Chapter 2 (1)

Chapter 2 (1) - Chapter 2: Atoms, Molecules and Ions I....

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Chapter 2: Atoms, Molecules and Ions I. Historical Background Prior to 600 B.C, many chemical processes were practiced such as the extraction of metals from ores, preparation of medicines, dyes, brewing and baking, but why or how these occurred was not known. Between 600 and 300 B. C., the Greek philosophers proposed that all terrestrial substances were composed of four elements, earth, air, fire and water in varying proportions. A theory that matter consists of separate and distinct units called atoms was proposed by Leucippus and extended by Democritus in the 5 th century B.C. Plato suggested that the atoms of one element differ in shape from atoms of another and that one element can be changed into another by changing its shape. Aristotle believed that all substances are composed of the same primary matter and differ not only in shape but also in qualities that distinguish one substance from another. Between 300 B.C. and 1808 A. D., various theories about matter and chemistry were advanced, among them alchemy and the phlogiston theory. Modern chemistry began in the early 19 th century with John Dalton, an English scientist and teacher, who presented his atomic theory in the following postulates: 1. Elements are composed of extremely small particles called atoms . 2. All atoms of a given element are identical with the same properties, but differ from atoms of other elements. 3. Compounds are composed of atoms of two or more different elements in a definite ratio of integers or simple fractions. 4. In chemical reactions atoms are neither created nor destroyed, but rearranged. (Law of Conservation of Mass) 1
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II. The Structure of the Atom A. General Information 1. Atom - the basic unit of an element that can enter into chemical combination. 2. Subatomic particles - electrons, protons and neutrons B. Subatomic Particles 1. Electrons - negatively charged particles with negligible mass 2. Protons - positively charged particles in the nucleus of the atom 3. Neutrons - particles with no charge residing in the nucleus of the atom III. Atomic Number, Mass Number and Isotopes A. Atomic Number : Z - the number of protons in the nucleus of an atom B. Mass Number: A - the total number of protons and neutrons in the nucleus of an atom C. Isotopes - atoms that have the same atomic number but different mass number D. Representation of an atom: A - mass number (p + n) X Z - atomic number (p) E. Exercises: 1. How many protons, neutrons and electrons are in the following atoms? 235 92 U p = 238 92 U p = n = n = e = e = 2
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63 29 Cu p = 56 26 Fe p = n = n = e = e = 2. Give the correct symbol for the element with atomic number 30 having 35 neutrons. IV. Atomic Weight and Isotope Abundance Definitions 1. Atomic weight - the weighted average of all the naturally occurring isotopes of an element. 2.
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Chapter 2 (1) - Chapter 2: Atoms, Molecules and Ions I....

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