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Molecules and Compounds

Molecules and Compounds
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Dr.SamuelHunt,United Arab Emirates,Teacher
Published Date:21-07-2017
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Chapter 5 Molecules and Compounds www.ThesisScientist.comMolecules and Compounds • Salt  Sodium—shiny, reactive, poisonous.  Chlorine—pale yellow gas, reactive, poisonous.  Sodium chloride—table salt. • Sugar  Carbon—pencil or diamonds.  Hydrogen—flammable gas.  Oxygen—a gas in air.  Combine to form white crystalline sugar. www.ThesisScientist.comLaw of Constant Composition • All pure substances have constant composition. All samples of a pure substance contain the same elements in the same percentages (ratios). Mixtures have variable composition. www.ThesisScientist.comCompounds Display Constant Composition If we decompose water by electrolysis, we find 16.0 grams of oxygen to every 2.00 grams of hydrogen. Water has a constant mass ratio of oxygen to hydrogen of 8.0. mass of oxygen Mass Ratio  mass of hydrogen 16.0 g  8.0 2.0 g www.ThesisScientist.comPractice—Show that Hematite Has Constant Composition if a 10.0 g Sample Has 7.2 g Fe and the Rest Is Oxygen; and a Second Sample Has 18.1 g Fe and 6.91 g O. www.ThesisScientist.comExample 5.1—Show that Two Samples of Hematite Are Consistent with the Law of Constant Composition. Given: Sample 1: 7.2 g Fe, (10.0-7.2) = 2.8 g O; Sample 2: 18.1 g Fe, 6.91 g O Find: proportion Fe:O Solution Map: element masses compound composition composition = mass Fe : mass O Relationships: Solution: Sample 1 Sample 2 7.2 g Fe 18.1 g O  2.6 2.61 2.8 g O 6.91 g C Since both samples have the same Compare: proportion of elements, hematite shows constant composition.Why Do Compounds Show Constant Composition? • The smallest piece of a compound is called a molecule. • If you have a pure substance, then every molecule will have the same number and type of atoms. • Therefore, your compound have the same predictable properties (physical & chemical). www.ThesisScientist.comFormulas Describe Compounds • Elements are represented by a letter symbol. • A pure compound is composed of atoms of two or more elements. • The number of each element is written to the right of the element as a subscript.  If there is only one atom, the 1 subscript is not written. • Polyatomic groups are placed in parentheses.  If more than one. www.ThesisScientist.comFormulas Describe Compounds, Continued Water = H O \ two atoms of 2 hydrogen and 1 atom of oxygen Table salt = NaCl, one sodium and one chlorine atom: notice that pure substances have consistent structures, these pure substance will also have consistent physical and chemical properties.Order of Elements in a Formula • Metals are written first.  NaCl • Nonmetals are written in order from Table 5.1.  CO 2  There are occasional exceptions for historical or informational reasons. H O, but NaOH . 2 Table 5.1 Order of Listing Nonmetals C P N H S I Br Cl O F in Chemical Formulas www.ThesisScientist.comPractice—Write Formulas for Each of the Following Compounds. • Hematite—Composed of four oxide ions for every three iron ions. Fe O 3 4 • Acetone—Each molecule contains six hydrogen atoms, three carbon atoms, and one oxygen atom. C H O 3 6 www.ThesisScientist.comMolecules with Polyatomic Ions Symbol of the polyatomic Symbol of the polyatomic ion called nitrate. ion called sulfate. Mg(NO ) CaSO 3 2 4 Compound called Compound called magnesium nitrate. calcium sulfate. Implied ―1‖ subscript Implied ―1‖ subscript on magnesium. on calcium. Parentheses to group two NO s. No parentheses for one SO . 3 4 www.ThesisScientist.comMolecules with Polyatomic Ions, Continued Subscript indicating No subscript indicating two NO groups. 3 one SO group. 4 Mg(NO ) CaSO 3 2 4 Compound called Compound called magnesium nitrate. calcium sulfate. Implied ―1‖ subscript Implied ―1‖ subscript on nitrogen, total 2 N. on sulfur, total 1 S. Stated ―4‖ subscript Stated ―3‖ subscript on oxygen, total 4 O. on oxygen, total 6 O.Practice—Determine the Total Number of Atoms or Ions in One Formula Unit of Each of the Following. • Mg(C H O ) 1 Mg + 4 C + 6 H + 4 O = 15 2 3 2 2 • (Hg ) (PO ) 6 Hg + 2 P + 8 O = 16 2 3 4 2 www.ThesisScientist.comClassifying Materials • Atomic elements = are single atoms. • Molecular elements = multi-atom molecules. • Molecular compounds = molecules made of only nonmetals. • Ionic compounds = Compounds made of cations and anions. www.ThesisScientist.comMore about Molecular Elements • Certain elements occur as diatomic molecules. • 7 diatomic elements—The Rule of 7s Find the element with atomic number 7, N. Make a figure 7 by going over to Group 7A, then down. The seventh element is H . 2 7A 7 H N O F 2 2 2 2 Cl 2 Br 2 I 2 www.ThesisScientist.comMolecular Compounds • Two or more nonmetals. • Smallest unit is a molecule. www.ThesisScientist.comIonic Compounds • Metals + nonmetals. • No individual molecule units, instead have a 3-dimensional array of cations and anions made of formula units. www.ThesisScientist.comMolecular View of Elements and Compounds www.ThesisScientist.comClassify Each of the Following as Either an Atomic Element, Molecular Element, Molecular Compound, or Ionic Compound, Continued. • Aluminum, Al = Atomic element. • Aluminum chloride, AlCl = Ionic compound. 3 • Chlorine, Cl = Molecular element. 2 • Acetone, C H O = Molecular compound. 3 6 • Carbon monoxide, CO = Molecular compound. • Cobalt, Co = Atomic element.