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SGCh02 - Chapter Two ATOMS MOLECULES AND IONS Dalton's...

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23 Chapter Two ATOMS, MOLECULES, AND IONS Dalton's Atomic Theory Subatomic Particles and the Structure of the Atom Molecules and Ions Chemical Formulas Naming Inorganic Compounds DALTON'S ATOMIC THEORY STUDY OBJECTIVES 1. State the laws of definite proportions, multiple proportions, and conservation of mass. 2. List and discuss the postulates of Dalton's atomic theory. 3. Describe how Dalton's atomic theory explains these three laws. Atoms. According to John Dalton's atomic theory, which he proposed in 1808, elements are composed of extremely small, indivisible particles called atoms. Dalton assumed that atoms were the smallest unit of an element that can enter into chemical combination. Atoms of the same element all have the same mass, and atoms of different elements have different masses. His theory explained three laws that were known at that time. The atomic theory consists of a set of postulates which can explain the law of definite proportions, the law of multiple proportions, and law of conservation of mass . The postulates are summarized below. Dalton's Atomic Theory 1. All matter is made of extremely small, indivisible particles called atoms. An atom is the smallest unit of an element that can enter into chemical combination. Atoms of the same element are identical in size, mass, and chemical properties. 2. Atoms of one element are different from atoms of another element. 3. Compounds are formed when atoms from two or more elements combine. Atoms combine in the ratio of small whole numbers. For a given compound, the number of atoms of one element that combine per atom of the other element is a fixed ratio. The smallest particle that has the properties of the compound is called a molecule. 4. Atoms are the smallest units of chemical change. Chemical change involves the combination, separation, or rearrangement of atoms. Atoms are not destroyed or created in chemical processes. The law of definite proportions refers to the elemental constituents of compounds and states that all purified samples of a compound contain its constituent elements combined in the same proportions by mass. For example, all samples of the compound carbon monoxide, no matter what size, show that it is 43% carbon and 57% oxygen by mass. Dalton's proposal was that the smallest particle of carbon monoxide was a molecule consisting of one carbon atom and one oxygen atom. If all carbon atoms had the same mass, and all oxygen Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website
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24 / Atoms, Molecules, and Ions atoms had a mass about 1.3 times greater than carbon atoms, then the composition of carbon monoxide would be exactly as given above. While working on the atomic theory, Dalton discovered the law of multiple proportions : When two elements form more than one compound, the various masses of one element combining with a fixed mass of another element are related by small whole-number ratios. This law can be applied to two compounds of nitrogen and oxygen; nitrogen monoxide, and nitrogen dioxide. The mass of oxygen combining with nitrogen in
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