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ChemMod7Naming - Module 7 – Writing Names and Formulas...

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Module 7 – Writing Names and Formulas © 2011 www.ChemReview.Net v.n4 Page 120 Module 7 – Writing Names and Formulas Lesson 7A: Naming Elements and Covalent Compounds Pretest : If you think you know this topic, try the last letter of each question in Practice A and Practice B. If you get those right, skip the lesson. * * * * * Systems for Naming Substances Chemical substances are identified by both a unique name and a chemical formula. For names and formulas that both identify and differentiate substances, a system for writing formulas and names is required. 1. Some compounds have names that are non-systematic but familiar: Water (H 2 O) and ammonia (NH 3 ) are examples. 2. Historically, chemical substances have been divided into two broad categories. Compounds containing carbon and hydrogen are studied in organic chemistry , which has its own system for naming compounds. All other substances are part of inorganic chemistry , which is the focus of most first-year courses. 3. Different types of inorganic substances have different naming systems. We will begin with the rules for naming elements, ions, and binary covalent compounds. Naming Elements An element is a stable, electrically neutral substance that contains of only one kind of atom. The name of an element is simply the name of its atoms . Examples The element comprised of neutral atoms with 20 protons is called calcium . Calcium is a metal, and the formulas of metals are written as if they are monatomic elements. The formula for the element calcium is therefore written as Ca . Neutral oxygen atoms, at room temperature, are stable when they exist in diatomic molecules. For the element oxygen, the formula is O 2 . At room temperature, sulfur atoms tend to form molecules with 8 bonded atoms. The formula for the elemental form of sulfur is S 8 . Note that for elements, the formula easily distinguishes between monatomic, diatomic, or polyatomic structures, but the name does not. This is only an issue for a few of the elements, but for the millions of chemical compounds, a more systematic nomenclature (naming system) is needed.
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Module 7 – Writing Names and Formulas © 2011 www.ChemReview.Net v.n4 Page 121 Compounds In compounds, there is more than one kind of atom, but all the neutral molecules or formula units have the same atoms and structure. Most compounds can be classified as either ionic or covalent . Covalent compounds are molecules. They contain atoms bonded together by electrons shared between atoms. The attractive forces (bonds) within molecules are strong compared to the attractions between molecules. Compounds that are gases or liquids at room temperature are nearly always covalent compounds. At room temperature, compounds that are solids may be ionic or covalent, but ionic compounds are always solids. Ionic compounds are composed of an array of ions bonded strongly by electrostatic attraction.
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