CHEMISTRY

THE STRUCTURE OF THE ATOM

Many people have conducted various experiments about the structure of the atom. John  Dalton's hypothesis (which was later accepted as a theory) was the first to give a clear and precise scientific suggestion on the chemical and physical features of the atom. From the works of J.J. Thompson, Rutherford, Moseley, Millikan, Faraday, Stoney etc, the atom is known to be made up of smaller particles called electrons, protons and neutrons. These are sometimes referred to as subatomic particles. 

Recent experiments have shown that some of the subatomic particles can be broken further into two elementary particles called quarks and leptons. Leptons and quarks are called elementary or fundamental particles. Under this topic, we are interested in the major experiments that led to the establishment of the main part (gross features) of the atom.

	Dalton's Atomic Theory John Dalton (1766-1844) was an English scientist, one of the founders of modern chemistry — through his quantitative formulation of an atomic theory — and a pioneer founder of modern meteorology. He taught mathematics and physical sciences at New College, Manchester, resigning his position in favor of lecturing, private tutoring and research, having begun his teaching career at the age of 12 years as founder and teacher of an elementary school in his local community. He developed experimental methods for determining the relative weights of the atoms of different elements, he formulated the 'law of partial pressures' (Dalton's law) — "....where the pressure exerted by each gas in a mixture [of gases] is independent of the pressure exerted by the other gases, and where the total pressure is the sum of the pressures of each gas."  — and he formulated a law about the combining of elements into compounds, the 'law of multiple proportions', a law that made sense only in the light of Dalton's atomic theory.Thinking atomisticly early on in his researches, Dalton inferred from experimental studies of the atmosphere, and other researches on gases, liquids and solids, an atomic theory of matter, the idea first suggested by the ancient Greeks, the Greek idea accepted without evidence by Robert Boyle and Isaac Newton earlier than Dalton, however, the first to provide its experimental support, establishing that all elements did not have the same mass and size, a finding that not only contributed to the development of his atomic theory but also led to an explanation for the 'law of conservation of mass' and the 'law of definite proportions'. (see text box at below right). Although Dalton's atomic theory received wide recognition early on for its important contribution to chemical understanding, skepticism persisted for many decades after its introduction, even in England. Dalton suffered from color-blindness and studied that affliction, which later physicians referred to as Daltonism.
	Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. He stated his theory in a lecture to the Royal Institution in 1803. The theory proposed a number of basic ideas; All matter is composed of atoms Atoms cannot be made or destroyed All atoms of the same element are identical Different elements have different types of atoms Chemical reactions occur when atoms are rearranged Compounds are formed from atoms of the constituent elements. Using his theory, Dalton rationalised the various laws of chemical combination which were in existence at that time. However, he made a mistake in assuming that the simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio, and his system of atomic weights was not very accurate - he gave oxygen an atomic weight of seven instead of eight. Despite these errors, Dalton's theory provided a logical explanation of concepts, and led the way into new fields of experimentation. This theory was very useful particularly in the formulation of the laws of chemical combinations. Points two and three  of the theory are now found not to be very correct, though the others are still important and useful. J.J. Thompson and Rutherford made important discoveries of the particles inside the atom. Both scientists proposed models of the atom. Dalton's Law Of Multiple Proportions Consider two elements, hypothetically symbolized as Y and Z, able to form a variety of different compounds with each other, say compounds A, B, and C, the different compounds consisting only of Y and Z, each with a different quantity of Y relative to a fixed quantity of Z. Dalton's law of multiple proportions states that the ratio of the quantity of Y in compound A to Y in compound B, or to Y in compound C, will compute as ratios of small integral (whole) numbers — for example, 2:1 or 3:1 or 3:2, etc. Using modern values for the atomic masses of elements, the existence of the two compounds of carbon (C) and oxygen (O), carbon monoxide (CO) and carbon dioxide (C02), offers a simple example to illustrate the law of multiple proportions. Carbon, the element of fixed mass in the two compounds, which today chemists take as having an atomic mass of 12, combines with two different masses of oxygen, namely 16 (atomic mass of oxygen) in carbon monoxide, 32 (sum of atomic mass of two oxygen atoms) in carbon dioxide. Those two masses of oxygen in the two compounds with a fixed mass of carbon relate to each other in the ratio 16-to-32, which equals 1-to-2, a ratio of two whole small numbers. Dalton would have determined the actual weights in grams of oxygen in the two compounds, adjusting them to a fixed weight of carbon, and calculated the ratio of oxygen in the two compounds therefrom. Thus, empirically observed, 10 grams of carbon combines with 13.3 grams of oxygen to form carbon monoxide, and 26.6 grams of oxygen to form carbon dioxide, the ratio 13.3:26.6 = 1:2 for oxygen in the two compounds. For another example, consider the elements nitrogen, N, and oxygen O. The element oxygen occurs in the compounds NO (nitric oxide) and NO2 (nitric dioxide). The ratio of oxygen weights (1:2) in those compounds contains the small whole numbers 1 and 2. Note that in modern chemistry the concept "number of atoms" replaces "weight", used by Dalton. Now chemists say that the ratio of numbers of O-atoms in different NOx compounds (or in different N2Ox compounds) is expressible as a ratio of small whole numbers. Bernard Jaffe, in his popular book on the history of chemistry, Crucibles, The Story of Chemistry: From Ancient Alchemy to Nuclear Fission, describes the history of the law of multiple proportions: While working on the relative weights of the atoms, Dalton noticed a curious mathematical simplicity. Carbon united with oxygen in the ratio of 3 [parts by weight] to 4 [parts by weight] to form carbon monoxide, that poisonous gas which is used as a fuel in the gas-range. Carbon also united with oxygen to form gaseous carbon dioxide in the ratio of 3 [parts by weight] to 8 [parts by weight]. Why not 3 to 6, or 3 to 7? Why that number 8 which was a perfect multiple of 4 [a ratio of 2-to-1]? If that were the only example, Dalton would not have bothered his head. But he found a more striking instance among the oxides of nitrogen, which Cavendish and Davy had investigated. Here the same amount of nitrogen united with one, two and four parts of oxygen to form three distinct compounds. Why these numbers which again were multiples of each other? He had studied two other gases, ethylene and methane, and found that methane contained exactly twice as much hydrogen as ethylene. Why this mathematical simplicity?  Thinking atomistically, Dalton literally figured out the answer, using the figures, or symbols, he had invented for an atom of each of the known elements: (see accompanying illustration.) See text. If, as Dalton's atomic theory proposed, elements consisted of atoms differing in weight among elements but of equal weights for the atoms of a given element, and compounds consisted of atoms of different elements, then when a fixed bulk weight of one element combined to form different compounds with another element, the ratio of the bulk weights of that other element in the different compounds must relate as ratios of small whole numbers, as those bulk weights reflect the accumulated weight of the atoms of that other element in single particles of the compounds, which themselves relate as ratios of small whole numbers. Jaffe reports that the Swedish chemist, Jöns Jacob Berzelius (1779-1848), a contemporary of Dalton, stated Dalton´s explanation as: In a series of compounds made up of the same elements, a simple ratio exists between the weights of one and the fixed weight of the other element. And in a letter to Dalton, Berzelius wrote: ….this Law of Multiple Proportions was a mystery without the atomic hypothesis. By extension of Dalton's law of multiple proportions, the subscripts m, n, k, ... in a compound AmBnCk⋅⋅⋅ are integral numbers (integers). In other words, the law applies to the ratio of the differing elements in a given compound as well as the ratio of the same element in differing compounds. The law of multiple proportions may have helped lead Dalton to his theory that an element consists of atoms of the same weight and that the weights of atoms of different elements differ, inasmuch as when the weights of element O, say, when it forms different compounds with a fixed weight of element N, say, the differing weights of O in the differing compounds relate as the ratio small whole numbers. The law of multiple proportions also helped lay the foundation for writing formulas for chemical compounds. Exactly how Dalton arrived at his theory of chemical atoms has many differing scenarios by historians of chemistry. The true scenario may not proceed through any logical, ordered sequence. Dalton assigned the mass of the lightest element, hydrogen, 1, as the unit of atomic mass, enabling him to determine the relative atomic weights of different elements — relative to the unit weight of hydrogen.  The law of multiple proprtions upholds the validity of the theory of matter's nature as comprising atoms of different masses, and therefore of different shapes and sizes. Nevertheless, as philosopher of science, Maureen Christie, points out, one cannot understand the law of multiple proportions as a universal and unexceptioned law of nature, such as the law of conservation of energy. She notes that one cannot express the law of multiple proportions as a precise proposition owing to the use of such imprecise words as 'small' and 'simple' in its expression: The law of multiple proportions as understood by chemists, includes ‘simple’ as an essential descriptor of the whole number ratios. The law therefore cannot be formulated as a precise proposition. It is clearly instanced, and exactly instanced, but it also has clear exceptions, and there are cases where it cannot be decided how the law applies....The hydrocarbons provide a series of examples of the difficulty: there are thousands of different compounds which contain just the two elements carbon and hydrogen. If a comparison is made between ethyne (acetylene - C2H2) and ethene (ethylene - C2H4), the law of definite proportions is very simply and obviously instanced. A sample of ethene containing the same mass of carbon as a sample of ethyne always has just exactly twice as much hydrogen as the ethyne, but if we compare pentane (C5H12) with ethyne, the ratio is 12:5 rather than the simpler 2:l. We could go on to compare heptane (C7H16)with butane (C4H10), obtaining a ratio of 35:32, or even C25H52 with C33H8, which gives 429:425! [footnote]. It can be seen that although the first comparison is in the ratio of simple whole numbers, the large range of compounds provides many comparisons which can be chosen to give ratios of almost any desired complexity. Certainly the last comparison could not fairly be regarded as providing an analysis in the ratio of simple [or small] whole numbers at all. J.J. Thompson's Experiment  (The discovery of electrons) J. J. Thomson was one of the great scientists of the 19th century; his inspired and innovative cathode ray experiment greatly contributed to our understanding of the modern world. Like most scientists of that era, he inspired generations of later physicists, from Einstein to Hawking. His better-known research proved the existence of negatively charged particles, later called electrons, and earned him a deserved Nobel Prize for physics. This research led to further experiments by Bohr and Rutherford, leading to an understanding of the structure of the atom. The cathode Ray Tube A cathode ray tube, or CRT, is "an vacuum tube in which a beam of electrons can be focused to a small cross section and varied in position and intensity on a dispaly surface". (Patrick, Norman W, "Cathode Ray Tube").This vacuum tube (a vacuum tube is an electron tube consiting of a sealed glass enclosure with no air) creates images when its phosphorescent surface is hit by electron beams. In the cathode ray tube device is a cathode, an electrode where electrons enter a system. The cathode rays are "streams of electrons emittted from the cathode of a dishcarege tube containing a gas in a vacuum". (Hemenway, C., "Cathode Rays".). Cathode Ray tubes can be found in everyday devises such as televisions, video game machines, computers, video cameras, monitors, auomated teller machines, oscilloscopes, and radar displays. (Bellis, Mary, "The History of the Cathode Ray Tube"). Who discovered the Electron? J.J. Thomson discovered the negatively charged subatomic particles of an atom called electrons in 1897. He determined that electrons are in all atoms of all elements. J.J. Thomson did various experiments that "involved passing electric current trhough gases at low pressure". ("Matta, Michael S., "Structure of the Nuclear Atom"). These gases were sealed in glass tubes that were closed by electrodes, and these electrodes were connected to a power source, which was high-voltage electricity. (Mary, Bellis, "The History of the Cathode Ray Tube"). One electrode, the anode, became positively charged while the cathode, which was the other electrode, became negatively charged. A cathode ray, which was a "glowing beam", formed between the two electrodes. A positive electrical charge attracted the cathode rays, where as a negative charge refused the rays. Since the cathode rays refused a negative charge, but were attracted by a positive charge, Thomson concluded that particles that make up the cathode rays are negative because opposite charges attract. (Matta, Michail S., "Structure of the Nuclear Atom"). (Hemenway, C. "Cathode Rays"). Parts of a Cathode Ray Tube: Cathode-The cathode is the metal electrode from which the electrons originate. The cathode is the negative electrode. Tube-The tube is a sealed glass tube from which most of the air has been removed. If the tube is full of air, it will not work. In order for the metal cathode to emit electrons, a strong electical current is needed; thus, a cathode ray tube needs a power source. A thin piece of metal coated with a material that emits light when struck by electrons is sealed inside the tube to detect the path of the electrons. Anode-The anode is the positive electrode and is the metal electrode towards which the electrons travel. Glass Tube-The glass tube connects the cathode ray tube to the stand. Stand-The stand supports the cathode ray tube. Alligator Clip-The alligator clip connects the metal electrode to the power source. ("Cathode Ray Tube".Chemistry Learning Center. Sept. 23, 2006.  The Cathode Ray Tube in Daily Life: CRT's are used in modern television. A CRT that is found in a television set would look far different from the simplier version seen above in the previous pictures. A televisions's CRT is far more complex. Gases conduct electric current at very low pressure (or in vacuum) at high voltages. Air was pumped out from the discharge tube and a high voltage (potential difference) was applied across the two metal plates or electrodes 2 and 3. When the electricity was switched on, a beam of rays (called cathode rays) was emitted from the cathode. Thompson observed that the cathode rays: 1. moved straight to the anode, passing through the hole in it to the zinc sulphide fluorescent screen and were detected as a glow at 'A'. 2. were deflected by both magnetic and electric fields. 3. were deflected away from the negative electric plate (3) and attracted towards the positive plate (4) of the secondary electrode when switched on. The rays landed at point B on the screen. 4. behave the same way when different gases, material of tube and the electrodes were changed. Explanation/Conclusions/Deductions 1. Deflection by electric and magnetic fields means that the cathode rays were electrical carriers. 2. Repelled by negative and attracted towards positive plate implies that the rays were negative charged. 3. Produced by all types of matter indicates that the cathode rays were present in all matter. Thompson identified and named the negative particles as electrons. He measured the charge to mass ratio (e/m) of the electron and found out that it was always the same for all matter. Upon these facts, Thompson proposed that An atom is a solid sphere of positive charge with negative electrons stuck uniformly in it, the negative charges balancing the positive charges to make the atom neutral. Rutherford's Alpha Particle Scattering Experiment In the early part of the last century, the accepted model of the atom was proposed by J J Thompson in his plum pudding model. This consisted of a matrix of protons in which were embedded electrons. Ernest Rutherford (1871 – 1937) used alpha particles to study the nature of atomic structure with the following apparatus: Instead of bits of atom, Rutherford found that a small proportion of the alpha particles were deflected, while an even smaller proportion bounced right back. From analysis of these observations he concluded: Most of the atom was empty space. The positive charge was concentrated in a very small space. The radius of the nucleus was in the order of 3x10-14m. The alpha particles that were deflected back had to be traveling in a line with the nucleus. Rutherford’s estimates were not far out. Later research has shown the nuclear radius to be in the order of 1.5x10-14m. However the boundary is not sharp, but rather fuzzy, as the nucleus is a very dynamic entity. Summary All atoms are made of a nucleus with orbiting electrons. The nucleus is made up of protons and neutrons, the nucleons. Isotopes have the same number of protons but numbers of different neutrons. Atoms are represented as:Where A is the nucleon number and Z is the proton number. Number of neutrons = A-Z The nucleus is positively charged and very small. Notes: Alpha particles are ionised helium atoms (He^2+) and are emitted by radioactive substances or elements.  It is by far heavier than an electron and ionises other particles easily. A thin film of alpha particles was directed straight onto the screen by placing a slit on their path. The rotating screen detects any alpha particles that fall on it by giving a flash or a glow at any point of the glass vessel. When the thin gold foil was placed between the slit and the screen, three observations were made: 1. Most of the alpha particles went through the thin foil without any change in path, 'A'. 2. A few alpha particles were deflected through small angles, 'B'. 3. Very few alpha particles were deflected backwards, 'C'. Explanation/Deductions 1. Scattering 'A' suggests that the negative electrons in atoms (as proposed by Thompson) are light and could easily be pushed out of the way of the heavy alpha particles. OR if attractions could occur between the electrons and the alpha particles, then the alpha particles passed through empty spaces and hence did not meet any attraction or repulsion. Since most alpha particles went through 'A', it means that most of the volume of an atom is occupied by electrons in empty space. 2. Scattering 'B' means that the few alpha particles came near a positively charged particle with a heavy mass (nucleus) and so were deflected (repelled). 3. Scattering 'C' was due to deflection by the heavy mass positively charged particles. The very few alpha particles scattered means that the volume occupied by the heavy mass center (nucleus) was very small. Rutherford conducted several experiments and predicted the presence of neutrons in the nucleus of atoms. Rutherford's model of the atom 1. The atom has a central nucleus, which has positively charged particles called protons and neutral particles called neutrons. The whole (bulk) of the mass of the atom is concentrated in the nucleus; and the nucleus occupies a small volume compared to the whole of the atom. 2. Electrons have negative charges; move around the nucleus in orbits; have very small mass compared with protons and are spread in an area of empty space. Present Day structure of the Atom