2. STRUCTURE OF ATOM 2021
2. STRUCTURE OF ATOM
Electron: Discovered by J J Thomson by Cathode ray discharge tube experiment. These rays start
from the cathode and move in straight lines are called cathode rays or cathode ray particles.
Properties of Cathode Rays: e/me = 1.758 × 1011 C kg–1, me is the mass of the electron in kg and e
is the magnitude of the charge on the electron in coulomb (C).
Charge on the Electron (e): Charge on the electron is – 1.6022 × 10–19 C.
Mass of electron (me): Mass of electron (me) = 9.1 ×10–31 kg
Protons : E. Goldstein discovered anode rays or canal rays.
The smallest and lightest positive ion was obtained from hydrogen and was called proton.
Neutrons: Discovered by Chadwick by bombarding a thin sheet of beryllium by α-particles. They
are electrically neutral particles having a mass slightly greater than that of the protons.
Rutherford’s Nuclear Model of Atom: Rutherford proposed an atom model based on his α–
particle scattering experiment. He bombarded a very thin gold foil with α–particles.
The Experiment: A stream of high energy α–particles from a radioactive source was directed at a
thin gold foil. The thin gold foil had a circular fluorescent screen around it. Whenever α–particles
struck the screen, a tiny flash of light was produced at that point.
Observations:
1. Most of the α– particles passed through the gold foil without any deviation.
2. A small fraction of the α–particles was deflected by small angles.
3. A very few α– particles bounced back, that is, were deflected by nearly 180°.
Conclusions:
1. Since most of the α–particles passed through the foil without any deviation, most space in
the atom is empty.
2. A few positively charged α– particles were deflected. This is because the positive charge of
the atom is concentrated in a very small volume at the centre called nucleus.
3. The volume occupied by the nucleus is negligibly small as compared to the total volume of
the atom.
Nuclear model of atom.
1. All the positive charge and most of the mass of the atom were concentrated in an extremely
small region called nucleus.
Focus area Page 1
, 2. STRUCTURE OF ATOM 2021
2. Electrons are revolving round the nucleus with a very high speed in circular paths called
orbits.
3. Electrons and the nucleus are held together by electrostatic forces of attraction.
Drawbacks or Limitations of Rutherford’s atom model:
1. Rutherford model cannot explain the stability of the atom.
2. He cannot explain the electronic structure of atom.
Q 1. What are the important observations and conclusions made by Rutherford from his alpha ray scattering
experiment? Give any two limitations of Rutherford‟s nuclear model of atom.
Particle Nature of Electromagnetic Radiation:
Black body radiation: An ideal body which emits and absorbs all frequencies is called a black
body and the radiation emitted by such a body is called black body radiation.
The phenomenon of black body radiation was first explained by Max Planck by his Quantum
theory.
Q 2. Mention two observations which could not be explained by wave nature of electromagnetic radiations.
(Hint: Black body radiation, photoelectric effect)
Planck’s Quantum Theory:
Atoms and molecules could emit (or absorb) energy only in discrete quantities and not in a
continuous manner, but in small packets of energy called quantum.
The energy (E) of each quantum of radiation is proportional to its frequency (ν). It is expressed by
the equation,
E = hν, Where „h‟ is known as Planck‟s constant and its value is 6.626×10–34 J s.
Photoelectric effect: (H. Hertz). It is the ejection of electrons by certain metals (like potassium,
rubidium, caesium etc.) when light of suitable frequency incident on them. The electrons ejected are
called photoelectrons.
The results observed during photoelectric effect are:
1. The electrons are ejected from the metal surface as soon as the beam of light strikes the
surface.
2. The number of electrons ejected is proportional to the intensity or brightness of light.
3. For each metal, there is a minimum frequency (known as threshold frequency [ν0]) below
which photoelectric effect is not observed.
4. The kinetic energy of the ejected electrons is directly proportional to the frequency of the
incident light.
Focus area Page 2
2. STRUCTURE OF ATOM
Electron: Discovered by J J Thomson by Cathode ray discharge tube experiment. These rays start
from the cathode and move in straight lines are called cathode rays or cathode ray particles.
Properties of Cathode Rays: e/me = 1.758 × 1011 C kg–1, me is the mass of the electron in kg and e
is the magnitude of the charge on the electron in coulomb (C).
Charge on the Electron (e): Charge on the electron is – 1.6022 × 10–19 C.
Mass of electron (me): Mass of electron (me) = 9.1 ×10–31 kg
Protons : E. Goldstein discovered anode rays or canal rays.
The smallest and lightest positive ion was obtained from hydrogen and was called proton.
Neutrons: Discovered by Chadwick by bombarding a thin sheet of beryllium by α-particles. They
are electrically neutral particles having a mass slightly greater than that of the protons.
Rutherford’s Nuclear Model of Atom: Rutherford proposed an atom model based on his α–
particle scattering experiment. He bombarded a very thin gold foil with α–particles.
The Experiment: A stream of high energy α–particles from a radioactive source was directed at a
thin gold foil. The thin gold foil had a circular fluorescent screen around it. Whenever α–particles
struck the screen, a tiny flash of light was produced at that point.
Observations:
1. Most of the α– particles passed through the gold foil without any deviation.
2. A small fraction of the α–particles was deflected by small angles.
3. A very few α– particles bounced back, that is, were deflected by nearly 180°.
Conclusions:
1. Since most of the α–particles passed through the foil without any deviation, most space in
the atom is empty.
2. A few positively charged α– particles were deflected. This is because the positive charge of
the atom is concentrated in a very small volume at the centre called nucleus.
3. The volume occupied by the nucleus is negligibly small as compared to the total volume of
the atom.
Nuclear model of atom.
1. All the positive charge and most of the mass of the atom were concentrated in an extremely
small region called nucleus.
Focus area Page 1
, 2. STRUCTURE OF ATOM 2021
2. Electrons are revolving round the nucleus with a very high speed in circular paths called
orbits.
3. Electrons and the nucleus are held together by electrostatic forces of attraction.
Drawbacks or Limitations of Rutherford’s atom model:
1. Rutherford model cannot explain the stability of the atom.
2. He cannot explain the electronic structure of atom.
Q 1. What are the important observations and conclusions made by Rutherford from his alpha ray scattering
experiment? Give any two limitations of Rutherford‟s nuclear model of atom.
Particle Nature of Electromagnetic Radiation:
Black body radiation: An ideal body which emits and absorbs all frequencies is called a black
body and the radiation emitted by such a body is called black body radiation.
The phenomenon of black body radiation was first explained by Max Planck by his Quantum
theory.
Q 2. Mention two observations which could not be explained by wave nature of electromagnetic radiations.
(Hint: Black body radiation, photoelectric effect)
Planck’s Quantum Theory:
Atoms and molecules could emit (or absorb) energy only in discrete quantities and not in a
continuous manner, but in small packets of energy called quantum.
The energy (E) of each quantum of radiation is proportional to its frequency (ν). It is expressed by
the equation,
E = hν, Where „h‟ is known as Planck‟s constant and its value is 6.626×10–34 J s.
Photoelectric effect: (H. Hertz). It is the ejection of electrons by certain metals (like potassium,
rubidium, caesium etc.) when light of suitable frequency incident on them. The electrons ejected are
called photoelectrons.
The results observed during photoelectric effect are:
1. The electrons are ejected from the metal surface as soon as the beam of light strikes the
surface.
2. The number of electrons ejected is proportional to the intensity or brightness of light.
3. For each metal, there is a minimum frequency (known as threshold frequency [ν0]) below
which photoelectric effect is not observed.
4. The kinetic energy of the ejected electrons is directly proportional to the frequency of the
incident light.
Focus area Page 2
