CLASS 12 PHY - ATOMS NOTES
Introduction
• 1897 - experiments on electric discharge through gases - English physicist J. J.
Thomson
Outcome:
➢ atoms of different elements contain negatively charged constituents (electrons)
that are identical for all atoms
➢ atoms on the whole are electrically neutral
• The first model of the atom - J. J. Thomson – 1898
Inference:
➢ An atom's positive charge is uniformly distributed throughout the atom's
volume and the negatively charged electrons are embedded in it like seeds in a
watermelon - plum pudding model of the atom
➢ No. of electrons equals no. of protons
Disadvantages:
➢ Couldn’t explain the origin of spectral series
➢ Couldn’t explain the angle scattering of alpha particles
• In the early nineteenth century - element is associated with a characteristic spectrum
of radiation, for example, hydrogen always gives a set of lines with fixed relative
position between the lines
• 1885 - Johann Jakob Balmer (1825 – 1898) obtained a simple empirical formula that
gave the wavelengths of a group of lines emitted by atomic hydrogen
• Ernst Rutherford - experiments on alpha-particles emitted by some radioactive
elements – 1906 - experiment of scattering of these alpha-particles by atoms to
investigate the atomic structure.
• This experiment was later performed around 1911 by Hans Geiger (1882–1945) and
Ernst Marsden
• Rutherford’s planetary model of the atom (nuclear model of the atom)
➢ the entire positive charge and most of the mass of the atom is concentrated in a
small volume called the nucleus
➢ electrons revolving around the nucleus just as planets revolve around the sun
ALPHA-PARTICLE SCATTERING AND RUTHERFORD’S NUCLEAR MODEL OF
ATOM
• At the suggestion of Ernst Rutherford, in 1911, H. Geiger and E. Marsden performed
some experiments.
• In one of their experiments:
➢ Directed a beam of 5.5 MeV alpha-particles emitted from a 214 Bi 83
radioactive source at a thin metal foil made of gold
➢ Alpha particles emitted by a radioactive source were collimated into a narrow
beam by their passage through lead bricks.
➢ The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10–7 m
➢ The scattered alpha particles were observed through a rotatable detector
consisting of a zinc sulphide screen and a microscope
1
, ➢ The scattered alpha-particles striking the screen produced brief light flashes
or scintillations - viewed through a microscope and the distribution of the
number of scattered particles may be studied as a function of the angle of
scattering
➢ Alpha particles – mass = 4 * mass of proton
- Charge = 2e
- He ion where both electrons are removed
Observations:
➢ A typical graph of the total number of a-particles scattered at different angles,
in a given interval of time:
➢ The dots - data points – experimental
➢ solid curve - theoretical prediction based on the assumption that the target
atom has a small, dense, positively charged nucleus.
2
Introduction
• 1897 - experiments on electric discharge through gases - English physicist J. J.
Thomson
Outcome:
➢ atoms of different elements contain negatively charged constituents (electrons)
that are identical for all atoms
➢ atoms on the whole are electrically neutral
• The first model of the atom - J. J. Thomson – 1898
Inference:
➢ An atom's positive charge is uniformly distributed throughout the atom's
volume and the negatively charged electrons are embedded in it like seeds in a
watermelon - plum pudding model of the atom
➢ No. of electrons equals no. of protons
Disadvantages:
➢ Couldn’t explain the origin of spectral series
➢ Couldn’t explain the angle scattering of alpha particles
• In the early nineteenth century - element is associated with a characteristic spectrum
of radiation, for example, hydrogen always gives a set of lines with fixed relative
position between the lines
• 1885 - Johann Jakob Balmer (1825 – 1898) obtained a simple empirical formula that
gave the wavelengths of a group of lines emitted by atomic hydrogen
• Ernst Rutherford - experiments on alpha-particles emitted by some radioactive
elements – 1906 - experiment of scattering of these alpha-particles by atoms to
investigate the atomic structure.
• This experiment was later performed around 1911 by Hans Geiger (1882–1945) and
Ernst Marsden
• Rutherford’s planetary model of the atom (nuclear model of the atom)
➢ the entire positive charge and most of the mass of the atom is concentrated in a
small volume called the nucleus
➢ electrons revolving around the nucleus just as planets revolve around the sun
ALPHA-PARTICLE SCATTERING AND RUTHERFORD’S NUCLEAR MODEL OF
ATOM
• At the suggestion of Ernst Rutherford, in 1911, H. Geiger and E. Marsden performed
some experiments.
• In one of their experiments:
➢ Directed a beam of 5.5 MeV alpha-particles emitted from a 214 Bi 83
radioactive source at a thin metal foil made of gold
➢ Alpha particles emitted by a radioactive source were collimated into a narrow
beam by their passage through lead bricks.
➢ The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10–7 m
➢ The scattered alpha particles were observed through a rotatable detector
consisting of a zinc sulphide screen and a microscope
1
, ➢ The scattered alpha-particles striking the screen produced brief light flashes
or scintillations - viewed through a microscope and the distribution of the
number of scattered particles may be studied as a function of the angle of
scattering
➢ Alpha particles – mass = 4 * mass of proton
- Charge = 2e
- He ion where both electrons are removed
Observations:
➢ A typical graph of the total number of a-particles scattered at different angles,
in a given interval of time:
➢ The dots - data points – experimental
➢ solid curve - theoretical prediction based on the assumption that the target
atom has a small, dense, positively charged nucleus.
2
