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userdata-paziras-Chem101-Chap_02A - Chemistry101 Chapter2...

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Chemistry 101 Chapter 2 1 ATOMIC MODEL All forms of matter are made up of PARTICLES These PARTICLES may be: ° atoms (Carbon atoms in diamond) ° molecules (H 2 O molecules in water) ° ions (Na + and Cl - ions in sodium chloride) The particulate nature of matter as we know it today evolved over a very long period of time. I. EARLY THOUGHTS (ancient Greek philosophers) Empedocles (440 B.C.) stated that all matter was composed of “four elements”: earth, air, water, and fire. Democritus (about 470­370 B.C.) thought that all forms of matter were finitely divisible into very small particles which cannot be divided further. He called these particles atoms.( atomos = indivisible in Greek) Democritus idea was a speculative hypothesis which was not based on scientific observations. Aristotle (384­322 B.C.) opposed the theory of Democritus, and the existence of atoms was no longer considered until the 17 th century II. DALTON’S ATOMIC THEORY John Dalton revived the concept of atoms and proposed an atomic theory based on indirect experimental evidence (1803­1810) Dalton’s Atomic Theory: ° explains the difference between an element and a compound ° explains two scientific laws, and ° predicts a new scientific law.
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Chemistry 101 Chapter 2 2 DALTON’S ATOMIC THEORY POSTULATES DEDUCTIONS All matter is composed of indivisible particles called atoms. An atom is a very small particle of matter that retains its identity during a chemical change. An element is a type of matter composed of only one kind of atom. All atoms of a particular element are identical (e.g. they have the same mass) Gives a more precise definition for an element. A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. Give a more precise definition for a compound. Supports the “Law of Definite Composition”. Predicts the “Law of Multiple Proportions. A chemical reaction consists of the rearrangement of atoms from one set of combination to another.
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