Project On Atomic Structure

DISCOVERY OF ELECTRONS •

Hundred years ago, amidst glowing glass tubes and the hum of electricity, the British physicist J.J Thomson. was venturing into the interior of the atom. At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as "cathode rays." His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most fundamental unit of matter.

J.J Thomson

Cathode ray tube

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During the 1880s and ’90s scientists searched cathode rays for the carrier of the electrical properties in matter. Their work culminated in the discovery by English physicist J.J. Thomson of the electron in 1897. The existence of the electron showed that the 2,000-year-old conception of the atom as a homogeneous particle was wrong and that in fact the atom has a complex structure. Cathode-ray studies began in 1854 when Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker, improved the vacuum tube. Plücker discovered cathode rays in 1858 by sealing two electrodes inside the tube, evacuating the air, and forcing electric current between the electrodes. He found a green glow on the wall of his glass tube and attributed it to rays emanating from the cathode. In 1869, with better vacuums, Plücker’s pupil Johann W. Hittorf saw a shadow cast by an object placed in front of the cathode. The shadow proved that the cathode rays originated from the cathode. The English physicist and chemist William Crookes investigated cathode rays in 1879 and found that they were bent by a magnetic field; the direction of deflection suggested that they were negatively charged particles. As the luminescence did not depend on what gas had been in the vacuum or what metal the electrodes were made of, he surmised that the rays were a property of the electric current itself. As a result of Crookes’s work, cathode rays were widely studied, and the tubes came to be called Crookes tubes

CANAL RAYS Anode rays (or Canal rays) were observed in experiments by a German scientist, Eugen Goldstein, in 1886. Goldstein used a gas discharge tube which had a perforated cathode. A "ray" is produced in the holes (canals) in the cathode and travels in a direction opposite to the "cathode rays," which are streams of electrons. Goldstein called these positive rays "Kanalstrahlen" canal rays because it looks like they are passing through a canal. In 1907 a study of how this "ray" was deflected in a magnetic field, revealed that the particles making up the ray were not all the same mass. The lightest, formed when there was a little hydrogen in the tube, was calculated to be 1837 times as massive as an electron. They were protons

Discovery of the Neutron •

It is remarkable that the neutron was not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Since the time of Rutherford it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus. As of about 1930 it was presumed that the fundamental particles were protons and electrons, but that required that somehow a number of electrons were bound in the nucleus to partially cancel the charge of A protons. But by this time it was known from the uncertainty principle and from " particle-in-a-box" type confinement calculations that there just wasn't enough energy available to contain electrons in the nucleus.

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.

RUTHERFORD’S ALPHA SCATTERING EXPERIMENT •

In the years 1909-1911 Ernest Ruthefordand his students - Hans Geiger (1882-1945) and Ernest Marsden conducted some experiments to search the problem of alpha particles scattering by the thin gold-leaf. Rutheford knew that the particles contain the 2e charge. The experiment caused the creation of the new model of atom - the "planetary" model. Rutheford suggested to hit the gold-leaf (picture no. 1) with fast alpha particles from the source 214Po. (The source R was in the lead lining F). The particles felt from the source on the gold-leaf E and were observed by the microscope M. The whole experiment was in the metal lining A and was covered with the glass plate P. The instrument was attached to the footing B. The gold leaf was about 5*10-7 meter thick. The scientist knew that reckoning the scattering angle could say much about the structure of atoms of the gold-leaf.

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Rutheford made a theoretical analysis of angles of scattering in accordance with Thomson's theory of atom and in accordance with his own theory. He assumed that atom consisted of positive charged nucleus and negative charged electrons circling around the nucleus. Then his theoretic calculations he compared with the experiment result. Alpha particles going through atom created in accordance with the "plum cake" model wouldn't be strong abberated because the electric field in that atom wouldn't be strong. In the model created by Rutheford the field is much stronger near to the nucleus, so some of alpha particles are much more abberated. The other going in the far distance to the nucleus are almost not at all abberated. The probability that any alpha particle will hit the nucleus is small but there is such a chance.

PLANETARY MODEL OF ATOM BY BOHR •

The Bohr Model is probably familar as the "planetary model" of the atom illustrated in the adjacent figure that, for example, is used as a symbol for atomic energy (a bit of a misnomer, since the energy in "atomic energy" is actually the energy of the nucleus, rather than the entire atom). In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). The adjacent image is not to scale since in the realistic case the radius of the nucleus is about 100,000 times smaller than the radius of the entire atom, and as far as we can tell electrons are point particles without a physical extent

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This similarity between a planetary model and the Bohr Model of the atom ultimately arises because the attractive gravitational force in a solar system and the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons in an atom are mathematically of the same form. (The form is the same, but the intrinsic strength of the Coulomb interaction is much larger than that of the gravitational interaction; in addition, there are positive and negative electrical charges so the Coulomb interaction can be either attractive or repulsive, but gravitation is always attractive in our present Universe.)

QUANTUM MECHANICAL MODEL •

According to the Principles of Quantum Mechanics electrons are distributed around the nucleus in "probability regions". These probability regions are called "atomic orbitals". According to Quantum Mechanics, these orbitals are mathematically defined and are described by a uniquely different math function for each electron in the atom called an "eigen function" and a differential equation generated by the following equation:

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H(eigen function) = Energy ( eigen function) The H in the above equation stands for a mathematical operator called the Hamiltonian. We should be familiar with math operators since we have been dealing with them since grade school. The addition operator has to operate upon two numbers one that appears on its left and the other on its right. For example the addition operator operates upon the number 4 and 3 and the result of that operation as all knows would be 7. We have subtraction, multiplier, division,common log, natural log,exponentiation,etc. The Hamiltonian operator is kinda like these but much more complex. The result of the Hamiltonian operator operating on the eigen function of an electron is to generate a differential equation. Differential equations often have more than one root or solution which is not new to those who have had a first year algebra course where quadratic equations are studied. However, one property of differential equations that might be new to you is the fact that differential equations cannot be solved exactly. We must use approximation methods to extract any roots out of the equation, and those roots or solutions will be approximate solutions.