CH05_Tro_LectureNotes - Principles of Chemistry Online...

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Principles of Chemistry Online Lecture Notes Introductory Chemistry , 2 nd ed, by Nivaldo Tro Chapter 5: Molecules and Compounds In this chapter we will learn how compounds are formed, the types of chemical bonds in the compounds, how to write correct formulas and name the two types of compounds. Molecules and Compounds A molecule is a particle of matter in which there are two or more atoms combined together chemically. In Chapter 3, we learned that pure substances could be classified either as elements or compounds. Elements may exist as individual atoms, or as molecules, depending on the element. Elements that exist as molecules have more than one atom of the same type chemically joined together. A compound is made up of two or more elements (two or more types of atoms) which have been chemically combined and therefore exists as molecules. Examples of compounds are water, H 2 O; sulfuric acid, H 2 SO 4 ; carbon monoxide, CO. Compounds generally have completely different properties than the elements from which they are formed. For example, table salt, NaCl (sodium chloride), is commonly used to make our food taste better, but both sodium metal and chlorine gas can be quite harmful as individual elements. Law of Constant Composition In a compound, there are two or more different types of atoms present. However, it is important to realize that a compound has a fixed composition, whereas a mixture has variable composition. The Law of Constant Composition (sometimes called the Law of Definite Proportion) states that a compound will always be made up of the same elements combined in the same ratios by mass. For example, water will always have eight parts of oxygen for every part of hydrogen by mass. Chemical Formulas Chemical formulas provide us with a shorthand way of describing the makeup of a compound by listing the type of atoms present as well as the number of atoms in the smallest unit of the compound. In a chemical formula, the atom types are represented by the element symbol from the periodic table. If more than one atom of a particular type is present, a subscript numeral to the right of the element symbol specifies how many are present. For example, the formula of the compound ammonia is NH 3 ; one unit of ammonia contains one nitrogen atom and three hydrogen atoms. If there is only one atom of a particular type, the subscript “1” is not used. Use parentheses around a repeating group of atoms in a formula. Elements can be atomic or molecular As we stated previously, elements can exist as individual atoms ( atomic elements ) or as molecules ( molecular elements .) Molecular Elements: Rule of Sevens There are seven common molecular elements that occur naturally as 2-atom molecules: H 2 , N 2, O 2, F 2, Cl 2, Br 2, and I 2 . If you locate these elements on the periodic table, you will notice that beginning at nitrogen, six of these elements form a figure “7”. You must remember to add hydrogen into this group to account for all seven elements. This is called the rule of 7’s.
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This note was uploaded on 02/28/2012 for the course CSCE 3510 taught by Professor Unt during the Spring '12 term at North Texas.

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CH05_Tro_LectureNotes - Principles of Chemistry Online...

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