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Chapter 5 - Chapter 5 Molecules and Compounds 5.1 o Sugar...

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Chapter 5: Molecules and Compounds 5.1 o Sugar and salt When elements combine to form compounds their properties change completely 5.2 o Compounds display constant composition The first chemist to formally state the idea that elements combine in fixed proportions to form compounds was Joseph Proust in the law of constant composition Law of constant composition: all samples of a given compound have the same proportions of their constituent elements. Even though atoms combine in whole-number ratios, their mass ratios are not necessarily whole numbers. 5.3 o Chemical formulas: how to represent compounds H 2 O is the formula for water; it indicates that water consists of hydrogen and oxygen atoms in a 2:1 ratio The subscripts in a chemical formula are part of the compound’s definition —if they change, the formula no longer specifies the same compound The subscripts in a chemical formula represent the relative numbers of each type of atom in a chemical compound; they never change for a given compound. Chemical formulas normally list the most metallic elements first. Therefore the formula for table salt is NaCl not ClNa. Metals are found on the left side of the periodic table and nonmetals on the upper right side. Within a single column in the periodic table, elements toward the bottom or more metal like than elements toward the top. When several groups of the same kind are present, their formula is set off in parentheses, with a subscript to indicate how many there are. For
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example Mg(NO 3 ) 2 indicates a compound containing one magnesium atom and two NO 3 groups. 5.4 o A molecular view of elements and compounds Atomic element; an element that exists in nature with single atoms as their basic units Molecular element: an element that does not normally exist in nature with single atoms as their basic unit, exist as diatomic molecules Molecular compound: a compound formed from two or more nonmetals. Base units are molecules.
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