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Chemical Formulas: The Basics Subscripts in Chemical Formulas Numbers inside formulas. Smaller than the font size & lowered below the line of type. Subscripts show the number of atoms of each element in a representative unit. Rules for Subscripts in Formulas No subscript – it’s understood to be 1. Subscripts refer to the element immediately preceding them. Remember – every element symbol starts with an upper case letter. Subscripts after parentheses refer to everything inside ( ). Chemical Formulas H2O has 2 hydrogens and 1 oxygen NH3 has 1 nitrogen and 3 hydrogens CH4 has ? 1 carbon and 4 hydrogens CO2 has ? 1 carbon and 2 oxygens AgNO3 has ? 1 silver, 1 nitrogen, 3 oxygens Al(NO3)3 has ? 1 aluminum, 3 nitrogens, and 9 oxygens No. of atoms in formulas Al(NO3)3 Al = 1 N = 1 X 3 = 3 O = 3 X 3 = 9 Total = 13 Subscript outside ( ) X subscript inside ( ) = # of atoms Hint: Look for uppercase letters! No. of atoms in formulas Mg3(PO4)2 Mg = 3 P = 2 X 1 = 2 O = 4 X 2 = 8 Total = 13 Coefficients Numbers in front of formulas Refer to everything in the formula immediately following No coefficient? It’s understood to be 1. Coefficients 2H2O means 2(H2O) To count atoms take coefficient X subscript 2 X 2 = 4 H’s 2 X 1 = 2 O’s Hydrates A group of salts that have water molecules stuffed in the empty spaces Formulas are distinctive Ex: CuSO45H2O means “is associated with” or PLUS Not a true chemical bond, but the structure is definite so the number of empty spaces is definite and the number of water molecules is definite CuSO45H2O Count up the atoms! 1 X Cu 1XS 4XO 10 X H 5XO 5[CuSO45H2O] Count up the atoms! 5 X Cu 5XS 20 X O 50 X H 25 X O Coefficients X Subscripts Representative Unit Depends on the structure of the substance. For covalent substances, the representative unit is the molecule. For ionic substances, which make a crystal lattice, the representative unit is the unit cell. Covalently bonded substances make molecules. If you have more raw materials, you make more molecules. NaCl is an ionic compound. The ions are arranged in a crystal lattice. There is one Na+ for every Cl-. They are arranged in a repeating pattern. The more raw materials you have, the larger the crystal. It is not useful to have a “molecular” formula for ionic compounds – every crystal in the world is different. This diagram shows the relative sizes of the two ions. Use empirical formulas instead – give smallest whole number ratio. When all the ions are in the correct location, you can get a large “single crystal.” This model shows the position of the ions but not the relative sizes. Covalent vs. Ionic All Non-metals Molecules Definite size Molecular formula gives exact composition of molecule Metal & Nonmetal Crystal Lattice Indefinite size Empirical formula gives smallest whole number ratio of elements