Question 1B

Niels Bohr- A model of the atom, first described by Niels Bohr, that explains the emission and absorption of radiation as transitions between stationary electronic states in which the electron orbits the nucleus at a definite distance. The Bohr model violates the Heisenberg uncertainty principle, since it postulates definite paths and momenta for electrons as they move around the nucleus. Modern theories usually use atomic orbitals to describe the behavior of electrons in atoms.

Ernest Rutherford- Ernest Rutherford explains his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (mostly gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'. His model didn't make any headway in the explanation of the electrons but just kinda helped in the understanding of the past and present atomic structures.

William Crookes- This fascinating scientist discovered the phenomenon upon which depends the action of the well-known little instrument, the Crookes radiometer, in which a system of vanes, each blackened on one side and polished on the other, is set in rotation when exposed to radiant energy. He did not, however, provide the true explanation of this apparent attraction and repulsion resulting from radiation. Of more fundamental importance were his researches on the passage of the electrical discharge through rarefied gases. He found that as the attenuation of the gas was made greater the dark space round the negative electrode extended, while rays, now known as cathode rays, proceed from the electrode. He investigated the properties of the rays, showing that they travel in straight lines, cause phosphorescence in objects upon which they impinge, and by their impact produce great heat. He believed that he had discovered a fourth state of matter, which he called "radiant matter". But his theoretical views on the nature of "radiant matter" proved to be mistaken. He believed the rays to consist of streams of particles of ordinary molecular magnitude. It remained for J. J. Thomson to discover their subatomic nature, and to prove that cathode rays consist of streams of negative electrons, that is, of negatively electrified particles whose mass is only 1/1,800 that of the atom of hydrogen.

J.J. Thompson-Thomson's work suggested that the atom was not an "indivisible" particle as John Dalton had suggested but, a jigsaw puzzle made of smaller pieces.
Thomson's notion of the electron came from his work with a nineteenth century scientific curiosity: the cathode ray tube. For years scientists had known that if an electric current was passed through a vacuum tube, a stream of glowing material could be seen; however, no one could explain why. Thomson found that the mysterious glowing stream would bend toward a positively charged electric plate. Thomson theorized, and was later proven correct, that the stream was in fact made up of small particles, pieces of atoms that carried a negative charge. These particles were later named electrons. The present day atom has electrons in it so that has not changed. His discovery played a huge part in the constructing of the present day model.

John Dalton-a study of Dalton's own laboratory notebooks, discovered in the rooms of the Lit & Phil, concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases. Dalton proceeded to print his first published table of relative atomic weights. Six elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. Dalton provided no indication in this first paper how he had arrived at these numbers. However, in his laboratory notebook under the date 6 September 1803 there appears a list in which he sets out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time. His discoveries led to the discovery of a few elements his atomic theory has also played a major role in the new discoveries.


H.G.J Mosley-Before Moseley and his created law, atomic numbers had been thought of as a semi-arbitrary ordering number, vaguely increasing with atomic weight but not strictly defined by it. Moseley redefined the idea of atomic numbers from its previous status as an ad hoc numerical tag to help sorting the elements, in particular in the Periodic Table, into a real and objective whole-number quantity that was experimentally measurable. Furthermore, as noted by Bohr, Moseley's law provided a reasonably complete experimental set of data that supported the conception by Ernest Rutherford and Antonius Van den Broek of the atom, with a positively-charged nucleus surrounded by negatively-charged electrons in which the atomic number is understood to be the exactly physical number of positive charges in the central atomic nuclei of the elements. Simple modification of Rydberg's and Bohr's formulas were found to give theoretical justification for Moseley's empirically-derived law for determining atomic numbers. The atomic number is one of the most important things of an atom. It has helped us understand so much about current atoms. His dicovery was a very important and useful discovery.

Dmitri Mendeleev- Developed the first periodic table when trying to classify elements not by accidental or instinctive reasons, but by a set principle. He believed it should be numerical in nature to eliminate any margin of arbitrariness. The trend of increasing atomic mass allowed him to discover a periodicity of elemental properties. The first model used vertical columns and showed that there were some missing places where there could be undiscovered elements.

James Chadwick- discovered a third type of subatomic particle, which he named the neutron. Neutrons help stabilize the protons in the atom's nucleus. Because the nucleus is so tightly packed together, the positively charged protons would tend to repel each other normally. Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms and they are about the same size as protons. However, neutrons do not have any electrical charge; they are electrically neutral.
Atoms are electrically neutral because the number of protons + charges is equal to the number of electrons - charges and thus the two cancel out. As the atom gets larger, the number of protons increases, and so does the number of electrons in the neutral state of the atom. The illustration linked below compares the two simplest atoms, hydrogen and helium. The present day model still consists of the protons. His discovery is still a very important factor today and for the present day atom.