Points

• Canal rays are positively charged radiations which led to discovery of protons
• Thomson proposed that:

  ¹ an atom consists of A positively charged sphere and electrons are embedded in it

  ² then negative and positive charges are equal in magnitude so the atom as a whole is electrically neutral
• Rutherford's model:

  ¹ selected a cold foil is he wanted a thin layer gold for was about 100 atoms stick

  ² Alpha particles are doublely charged helium ions they have mass of 4 u fast moving Alpha particles have a considerable amount of energy

  ³ it was expected that Alpha particles would be deflected by subatomic particles since the Alpha particles were much heavier than the protons he did not expect to see large deflections
• Rutherford model observation

  ¹ most of the fast movie Alpha particles passed straight line through the gold foil

  ² some of the Alpha particles were diffracted by the foil by small angles

  ³ one out of every 12000 particles appear to rebound
• Words of Rutherford

  This result was almost as incredible as if you fire A 15 inch shell at a piece of tissue paper and it comes back and hits you
• Conclusion of the Rutherford's model

  ¹ most of the space inside the atom is empty because most of the alpha particles pass through the cold foil without getting defected

  ² very few particles were deflected from their back indicating that positively charged of atom occupies very little space

  ³ a small fraction of Alpha particles were deflected by 180 degree indicating that all the positive charge and mass of gold atom were concentrated in the small volume within the atom

  ⁴ calculated that radius of nucleus is about 10⁵ times less than the radius of the atom
• Rutherford's model

  1. There is a positive charge centre in an atom called nucleus nearly all the mass of the atom resides in the nucleus
  2. The electrons revolves around the nucleus in circular paths
  3. The size of the nucleus is very small as compared to size of the atom
• Drawback of rutherford's model

  Any particle in a circular orbit would undergo acceleration during acceleration charge particles would radiate energy revolving electron would lose energy and finally fall into the nucleus the atom should be early and stable if this was so and hands matter would not exist in the form we know we know that matter are quit and stable
• Bohr's model of atom

  1. Only certain special orbits known as discrete orbits of electrons are allowed inside atom
  2. World revolving in discrete orbits the electrons do not radiate energy
  3. This orbits or shells are called energy levels
  4. They are represented by the letter k L M N or numbers 1 2 3
• Neutrons

  • JJ chadwick in 1932 discovered neutron

  • Mass was nearly equal to that of a proton

  • It is present in the nucleus of the atom except hydrogen

  •Sum of masses of protons and neutrons present in nucleus is equal to mass of an atom
• Rules for writing the number of electrons in different shells

  1. The maximum number of electrons present in the shell is given by formula 2n² where and is number of energy level index 1 2 3
  2. The maximum number of electrons that can be accommodated in the outermost orbit is 8
  3. Electrons are not accommodated in the given shell unless the internal shells are filled that is the shells are filled in this step wise
• Electronic configuration

  Arrangement of electrons in an atom is called electronic configuration
• Valency electrons

  The electrons present in the outermost shell of an atom are known as valency electrons
• Octet

  As outermost shell which has 8 electrons was said to process and octet
• Atomic number

  Number of protons present in an atom is called atomic number that is represented by z
• Mass number

  The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of the atom
• Isotopes

  Items of the same element which has same atomic number but different mass number are called isotopes

  example: hydrogen(protium, deuterium, tritium) carbon(4) chlorine(2)
• Applications of isotopes

  1. And isotope of Uranium is used as a fuel in nuclear reactors
  2. And isotopes of Cobalt is used in the treatment of cancer
  3. And isotope of iodine is used in a treatment of goitre
• Isobars

  Items of different elements with different atomic number which have the same mass number are known as isobars

  Examples: calcium and argon atomic number is 20 and 18 but their mass number is 40
• Atomic mass

  Atomic mass of an element which has no isotope is sum of the masses of protons and neutron

  Atomic mass of an element which occurs in isotopes is average mass of all naturally occurring atoms of that element