Chemical Formula of a Compound

Concepts you need to know to be preparedA compound’s chemical formula is made up of the symbols (from the Periodic Table) that represent each of theelements in that compound. And the numerical subscripts following these symbols indicate the fixed proportion inwhich the elements combine when that compound is formed by a chemical reaction.For an ionic compound (such as sodium chloride, NaCl), youshould NEVER think of that compound as just a PAIR ofoppositely-charged ions, as the chemical formula seems to imply.If you look closely at the illustration of a sodium chloride (NaCl)salt crystal to the right, you’ll see that it is definitely NOT just apair of Na + and Cl - ions, but an extensive, orderly, threedimensionalnetwork of charged particles.Each Na + ion is attracted to the Cl - ions immediately above,below, to the right, to the left, in front, and behind it. And eachCl - ion is attracted to the Na + ions surrounding it.So it’s important to realize that the chemical formula for ANYionic compound does NOT describe a small grouping of ions, itdescribes the fixed proportion in which vast numbers of theoppositely-charged ions combine in order to appropriatelybalance out the positive and negative charges.So – one way to think about the chemical formula for Copper (II) Chloride, CuCl 2 , the ionic compound in thisweek’s experiment, is that it expresses the “ION ratio” of the elements in the compound:The compound CuCl 2 contains 2 Cl - IONS for every 1 Cu 2+ ION.But a “bigger picture” way to think about that chemical formula is that it expresses the “MOLE ratio” of theelements in the compound:The compound CuCl 2 contains 2 MOLES of Cl - ions for every 1 MOLE of Cu 2+ ions.One mole of any pure substance (element or compound) is really just an Avogadro’s number of “particles” of thatsubstance (where “particles” = atoms, ions, or molecules). For example:Or in the case of Copper (II) ions:1 mole of Copper metal = 6.02 x 10 23 Cu atoms1 mole of Copper (II) ions = 6.02 x 10 23 Cu 2+ ionsAn Avogadro’s number of atoms for a particular element, one MOLE of the element, will ALWAYS have a mass inGRAMS equal to the Atomic Weight listed for that element on the Periodic Table:Or for Copper (II) ions:1 mole of Copper (Cu) = 6.02 x 10 23 atoms of copper = 63.55 grams of Copper metal1 mole of Copper (II) ions = 6.02 x 10 23 Cu 2+ ions = 63.55 grams of Copper (II) ionsNOTE: The mass in grams of 1 mole of any pure substance is commonly referred to as the “Molar Mass.”A similar unit relationship is easily determined for any other element on the Periodic Table. The first part of thisunit relationship is always the same:1 mole of any element = 6.02 x 10 23 atoms of that elementC. Graham Brittain Page 2 of 9 9/28/2010