Atomic structure A. Introduction: In 1808, an English scientist called John Dalton proposed an atomic theory based on experimental findings. (1) Elements are made of extremely small particles called atoms. (2) Atoms cannot be divided or destroyed. (3) Atoms of any elements are identical and have the same mass. (4) Atoms of different elements have different masses. (5) Compounds are formed by the combination of small whole numbers of different atoms. The theory stated that atoms are indivisible and indestructible. However, we all know that atoms can further sub-divided into sub-atomic particles -neutrons, protons and electrons. Ernest Rutherford was born in New Zealand but he carried out most of his research at British universities. He first worked with another famous scientist called J.J. Thomson. Thomson was the first to realize that all matter contained electrons. Rutherford discovered that atom is made of a "nucleus" containing protons and neutrons, surrounded by electrons. As electrons are negative, it meant that the atom must also contain positive particles. Rutherford set up an experiment to find out how these positive particles and electrons were arranged within the atom. The following diagram shows the set-up. Nearly all the alpha particles passed straight through the gold sheet, but to his great surprise, about one alpha particle in 10,000 bounced back! From this experiment, Rutherford deduced that the atom must be made mainly of empty space, but there must also be a very very tiny region with a very dense positive charge. He called this region the nucleus. The following diagram explained the deduction. For convenience, we may consider an atom in two parts: the nucleus and the electrons.
2 B. The nucleus: i. Radioactivity Besides the existence of isotopes, nuclei also produce three kinds of radiation. According to their response to the electric field, we call them α (alpha) ray (double positive charged helium ion), ß (beta) ray (a stream of electrons) and γ (gamma) ray. The following diagram shows the effect of electric field on these rays: The uranium and thorium are the natural radioactive atoms. 238 U 234 Th + 4 He (α decay) 92 90 2 234 Th 234 Pa + 0 e (ß decay) 90 91-1 The above equations are nuclear equation. In the equation for a nuclear reaction, the sum of the neutron numbers (mass number) is the same on both sides, and the sum of proton numbers (atomic numbers) is also the same on both sides of the equation. For example, the artificial radioactivity of 16 N. 7 16 N a O + o e 7 b -1 considering neutron numbers gives: 16 = a + 0, => a = 16 considering proton numbers gives: 7 = b + (-1), => b = 8 Classwork: That means after the ß decay, 16 N produce 16 O. 7 8 The overall reaction is : 16 N 16 O + o e 7 8-1 Find the values of x and y in the following nuclear equations. 1. 14 C x N + o e y 7-1 2. 73 As + o e x Ge 3. y -1 32 x Ra 4 He + 222 Rn 88 2 y Uses of radioactive isotopes:
3 a. Cancerous tissue is destroyed by radioactivity in preference to healthy tissue. b. Surgical instrument can be sterilised more effectively by radioactivity than by boiling. c. A production line use for radioactivity is to check whether cans have been correctly filled. d. To detect and regulate the thickness of aluminium foil during its production. e. Underground leaks in water or fuel pipes can be detected by introducing a short-lived radioisotope into the pipe. The level of radioactivity on the surface can be monitored. A sudden increase of surface radioactivity shows where water or fuel is escaping. f. Carbon-14 dating: The method is used to calculate the age of plant and animal remains. Living plants and animals take in carbon, which includes a small proportion of the radioactive isotope carbon-14. When a plant or animal dies, it takes in no more carbon-14, and that which is already present decays. The rate of decay decreases over the years, and the activity that remains can be used to calculate the age of the plant or animal material. g. Tracer studies use radioactive isotopes to track the path of an element through the body or in certain chemical reactions in which the mechanisms are being studied. h. Nuclear power is generated from the isotope of uranium. Energy from nuclear reactor is obtained by fission of 235 U, carried out in a controlled ways. The following diagram shows the chain reaction of fission of 235 U. (refer to page. 4)
4 The following diagram shows the simplified structure of nuclear reactor. ii. Isotopy Nucleus consists of protons and neutrons. The word nuclide is used to describe any atomic species of which the proton number and neutron number are specified. For example: 12 C and 9 Be are nuclides of carbon and beryllium. N.B. mass number = number of protons and neutrons atomic number = number of protons In an element, there may exist different atoms with different masses which are due to the different number of neutrons inside the nuclei. Isotopes are nuclides of the same element. For example, chlorine-35 ( 35 Cl) and chlorine-37 ( 37 Cl). Relative isotopic mass is the relative mass of a specific isotope of an element, for example, the relative isotopic mass of 35 Cl is 35, and the relative isotopic mass of 37 Cl is 37. Relative atomic mass is the relative mass of an atom of an element taken account of
all isotopes, for example, the relative atomic mass of Cl is 35.5. Such phenomenon has been studied by a device called Mass spectrometer. The technique is termed mass spectrometry. The following diagram shows a simple structure of mass spectrometer. 5 Scientists can change either the electric field or magnetic field or both, a particle with specific mass/charge value can be detected. By plotting a group (relative abundance against relative mass), scientists can 1. prove there exist different nuclides (isotopes) 2. calculate the relative atomic (molecular) mass of substances. For example, the diagram shows the mass spectrum of natural magnesium. The relative atomic mass of Mg = Classwork: 1. Referring to the diagram below,
6 a. State how many isotope(s) of neon are there; b. calculate the relative atomic masses of the neon atom. Class discussion: 1. Consider the following mass spectrum of an organic compound. Predict the molecular formula of the compound. relative abundance 94 96 15 79 81 95 97 m/z 2. The following diagram shows the spectrum of a hydrocarbon. Predict the structural formula of the compound. relative abundance 110 110 Hint : the ration of 12 C : 13 C = 89:1 100 5.62 15 57 n n+1
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