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Elements and Isotopes ELEMENTS AND ISOTOPES - VOCABULARY element A species of atom; each atom of a particular element has a specific number of protons in the nucleus which is the same for all atoms of the element. isotope A species of atom; each atom of a particular isotope has a specific number of protons and a specific number of neutrons in the nucleus which are the same for all atoms of the isotope, but are not necessarily equal to each other. atomic number The number of protons in the nucleus of one atom; the atomic number defines an element. mass number The number that represents the sum of the numbers of protons and neutrons in the nucleus of one atom; a combination of the atomic number and the mass number defines an isotope. atomic weight (relative atomic mass) The average mass of all atoms of a particular element found in nature. It is also called relative atomic mass. It is expressed in atomic mass units (amu). On the atomic mass scale, the mass of one atom of carbon-12 is set up as a standard and is exactly 12 amu. naturally occurring isotope An isotope whose atoms are present and can be detected in nature. Naturally occurring isotopes could be stable (e.g., oxygen-17) or radioactive (e.g., uranium-238). They could be abundant or present in tiny amounts in the Earth crust, oceans, or atmosphere. stable isotope An isotope whose atoms have an average half-life of more than 10 billion years (1010 years; the age of Earth is about 4.5 billion years; the age of the universe is about 15 billion years). The decay of atoms of a stable isotope cannot be detected by any modern technique. radioactive isotope An isotope whose atoms undergo a detectable radioactive decay. Radioactive isotopes can be naturally occurring or synthetic. radioactive decay Transformation of an unstable atom that produces a more stable atom (or atoms) as well as radiation (energy and energetic particles). synthetic element (isotope) An element (isotope) whose atoms have been created in nuclear reactions as a result of human activity: nuclear reactors, supercolliders, nuclear bomb explosions. All synthetic elements (isotopes) are radioactive. ATOMIC WEIGHT AND ATOMIC MASS UNIT (From College Chemistry by J.L. Rosenberg, L.M. Epstein, and P.J. Krieger) 1. What is atomic weight? 2. 12 grams of carbon-12 contain approximately 6.022×1023 atoms. What is the value of one atomic mass unit (amu or u) in grams? 3. Although there is only one naturally occurring isotope of iodine, 127I, the atomic mass of iodine given in the modern periodic table is 126.90447, but not 127. Explain. 4. At one time there was a chemical atomic mass scale based on the assignment of the value 16.0000 to naturally occurring oxygen. What would have been the atomic weight, on such a table, of silver? 5. On the old atomic mass scale used by physicists, the mass of oxygen-16 atom was assigned to be exactly 16 amu. What would be the atomic weight of magnesium on that scale? 6. In a chemical atomic mass determination, the tin content in 3.7692 g of tin tetrachloride was found to be 1.7170 g. If the atomic weight of chlorine is taken as 35.453 amu, what is the value for the atomic weight of the tin isolated during this experiment? ATOMIC THEORY WORKSHEET 1. Which of the following statements of the atomic theory proposed by John Dalton at the beginning of the 19th century are not quite true in light of modern atomic physics? Rewrite each statement to reflect the current understanding of the atomic theory. An element is made up of atoms. All atoms of Atoms cannot be created or destroyed. a given element are identical. All atoms of one element have the same Atoms of different elements may combine in mass. Atoms of two different elements have the ratio of small, whole numbers to form different masses. compounds. 2. Calculate the atomic weight of neon using the data on its isotopes provided in the table below. Natural Number of Number of Mass Atomic Mass Isotope Abundance Protons Neutrons Number (amu) (atom %) 20Ne 10 10 20 19.9924 90.48 21Ne 10 11 21 20.9938 0.27 22Ne 10 12 22 21.9914 9.25 ATOMS, ISOTOPES, ELEMENTS Number Symbol Number of Number of of of the Atomic Mass Neutrons Electrons Mass of Name of Name of Protons Atom Number Number in the in the the Atom the Element the Isotope in the (Isotope) Atom Atom Atom 235.0439 235 92 235 92 143 92 uranium uranium-235 92U amu 238 146 sulfur-34 20 chlorine 39 19K 12 amu (exactly) 7 6 sodium-23 35Cl+37Cl+ Intensity 8890 92 94 96 98 100 Atomic mass, amu.
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