400 B.C. – Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms. Democritus expanded the idea to state that matter was composed of small particles called “atoms” that could be divided no further. These atoms were all composed of the same primary matter with the only differences between them being their size, shape and weight. The differences in these characteristics explained the differences in the properties of the matter around us. Unfortunately for Democritus and mankind in general, his ideas were largely ignored for the next 2000 years.
1704 – Isaac Newton theorized a mechanical universe with small, solid masses in motion. 1803 – John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together. While others had proposed very similar theories, John Dalton is usually credited with developing the first coherent atomic theory. Dalton’s theory can be summarized as follows:
1. Matter is composed of small particles called atoms.
2. All atoms of an element are identical, but are different from those of any other element. 3. During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged. 4. Atoms always combine in whole number multiples of each other. For example, 1:1, 1:2, 2:3 or 1:3 1896 – Henri Becquerel discovered radiation by studying the effects of x-rays on photographic film. In 1896, Henri Becquerel discovered that a sample of uranium was able to expose a photographic plate even when the sample and plate were separated by black paper. He also discovered that the exposure of the plate did not depend on the chemical state of the uranium (what uranium compound was used) and therefore must be due to some property of the uranium atom itself. After Becquerel abandoned this work, it was continued by Pierre and Marie Curie who went on to discover other radioactive elements including polonium, radium and thorium. She (Pierre was hit by a truck and killed in the middle of this work) further suggested that the uranium, and the new elements, were somehow disintegrating over time and emitting radiation that exposed the plate. She called this phenomenon “radioactivity”. They are at least two important points to notice:
The Term Paper on Atomic Theory Energy Atom Elements
Throughout the ages the study of what we call today chemistry has evolved into a highly developed point of study. One distinct element of chemistry is atomic theory. Throughout the ages atomic theory has been developed and extended by many different men who were all well-known chemists and physicists in their day. They developed the study of atoms from pure conjecture into known facts. John Dalton ...
1. The positive particles are bent toward the negative plate, the negative particles are bent toward the positive plate and the neutral particles are not bent in either direction. 2. The extent to which the path of a particle is bent as it passes through an electric field depends on its mass and its charge * the larger the charge on the particle, the further it is bent. * the larger the mass of the particle, the less it is bent. While alpha particles were determined to have a larger charge than the beta particles (+2 vs. -1), they also have over 7000 times the mass of the beta particle. Therefore, their path is bent much less than that of the beta particle. 1897 – J.J. Thomson determined the charge to mass ratio of electrons. At approximately the same time as radioactivity was being investigated, J.J. Thomson and others were performing experiments with cathode ray tubes. A cathode ray tube is an evacuated tube that contains a small amount of gas between two metallic plates. When a potential is placed between the cathode (the negatively charged plate) and the anode (the positively charged plate) a “ray” of electric current passes from one plate to the other. Thomson discovered that this ray was actually composed of particles.
The Essay on Cathode Rays Thomson Charge Atom
Joseph John Thomson was born on December 18, 1856 near Manchester, England. His father died when "J. J. .' was only sixteen. The young Thomson attended Owens College in Manchester, where his professor of mathematics encouraged him to apply for a scholarship at Trinity College, one of the most prestigious of the colleges at Cambridge University. Thomson won the scholarship, and in 1880 finished ...
When a second set of plates is placed around the tube, the ray is bent toward the positive plate indicating that the ray is composed of negatively charged particles. By varying the potential on the plates, Thomson was able to determine the mass to charge ratio of these particles.
In further experiments he varied what metal was used to make the electrodes and what gas was used to filled the tube. In each case, the properties of the ray particles were exactly the same. He concluded that the negatively charged particles were subatomic particles that were part of every atom. He further surmised that, since atoms were electrically neutral, the atom must also contain some positive charge. Based on these conclusions Thomson proposed that an atom was composed of a spherical ball of positive charge with “corpuscles” of negative charge imbedded in it. The corpuscles would later become known as electrons.
1898 – Rutherford discovered alpha, beta, and gamma rays in radiation. In 1909, Rutherford set a fellow scientist, Hans Geiger, and a student, Ernest Marsden, to work on this problem. They devised a system that allowed alpha particles (the nuclei of helium atoms) to be shot at a very thin piece of gold foil and the trajectory of the particles monitored. They observed that while most of the particles passed through the foil with little or no deflection, some were deflected to a great degree.
In order to account for the fact that many of the alpha particles passed through the gold film, Rutherford discounted Thompson’s solid ball model of the atom, and believed that the central positive charge of the atom represented only a small fraction of the atom’s size, and that the remainder was primarily empty space. He calculated that, while an individual atom was about 1×10-10 meters in diameter, the nuclear diameter was only about 1×10-14 meters.
The nucleus, the red dot, is not shown to scale (it is really much smaller)
While solving the problem of the observed alpha particle deflection, Rutherford’s model created another. If the positive charge was located at the center of the atom, why were the negatively charged electrons not immediately drawn into it (opposite charges attract).
The Essay on Rutherford Alpha Particles
... large angles. Rutherford concluded that the deflections of the alpha particles were caused by a center of positive charge that contained most of the atom's mass ... (the nucleus). He also explained that the particles ...
Rutherford was not unaware of this problem but his model so adequately (and mathematically) explained the scattering results that it became widely accepted. Neils Bohr-1913—Bohr made numerous contributions to our understanding of atomic structure and quantum mechanics. Geiger-1925—Geiger was the first scientist to explain DNA.
Erwin Schroedinger-1926—Schroedinger explained movement of an electron in an atom as a wave. Chadwick-1931—British physicist James Chadwick measured the energy of the protons emerging from the hydrogen atoms.
Otto Hahn-1938—German physical chemist and Nobel laureate, best known for his contributions in the field of radioactivity.
Murray Gell-1964—United States physicist, noted for his classification of subatomic particles and his proposal of the existence of quarks.
