Atoms-1 Flashcards
jj thomson’s model of atom
The first model of atom was proposed by J. J. Thomson in 1898.
According to this model, the positive charge of the atom is uniformly
distributed throughout the volume of the atom and the negatively charged
electrons are embedded in it like seeds in a watermelon. This model was
picturesquely called plum pudding model of the atom.
We know that condensed matter (solids and liquids) and dense gases at
all temperatures emit electromagnetic radiation in which a continuous
distribution of several wavelengths is present, though with different
intensities. This radiation is considered to be due to oscillations of atoms and molecules, governed by the interaction of each atom or
molecule with its neighbours.
In contrast, light emitted from
rarefied gases heated in a flame, or excited electrically in a
glow tube such as the familiar neon sign or mercury vapour
light has only certain discrete wavelengths. The spectrum
appears as a series of bright lines. In such gases, the
average spacing between atoms is large. Hence, the
radiation emitted can be considered due to individual atoms
rather than because of interactions between atoms or
molecules
Geiger Marsden Experiment components
- they directed a beam of 5.5 MeV a-particles emitted from a 214- 83- Bi radioactive source at a thin metal foil made of gold.
-Alpha-particles emitted by a 214
83Bi 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 zinc
sulphide screen and a microscope.
-The scattered alpha-particles on striking
the screen produced brief light flashes
or scintillations. These flashes may be
viewed through a microscope
observations of alpha ray scattering experiment
i) Most of the alpha particles pass undeviated through the gold foil
ii) Very few alpha particles are scattered by by more than 1°
iii) About 1 in 8000 alpha particles by more than 90°
inferences of rutherford experiment
i) Since most the alpha particles pass undeviated, most of the space inside the atom is empty.
ii) Rutherford believed to defelect an alpha particle, it must have experienced a large repulsive force. So the entire positive charge of the atom must be concentrated at the central core of the atom. This small central core of each atom was called the nucleus.
iii) The order of the radius of the atom is 10-10 while it is 10-15 for the nucleus.
iv)The electrons would be moving in orbits
about the nucleus just as the planets
do around the sun. The centripetal force required by electrons for revolution is provided by the electrostatic force of attraction b/w nucleus and electrons.
assumptions in rutherford alpha scatterig experiment
-As the gold foil is very thin, it
can be assumed that a-particles will suffer not more than one scattering
during their passage through it.
-Since the nucleus of gold is about 50 times heavier than an
a-particle, it is reasonable to assume that it remains stationary
throughout the scattering process.
what is distance of closest approach
- The minimum distance from the nucleus upto which an energetic alpha particle reaches before coming to rest and retracing its path is the distance of closest approach.
Impact parameter
The impact parameter is the perpendicular distance of the
initial velocity vector of the a-particle from the centre of the nucleus, when the particle is far away from the nucleus of the atom.
It is seen that an a-particle
close to the nucleus (small impact
parameter) suffers large scattering. In case
of head-on collision, the impact parameter
is minimum and the a-particle rebounds
back (q @ p). For a large impact parameter,
the a-particle goes nearly undeviated and
has a small deflection (q @ 0).
emission spectrum
When an atomic gas or vapour is excited at
low pressure, usually by passing an electric current through it, the emitted
radiation has a spectrum which contains certain specific wavelengths
only. A spectrum of this kind is termed as emission line spectrum and it consists of bright lines on a
dark background.
absorption spectrum
When white light
passes through a gas and we
analyse the transmitted light
using a spectrometer we find
some dark lines in the
spectrum. These dark lines
correspond precisely to those wavelengths which were found in the
emission line spectrum of the gas. This is called the absorption spectrum
of the material of the gas.
drawbacks of rutherfords model of atom
- We know that an object which moves in a circle is being constantly accelerated – the acceleration being centripetal in nature. According to classical
electromagnetic theory, an accelerating charged particle emits radiation in the form of electromagnetic waves.
-The energy of an accelerating electron should therefore, continuously decrease. The electron would spiral inward and eventually fall into the nucleus. Thus, such an atom can not be stable.
-Further, according to the classical electromagnetic theory, the frequency of the electromagnetic waves emitted by the
revolving electrons is equal to the frequency of
revolution. As the electrons spiral inwards, their angular velocities and hence their frequencies would change continuously, and so will the frequency of the light emitted. Thus, they would emit a continuous spectrum, in contradiction to the line spectrum actually observed
