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                  Writing Formulas

 

Rules of writing formulas:

  • positive ion is written first this is usually a metal
  • negative ion is written second this is usually a nonmetal
  • subscripts are used to show how many ions of each part are in the compound. They are used to balance the charge of the ions.
  • criss-cross method:
  • Examples: sodium oxide sodium is the positive ion = +1
  • oxide is the negative ion = -2

    therefore it takes 2 sodium ions to balance the charge of the oxide

    Formula = Na2O

    calcium nitrate calcium is the positive ion = +2

    nitrate is the negative ion = -1

    therefore it takes 2 nitrates to balance the charge of calcium

    Formula = Ca(NO3)2

    aluminum sulfide aluminum is the positive ion = +3

    sulfide is the negative ion = -2

    therefore it takes 2 aluminum ions and 3 sulfide to balance the charge

    Formula = Al2S3

    Write the formulas for each of the chemical names.

    • barium chloride
    • potassium nitrate
    • aluminum oxide
    • silver bromide
    • calcium sulfate
    • magnesium fluoride
    • zinc hydroxide
    • barium carbonate
    • sodium nitride
    • lithium acetate
    • cesium chlorate
    • ammonium phosphide
    • calcium hydroxide
    • rubidium iodide
    • silver sulfide
  • CHEMICAL FORMULAS
  • Key Words: chemical symbols

    subscripts

    ions

    polyatomic ions

    cation

    anion

    Important Information: You must remember these things when writing chemical formulas.

    • Write the name of the element that comes first in the name
    • Write the name of the element that comes second in the name, but take off the ending and add ide
    • If the second part of the formula is a polyatomic ion, then write the name of the ion.
  • Examples: CaCl2 calcium chlorine calcium chloride
  • MgO magnesium oxygen magnesium oxide

    ZnS zinc sulfur zinc sulfide

    NaOH sodium hydroxide (notice this is one of your

    Polyatomic ions to know!!!)

    Write a word name for each of the following formulas.

    • LiOH
    • BeF2
    • SrS
    • KNO3
    • NaClO3
    • AlP
    • AgBr
    • NH4OH
    • ZnO
    • CaSO4
    • RbC2H3O2
    • NaCl
    • LiI
    • AlN
    • MgCO3
    • K2SO4

     

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