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Slide 1 Section 3.2 Atoms & Compounds Objectives To learn about Dalton’s Theory of atoms To understand and illustrate the Law of constant composition To learn how a formula describes a compound’s composition Slide 2 Dalton’s Atomic Theory Elements are made of tiny particles called atoms All atoms of a given element are identical The atoms of a given element are different from those of any other element Atoms of one element can combine with atoms of another element to form compounds. A given compound always has the same relative numbers and types of atoms. This is called the law of constant composition Slide 3 Dalton’s Atomic Theory Atoms are indivisible in chemical processes. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together. Slide 4 Formulas of Compounds A compound is a distinct substance that is composed of atoms of two or more elements A compound always contains the same relative masses of those elements In a chemical formula, the atoms are indicated by the element symbols and the number of each type of atom is indicated by a subscript Slide 5 Rules for Writing Formulas Each atom is represented by its element symbol The number of each type of atom is indicated by a subscript written to the right of the element symbol When only one atom of a given type is present, the subscript 1 is not written Slide 6 Formula Practice SO3 – 1 sulfur atom and 3 oxygen atoms N2O3 -- 2 nitrogen and 3 oxygen PCl3 -- 1 phosphorus, 3 chlorine CBr4 -- 1 carbon, 4 bromine A compound containing two iron atoms and three oxygen atoms Fe2O3 A compound containing three hydrogen atoms, one phosphorus atom and four oxygen atoms H3PO4 A compound containing one hydrogen, one chlorine, and four oxygen atoms HClO4