j.j thomson model of an atom explain in dettail with pic...for my project..

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Thomson's Atomic Model

[J. J. Thomson's raisin bread model (plum pudding model)]

J. J. Thomson

considered

that the structure

of an atom is

something like a

raisin bread,

so that his atomic model

is sometimes called

the raisin bread model.

He assumed

that the basic body

of an atom is

a spherical object

containing N

electrons confined

in homogeneous jellylike

but relatively massive positive

charge distribution

whose total charge cancels

that of the

N electrons.

The schematic drawing

of this model is shown

in the following figure.

Thomson's model is sometimes

dubbed a

plum pudding model.

[Thomson's Model and Alpha Particle Scattering]

As stated

on the preceding page,

Geiger and Marsden

carried out an experiment

in which

the alpha rays

were collided

against a thin metal foil.

Their results showed

that almost all incident alpha

particles penetrate

the foil and go straight

in forward direction

but a few are scattered in very large angles.

Is it possible

to explain this result

by using

Thomson's model?

This question will be

studied below,

but

before explaining it,

let us present

the answer in advance:

The results of the alpha particle scattering cannot be explained by Thomson's atomic model.

Thomson's raisin bread model

(plum pudding model)

therefore cannot

be valid

as an atomic model.

The reason will be

explained below.

Since we need somewhat

detailed mathematical expressions,

it will be given

on the other page,

2-4-A: Alpha Particle Scattering by Thomson's model

As seen

on the other page (2-4-A),

we can consider that

the scattering angle

of the alpha particle

by Thomson's model

is at most 0.01 degrees.

The thickness of

the metal foil

in the scattering experiment

of the alpha rays

is about 10^-6 m.

When assuming

that the atoms are

tightly packed

in the metal,

there are about 10000 atoms

lining up

in the direction

of thickness,

because the size

of an atom is

approximately 10^-10m.

(See the following figure.)

When the alpha particle

collides with these atoms

in the metal foil

10000 times successively,

the scattering angle

of each individual collision

in such a multiple scattering

is less than 0.01 degrees

as discussed above.

The resultant scattering angle

is obtained

by an accumulation of

these individual scatterings

of 10000 times.

One may expect that,

even if the scattering angle

of each individual scattering

is very small like 0.01 degrees,

we can have

as large resultant angle

This is however

unrealistic,

because the direction

of each individual scattering

must be random,

and an accumulation

of random values would give

nearly zero only.

So that we never obtain

such a large resultant scattering

angle after the multiple

scattering.

Accordingly,

such a large scattering angle

as those obtained

in Geiger and Marsden's experiment

cannot be reproduced

by such a multiple

scattering as stated above.

Thus we can conclude that

Thomson's atomic model is not held.

A schematic presentation of the plum pudding model of the atom. In Thomson's mathematical model the "corpuscles" (or modern electrons) were arranged non-randomly, in rotating rings.

The plum pudding model of the atom by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. In this model, the atom is composed of electrons (which Thomson still called "corpuscles", thoughG. J. Stoney had proposed that atoms of electricity be called electrons in 1894^[1]) surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons (see below). Instead of a soup, the atom was also sometimes said to have had a "cloud" of positive charge.

With this model, Thomson abandoned his earlier "nebular atom" hypothesis in which the atom was composed of immaterial vorticies. Now, at least part of the atom was to be composed of Thomson's particulate negative corpuscles, although the rest of the positively-charged part of the atom remained somewhat nebulous and ill-defined.

The 1904 Thomson model was disproved by the 1909 gold foil experiment, which was interpreted by Ernest Rutherford in 1911^[2] ^[3] to imply a very small nucleus of the atom containing a very high positive charge (in the case of gold, enough to balance about 100 electrons), thus leading to the Rutherford modelof the atom. Although gold has an atomic number of 79, immediately after Rutherford's paper appeared in 1911 Antonius Van den Broek made the intuitive suggestion that atomic number is nuclear charge. The matter required experiment to decide. Henry Moseley's work showed experimentally in 1913 (seeMoseley's law) that the effective nuclear charge was very close to the atomic number (Moseley found only one unit difference), and Moseley referenced only the papers of Van den Broek and Rutherford. This work culminated in the solar-system-like (but quantum-limited) Bohr model of the atom in the same year, in which a nucleus containing an atomic number of positive charge is surrounded by an equal number of electrons in orbital shells. Bohr had also inspired Moseley's work.

Thomson's model was compared (though not by Thomson) to a British dessert called plum pudding, hence the name. Thomson's paper was published in the March 1904 edition of the Philosophical Magazine, the leading British science journal of the day. In Thomson's view:

... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, ...^[4]

In this model, the electrons were free to rotate within the blob or cloud of positive substance. These orbits were stabilized in the model by the fact that when an electron moved farther from the center of the positive cloud, it felt a larger net positive inward force, because there was more material of opposite charge, inside its orbit (see Gauss's law). In Thomson's model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons, and spectra were to be accounted for by energy differences of different ring orbits. Thomson attempted to make his model account for some of the major spectral lines known for some elements, but was not notably successful at this. Still, Thomson's model (along with a similar Saturnian ring model for atomic electrons, also put forward in 1904 by Nagaoka after James Clerk Maxwell's model of Saturn's rings), were earlier harbingers of the later and more successful solar-system-like Bohr model of the atom.

hope dis 1 will help !!

In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum-physics–based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.

The model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants.

The Bohr model is a primitive model of the hydrogen atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, and its correct results for selected systems (see below for application), the Bohr model is still commonly taught to introduce students to quantum mechanics, before moving on to the more accurate but more complex valence shell atom. A related model was originally proposed by Arthur Erich Haas in 1910, but was rejected. The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a full-blown quantum mechanics (1925) is often referred to as the old quantum theory.

The Rutherford–Bohr model of the hydrogen atom (Z = 1) or a hydrogen-like ion (Z > 1), where the negatively charged electron confined to an atomic shell encircles a small, positively charged atomic nucleusand where an electron jump between orbits is accompanied by an emitted or absorbed amount of electromagnetic energy (hν).^[1] The orbits in which the electron may travel are shown as grey circles; their radius increases as n², where n is the principal quantum number. The 3 → 2 transition depicted here produces the first line of the Balmer series, and for hydrogen (Z = 1) it results in a photon of wavelength 656 nm (red light).