7. Chemical Formulas
A system of chemical notation was "invented" in 1811 by Jons Jakob Berzelius.
The system is based on the "law of definite proportions", stating that all samples of a given chemical compound have the same elemental composition.
Joseph Louis Proust
actually published a paper stating the law of definite proportions in 1794, but it was not widely accepted until Berzelius added his support to the idea.
Chemical formulas are short-hand representations of compounds using chemical symbols and oxidation numbers.The simplest chemical formulas are for binary compounds - compounds made up of two elements.
- Binary compounds have a positive half and a negative half.
- The sum of the oxidation numbers in a compound must equal zero.
- The oxidation number of many atoms can be read directly from the Periodic Table.
- The oxidation number of "transition metals", and any atoms that act in unusual ways, will be provided.
The name of the atom will be given with its positive oxidation number as a Roman numeral in parenthesis after the name. Example: iron (III) has an oxidation number of +3, while iron (II) has an oxidaiton number of +2
- The oxidation number of an ion is equal to its charge.
- The sum of the oxidation numbers in a polyatomic ion must equal its charge.
- Subscripts are small numbers to the lower right of a symbol. They represent the number of atoms of that element in the compound.
- Coefficients are large numbers to the left of formulas in chemical equations. They represent the ratio of molecules of each substance involved in a chemical reaction.
- Subscripts and coefficients of 1 are "understood" - NEVER WRITTEN.
- The subscripts indicate a "ratio" in which atoms combine to make compounds. For H2 O - the ratio is 2 to 1. 2 hydrogen atoms to 1 oxygen atom
Example: the chemical formula for water is H2 O
Hydrogen = ( Oxidation # ) ( Subscript ) = ( +1 ) ( 2 ) = + 2
Oxygen = ( Oxidation # ) ( Subscript ) = ( −2 ) ( 1 ) = −2
Total oxidation numbers in water = ( +2 ) + ( −2 ) = 0
The simplest way to make the total of the oxidation numbers in a formula equal zero is to use the oxidation number of one element as the subscript of the other element.
In the example above, calcium's 2 is used as the subscript for chlorine. Chlorine's 1 is not written as a subscript for calcium because subscripts of 1 are NEVER written - they are understood.
list of anions list of cations list of polyatomic ionsFormulas with polyatomic ions (ions made up of more than one atom)
- Although compounds containing polyatomic ions usually have more than two elements, write their formulas just like binary compounds.
- The cyanide ion is CN −.