CHM 201 Lecture Series 2022/2023 PDF

CHM 201 LECTURE SERIES. 2022/2023 SESSION DR T. A. AJAYEOBA. LECTURE II Anode (positive) rays: Discovery of Protons. Atom is electrically neutral, and has been found to have negatively charged particles known as electrons, it should have some particles which should be positively...

CHM 201 LECTURE SERIES. 2022/2023 SESSION DR T. A. AJAYEOBA. LECTURE II Anode (positive) rays: Discovery of Protons. Atom is electrically neutral, and has been found to have negatively charged particles known as electrons, it should have some particles which should be positively charged. Attempts were made by many scientists to show the presence of positively charged particles. In 1886, German physicist, Goldstein took a discharge tube provided with a cathode perforated with extremely fine holes and observed that when a large potential difference was applied between the electrodes, not only cathode rays or electrons were originated from cathode, but also new type of colored rays was simultaneously originated from the anode, moving from anode towards cathode through the fine holes or canals in the cathode and causing a glow on the wall opposite to anode. The search for the predicted positive charge continued in other to counterbalance the negatively charged electrons. However, it was Rutherford that was also credited with the discovery of proton through his Gold foil experiment.  Production of positive rays in the discharge tube. https://byjus.com/chemistry/proton-neutron-discovery CREDITING RUTHERFORD WITH DISCOVERY OF PROTON In 1909, Ernest Rutherford, along with collaborators Hans Geiger and Ernest Marsden, initiated the renowned gold foil experiment at the University of Manchester. Alpha particles were directed at a thin gold foil, with observations made on the scattering patterns. Contrary to expectations based on the prevailing model, some alpha particles experienced significant deflections or even rebounded. Rutherford astutely interpreted these outcomes as indicative of a concentrated, positively charged nucleus within the atom. Identification of the Proton: The subsequent step involved the identification of the specific particle responsible for the positive charge. Rutherford, in 1919, directed alpha particles at nitrogen gas, resulting in the emission of hydrogen nuclei. This established the presence of a positively charged particle, later confirmed as the proton. CREDITING RUTHERFORD WITH DISCOVERY OF PROTON CONT’D Ernest Rutherford observed that his scintillation detectors detected hydrogen nuclei when a beam of alpha particles was shot into the air. After investigating further, Rutherford found that these hydrogen nuclei were produced from the nitrogen atoms present in the atmosphere. He then proceeded to fire beams of alpha particles into pure nitrogen gas and observed that a greater number of hydrogen nuclei were produced. He concluded that the hydrogen nuclei originated from the nitrogen atom, proving that the hydrogen nucleus was a part of all other atoms. This experiment 14 was 17 the first to report a nuclear reaction, given by the equation: N + α → O + p [Where α is an alpha particle which contains two protons and two neutrons, and ‘p’ is a proton] The hydrogen nucleus was later named ‘proton’ and recognized as one of the building blocks of the atomic nucleus Properties of positive rays as observed by JJ Thomson 1. The particles constituting positive rays are positively charged. They were deflected towards the negatively charged plate in an electromagnetic field. 2. The charge-to-mass ratio (e/m) for positive rays is smaller than that for an electron. 3. The positive rays produces flashes on ZnS plate. 4. The rays travel in a straight line in a direction opposite the cathode. 5. Charge-to-mass ratio value: it was found that e/m values for different gases filling the discharge tube was not the same. When hydrogen gas was filling the tube, the e/m value was found to be highest. Since the value of the charge (e) was the same for all gases, it means that the value of the (m) should be lowest for the positive particle obtained from hydrogen gas. Thus, positive particle of hydrogen gas is the lightest among all positive particles obtained from different gases. These particles were called protons, given by Rutherford. 6. Charge on a proton. Charge on a proton is equal in magnitude but opposite -19 in sign to that of an electron. Thus, the charge carried by a proton is equal to +1.602 x 10 coulombs, which is taken to be equal to one units of positive charge (+1). Thus, a proton is said to carry one unit of positive charge on it. DISCOVERY OF NEUTRON Up till 1932, it was predicted that the atom is composed only of electrons and proton, and that the mass of an atom is due to the mass of proton present in the nucleus, since the mass of electron is negligible. Mass of each proton is equal to 1.00375 atomic mass. Unit. (a.m.u). Rutherford noticed that the atomic masses of different atoms could not be explained if it is composed of only protons and electrons, and therefore, he predicted in 1920 that some kind of neutral particles having mass equal to protons must be present. DISCOVERY OF NEUTRON, CONT’D. The discovery of neutrons can be traced back to the year 1930 when the German nuclear physicists Herbert Becker and Walther Bothe observed that a penetrating form of radiation was produced when the alpha particles emitted by polonium was incident on relatively light elements such as lithium, beryllium, and boron. This penetrating radiation was unaffected by electric fields and was, therefore, assumed to be gamma radiation. The British physicist Sir James Chadwick discovered neutrons in the year 1932. He was awarded the Nobel Prize in Physics in the year 1935 for this discovery. DISCOVERY OF NEUTRON, CONT’D DISCOVERY OF NEUTRON, CONT’D https://www.aakash.ac.in/important-concepts/chemistry/discovery-o f-protons-and-neutrons Properties of Neutrons Properties of Neutrons Henry Moseley and Atomic number. Moseley in 1913, bombarded a number of elements with cathode rays (electron). The energy provided by the cathode rays caused the elements to give off X-rays. Moseley investigated the connection between the frequency of the X-rays and the nature of the elements giving them off. He found that as the atomic mass increases, the wavelength of the resultant x-ray decreases. From this results, it was deduced that there must be a fundamental property of the atom of elements which increases regularly as the atomic mass increases. Moseley called this, the number of proton in the nucleus of the atom. 13 d Electrons Values of the charge and Mass protons, neutrons and electrons Particles Charge/coulomb Actual mass/Kg Ratio of mass Proton 1.602 x 10-19 positive (+) 1.673 x 10-27 1 Neutron 0.0 Nil 1.675 x 10-27 1 Electron -1.602 x 10-19 Negative (-) 9.109 x 10-31 14 Ernest Rutherford’s Gold Foil Experiment Ernest Rutherford and his co-workers in (1911) performed the gold foil experiment that gave direction to the modeling of the atomic structure He bombarded a thin gold foil with alpha particles (generated from a radioactive source). He found out that most of the alpha particles (α) passed through the foil in a straight line (undeflected) while a few of the particles (about 1 out of 8000) were deflected back at various degrees. A scintillation plate was used to locate the ray after passing through the foil. The explanation is that most alpha particles were able to pass through because the atom consist mainly of empty space. A few of α-particles were deflected because they approached the positively charged nucleus. The deflections were few because the nucleus is very small such that only few particles got near it. Ernest Rutherford’s Gold Foil Experiment cont’d. Rutherford then suggested an atomic model called the nuclear model in which an atom has a small positively charged centre (nucleus) where nearly all the mass are concentrated, and large space (extra nuclear space) that surrounds the nucleus containing the electrons. He posited that: (1) the mass and the positive charge must be concentrated in a tiny fraction of the atom called the nucleus. (2). Negatively charged electrons revolve around the nucleus in closed orbits with a fast sdpeed and hence, almost all the space round the nucleus is occupied by the revolving electrons. (a comparison with solar system). Thomson’s Plum pudding atomic model and Rutherford’s Nuclear model of the Atom. The two models suggested that atom is made up of largely empty space. Rutherford further Suggested the presence of a positively charged nucleus in the centre where most of the mass Is located. Culled from large.stamford.edu 17 Rutherford’s gold foil scattering experiment. Culled from Zigya.com 18 19 Limitations of Rutherford’s Nuclear Model of the Atom. As predicted by Rutherford that electrons are revolving round the nucleus, one could show that an electrically charged particles (such as electron) that revolved around a positively charged centre would continuously lose energy as electromagnetic radiation and it will spiral (collapse) into the nucleus (in about 10-10 sec). It then means atom would not be stable, hence there will be no matter. But that is actually not the situation. Atoms are stable hence we have matters. Limitations of Rutherford’s Nuclear Model of the Atom cont’d http://www.entelki.in/content/drawbacks-rutherford%E2%80%99s-model-atom 21 Bohr’s Atomic Model (1885-1962) Niels Bohr proposed an atomic model that helped solve the problem inherent in Rutherford’s model. Bohr put forward an atomic model which is based on Planck’s quantum theory of radiation. Bohr proposed a theory of atomic structure and provide a quantitative prediction of line spectra. He began with the planetry model but introduced certain assumptions which were: Bohr’s Atomic Model (1885-1962) cont’d (1). that the electron moves in an orbit around the central nucleus and only certain orbits are allowed. (II). Electron do not radiate energy when in these orbits, and it has associated with it, a definite amount of energy. (III). Under certain condition, transition between these stationary states (orbit) do occur and these transitions are accompanied by emmission or absorption. TO BE CONTINUED IN THE NEXT SLIDE