays are negatively charged. The e/m ratio was calculated and was observed to be the same irrespective of nature of the gas and cathode.
Conclusions : Conclusions Atoms are not the smallest particle of matter. The cathode rays consist of negatively charged particles. The e/m ratio of these rays is the same regardless of the gas taken in the discharge tube.
Therefore these particles form a fundamental unit of every atom.
Slide 11 : J.J. Thomson’s model Atom is spherical in shape. A lump of positive charge is distributed uniformly. Electrons are embedded in the positive charge Can you think of anything that you can compare this structure to?? However there was no experimental evidence to prove this theory.
Slide 12 : How did they find the charge of an electron? Millikan’s oil drop experiment. Charge on the oil drop is q = n × 1.6 × 10–19 C Thus the smallest charge possible is 1.6 × 10–19 C This is the charge of the electron.
Slide 13 : How did they prove the presence of positive charges? Goldstein’s Anode ray experiment
Slide 14 : Find the expected mass of Helium atom, given it has two protons and two electrons and, mass of one proton = 1.67 × 10 –27 kg and mass of one electron = 9.1 × 10 –31 kg. Calculated mass = 3.34 × 10 –27 kg What do you think could be the reason for this difference in mass? Experimental mass = 6.68 x 10-27 kg
Slide 15 : Experimentally, the mass of the atom was found to be heavier than the total mass of the protons and electrons. Discovery of neutrons The difference in mass was found to be equal to or a multiple of the mass of the proton. This led to the discovery of the third fundamental particle of an atom called Neutron (By James Chadwick).
Slide 16 : A neutron walks into a bar.
"I`d like a beer" he says.The bartender promptly serves up a beer. "How much will that be?" asks the neutron. "For you?" replies the bartender,
"no charge” !!
Slide 17 : Experimental proof for the existence neutron This was performed by James Chadwick neutron Lighter nuclear
Slide 18 : Bombarded a thin metal foil with positively charged alpha particles Rutherford’s Gold foil experiment
Slide 19 : What Rutherford Saw
Slide 20 : What did you observe?
What are your conclusions?
Based on the above conclusions what structure would you give to the atom?
Slide 21 : Most particles flew right through the atom as if there were nothing there
- The foil was mostly empty space
A small number of particles were bounced back to their source
- There must be a small, dense positive charge in the atom. Rutherford’s Gold foil experiment Observations and Inferences
Slide 22 : Postulates of Rutherford’s nuclear or planetary model Atom is hollow, and spherical in shape. Positive charge is concentrated at the centre which is called as nucleus. Electrons are revolving around the nucleus in circular orbits just like planets revolve around the sun. The electrons within an atom must revolve around the nucleus at high speeds in order to counter balance the electrostatic force of attraction between the protons and electrons.
Slide 23 : Drawbacks of Rutherford’s model According to classical electrodynamics, an accelerated electron continuously loses its energy acquiring a spiral path and eventually falling into the nucleus. But does this really happen??
If it does …what do you think the consequences would be? If this happens the energy should be radiated continuously by the electron. But experimentally it was found that energy was discontinuous. To get more information on the structure of the atom, interaction of light with matter should be known.
Slide 24 : Nature of light To understand nature of light we should first understand a wave. What is a wave? What are its characteristics? Wave length () Amplitude (a)
Slide 25 :
Slide 26 : White
light
beam
Slide 27 : What do you observe? What do you observe?
Slide 28 : Energy is absorbed / emitted in discrete packets called quanta How do we know the energy of each quanta E = hv (for each quanta) Or integral multiple of hv quanta
Slide 29 : Bohr’s model Electrons move around nucleus at high velocities in specified paths called orbits or shells. As long as the electron is in a particular orbit, its energy remains constant. These orbits are called stationary orbits. The orbits are designated as K, L, M, N which correspond to n = 1, 2, 3, 4 ……..
Slide 30 : Ek < EL < EM……….. Electrons can jump from one orbit to another i.e., either from a higher energy orbit to a lower energy orbit and vice versa.
Slide 31 : What is the angular momentum of the 4th orbit?