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ATOMS, ATOMIC THEORY, ELEMENTS, INTRO TO SYMBOLS AND FORMULAS
Lecture for 10/5/07
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Early ideas of the composition of matter
Democritus in 450 BC Aristotle and other well-known Greek
philosophers didn’t believe Democritus
Atoms are defined in terms of elements An atom is the smallest particle of an
element that retains the chemical identity of that element
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17th -18th century – new ideas emerge
1. Antoine Lavoisier2. Joseph Louis Proust: “Law of Constant
Composition”(1799) Compounds always contained the same ratio by weight
of their elements Concluded that a given compound always has the same
composition, no matter where it came from
3. John Dalton: Developed an atomic theory of matter in 1803 (called Dalton’s atomic theory)
1. Each element is composed of extremely small particles called atoms.
2. All atoms of a given element are identical, but they differ from those of any other element.
3. Atoms are neither created nor destroyed in any chemical reaction.
4. A given compound always has the same relative number and kinds of atoms.
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How can we be sure that atoms exist? Evidence that atoms exist is
overwhelming and conclusive Scanning tunneling microscope
provides pictures of atoms Chemistry is the study of two
worlds – macroscopic and submicroscopic Observations are made in the macroscopic
world, however, to understand this world one must try to understand the atoms that compose it.
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The Elements
Around 114 known elements and 92 are naturally occurring
The term “element” can mean different things:1. Microscopic form
2. Macroscopic form
3. Elements comprised of molecules Diatomic molecules: BrINClHOF
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Symbols and Formulas
Symbols for elements: Names are derived from Greek, Latin, and German Symbol is usually the first letter or first two letters of
the element name. Rule: first letter is capitalized, second is not
Formulas for compounds: Rules for writing formulas:
1. Each atom present is represented by its element symbol.2. The number of each type of atom is indicated by a subscript
written to the right of the element symbol.3. When only one atom of a given type is present, the subscript 1 is
not written.
hydrogen Hnitrogen Nboron Bargon Arcalcium Ca
nickel Ni
silver Aggold Au
tungsten Wcopper Cu
potassium Ksodium Nalead Pb