writing and naming chemical formula

Writing Chemical Formulas
		A chemical formula is a combination of elemental symbols and subscript numbers that is used to show the composition of a compound. Depending of the type of compound that the formula represents, the information that it provides will vary slightly. Before we go about learning how to write chemical formulas, it is important that you clearly understand the difference between molecular compounds and ionic compounds.

Binary Ionic Compounds

A binary compound is one that is composed of two elements that have been chemically combined. An ionic compound is one formed when a metal chemically combines with a non-metal. This chemical combination will always result in an ionic compound. In determining the formula of a binary ionic compound we will follow a set of steps until you are comfortable with the process. You will need to know or at least have access to a listing of common ions, their symbols, their charges (valences), and their names. Some elements have more than one possible charge or valence. These are referred to as the multi-valent elements. Iron for instance has a +2 charge (Iron II or Ferrous) in some compounds and a +3 charge (Iron III or Ferric) in other compounds so we will have to recognize in which state the Iron is in and write the formula accordingly.

• Mercury Hg⁺ (Mercury I) or Mercurous and Hg⁺² (Mercury II) or Mercuric
• Copper Cu⁺ (Copper I) or Cuprous and Cu⁺² (Copper II) or Cuprous
• Lead Pb⁺² (Lead II) or Plumbous and Pb⁺⁴ (Lead IV) or Plumbic
• Tin Sn⁺² (Tin II) or Stannous and Sn⁺⁴ (Tin IV) or Stannic
• Cobalt Co⁺² (Cobalt II or Cobaltous) and Co⁺³ (Cobalt III or Cobaltic)

Most transition state metals located in the middle of the Periodic Table are multi-valent.

All Group 1 elements in the Periodic Table are +1 in compounds. All Group 2 elements in the Periodic Table are +2 in compounds.

It is best to get a more complete listing of these common ions and their charges, symbols, and names.

The procedure that can be followed when confronted with the name of a compound and you wish to write its formula is as follows:

1. Identify the symbol of the cation (first part of the name) and the anion

2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol

3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero.

4. Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.

   Binary Molecular Compounds -

   The standard method for naming binary molecular compounds has changed over the years. Currently, the stock system is commonly used for naming molecular compounds. Names like "carbon dioxide", "carbon monoxide", and "dinitrogen pentoxide" are really remnants of an older system that used prefixes to identify the number of elements involved. When you are writing the formula for a molecular compound using the stock system, you will not really notice any difference from the methods described above, until you study bonding. You should be aware that you are not dealing with ions when you are working with molecular formulas, rather you are looking up what might be called the apparent charge on each atom.

   Ternary Ionic Compounds

   A Ternary compound is one that has three or more elements involved in the compound. These ionic compounds will have one or more polyatomic ions. These are ions that have two or more elements clustered together with a charge assigned to the cluster. The rules for writing the formulas of ternary ionic compounds are pretty much the same as for binary compounds except you will be using polyatomic ion names. One other thing to keep in mind is that if you have more than one polyatomic ion to indicate in the formula, you group the polyatomic ion is a set of parenthesis and then place the number of those ions on the outside of the parenthesis as a subscript. If you have only one polyatomic ion to indicate in the formula you do not use the parenthesis.

   IN SHORT...

   What are the rules in writing chemical formula?
   
   Read more: http://wiki.answers.com/Q/What_are_the_rules_in_writing_chemical_formula#ixzz1EVIpjMGZ

   
   

   Rule 1: CRISS-CROSS rule
   

   ---

   1. write the correct symbols of the elements and radicals
   2. Above each symbol, write the correct valence or oxidation numbers.
   Ex. Al⁺³ O^-2
   3. To obtain the proper subscript, criss-cross the valence or oxidation
   number,and drop the algebraic sign.
   Ex. Al⁺³O^-2 became Al₂O₃
   

   ---

   Rule 2: When the subscript is number 1, subscript is not written.
   Ex. Sodium Chloride- Na⁺¹ Cl⁺¹ --- NaCl
   
   

   ---

   Rule 3: When the oxidation # of both elements are numerically equal but grater than 1, the subscript are not also written.
   Ex. Calcium Oxide~ Ca⁺²O^-2 ----- CaO
   

   ---

   Rule 4: All radicals tke more than once( the subscript is 2 or more) must be
   enclosed in Pahrentheses().
   
   Ex. Ammonium Sulfate~~ NH_4⁺¹ SO_4^-2-----(NH₄)₂ SO₄
   

   ---

   RULE 4: All subscripts must be reduced to lowest term (except for molecular or
   covalent copound).
   
   Ex. Tin (IV) Chloride ~ Sn⁺⁴Cl^-1------ SnO₂

   
   aND ANOTHER

   When a subscript is used with an ion containing two or more elements, the ion must be enclosed in parentheses () before writing the subscript.
   
   Ex. Mg⁺² and NO₃^-1
   
   is written as Mg (NO)2