CONTRIBUTIONS OF J.J THOMAS, ERNST

RUTHERFORD AND NEIL BOHR IN THE FIELD OF CHEMISTRY

Prepared By- Meeran Ali Ahmad Class IX-A Roll No-08

What Is an ATOM? The smallest unit

of matter that retains the identity of the substance is

known as atom.

Sir Joseph John Thomson

Thomson’s model of atom(Plum pudding model) 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 this model, the atom is composed of electrons (which Thomson still called "corpuscles", though G. J. Stoney had proposed that atoms of electricity be called electrons in 1894 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.

• 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 ( 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.

Contributions- Thomson devised the famous plum pudding model of the atom, in

which electrons were compared to negative plums embedded in a positively charged pudding. The idea was wrong, and his successor at Cambridge, Ernest Rutherford, was soon to develop the nuclear model of the atom.

Thomson investigated positive rays, which consist of ionized atoms, beginning in 1906. He was able to use a combination of electric and magnetic fields to separate different charged atoms of elements on the basis of their charge/mass ratios.

Thomson was a great advocate of pure research, in contrast to applied research, declaring: Research in applied science leads to reforms, research in pure science leads to revolutions, and revolutions, whether political or industrial, are exceedingly profitable things if you are on the winning side.

Ernst Rutherford

Ernst Rutherford’s model of atom The Rutherford model or planetary model is a

model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909, which suggested on Rutherford's 1911 analysis that the so-called "plum pudding model" of J. J. Thomson of the atom was incorrect. Rutherford's new model for the atom, based on the experimental results, had the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and containing the bulk of the atomic mass (the nucleus of the atom).

• Rutherford's model did not make any new headway in explaining the electron-structure of the atom; in this regard Rutherford merely mentioned earlier atomic models in which a number of tiny electrons circled the nucleus like planets around the sun, or a ring around a planet (such as Saturn). However, by implication, Rutherford's concentration of most of the atom's mass into a very small core made a planetary model an even more likely metaphor than before, as such a core would contain most of the atom's mass, in an analogous way to the Sun containing most of the solar system's mass.

Contributions- After Rutherford's discovery, scientists started to realize that the

atom is not ultimately a single particle, but is made up of far smaller subatomic particles. Following research was done to figure out the exact atomic structure which led to Rutherford’s gold foil experiment. They eventually discovered that atoms have a positively-charged nucleus (with an exact atomic number of charges) in the center, with a radius of about 1.2 x 10−15 meters x [Atomic Mass Number]1/3. Since electrons were found to be even smaller, this meant that the atom consists of mostly empty space.

Later on, scientists found the expected number of electrons (the same as the atomic number) in an atom by using X-rays. When an X-ray passes through an atom, some of it is scattered, while the rest passes through the atom. Since the X-ray loses its intensity primarily due to scattering at electrons, by noting the rate of decrease in X-ray intensity, the number of electrons contained in an atom can be accurately estimated.

Neil Bohr

Bohr’s model of an atom In atomic physics, the Bohr model, introduced by Neil

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 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.

Contribution- The Bohr model of the atom, the theory that electrons travel in

discrete orbits around the atom's nucleus. The shell model of the atom, where the chemical properties of

an element are determined by the electrons in the outermost orbit.

The correspondence principle, the basic tool of Old quantum theory.

The liquid drop model of the atomic nucleus. Identified the isotope of uranium that was responsible for slow-

neutron fission – 235U. Much work on the Copenhagen interpretation of quantum

mechanics. The principle of complementarity: that items could be separately

analyzed as having several contradictory properties.

Conclusion Chemistry is the science of atomic matter,

especially its chemical reactions, but also including its properties, structure, composition, behavior, and changes as they relate the chemical reactions. Chemistry is centrally concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds. Chemistry is used by us in our daily lives though we do not realise. Chemistry is also known as “THE CENTRAL SCIENCE”.

It is difficult to imagine your life without the knowledge of chemistry