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Elements and Compounds. Section 3.4. Elements. An element is a pure substance that cannot be separated into simpler substances by physical or chemical means. Chemical names come from latin or other languages Chemical symbols are one two or (rarely)three letters long

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elements
Elements
  • An element is a pure substance that cannot be separated into simpler substances by physical or chemical means.
    • Chemical names come from latin or other languages
    • Chemical symbols are one two or (rarely)three letters long
    • First letter is always uppercase
    • Other letters are always lowercase
    • Sodium Na
    • Carbon C
periodic table
Periodic Table
  • 91 naturally occurring elements
  • Varying abundances
  • Mendeleev
    • Devised the first periodic table by atomic mass
    • Proposed that some elements were missing
    • Groups (columns) or families have similar chemical and physical properties
    • Rows are called periods
slide5

Argon and potassium should be reversed if arranged by atomic mass. Mendeleev correctly arranged them thinking mass was wrong.

compounds
Compounds
  • A compound is a combination of two or more different elements that are combined chemically.
    • Water, salt, aspirin and sugar
    • About 10 million know compounds
    • About 100,000 new compounds each year
    • Can be divided into simpler substances by chemical means
law of definite proportions
Law of Definite Proportions
  • The law of definite proportions states that, regardless of the amount, a compound is always composed of the same elements in the same proportion by mass.
slide10

Table 3-4 Sucrose analysis from bag sugar

Table 3-5 Sucrose analysis from sugar cane

practice problems
Practice Problems
  • 20-24 on page 76
law of multiple proportions
Law of Multiple Proportions
  • The law of multiple proportions states that when different compounds are formed by the combination of the same elements, different masses of one element combine with the same relative mass of the other element in a ratio of small whole numbers.
    • H2O
    • H2O2
slide13

Table 3-6 Analysis data of two Copper Compounds

1.793 g

Mass ratio compound 1

=

= 2.000

Mass ratio compound 2

0.8964 g