1. Introduction to Chemistry
2. The Periodic Table
3. Quantum Numbers
4. Electron Configuration
5. Chemical Families
6. Oxidation Numbers
7. Chemical Formulas
8. Chemical Names
9. Formula Mass
10. Percentage Composition
11. Reaction Types
12. Balancing Equations
13. The Mole Concept
14. Solution Concentration
16. Kinetic Theory
17. The Gas Laws
18. Enthalpy & Heat
19. Reaction Rates
20. Acids & Bases
21. pH Scale
23. Net Ionic Equations
24. Redox Reactions
25. Organic Chemistry
26. Nuclear Chemistry
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.
Oxidation numbers are used to write chemical formulas.
Indicating numbers of atoms and molecules in formulas and equations
- Binary compounds have a positive half and a negative half.
- The positive half is written first, the negative half second.
To determine the total oxidation number, multiply the subscript by the oxidation number.
- 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 H2O - the ratio is 2 to 1 . . . 2 hydrogen atoms to 1 oxygen atom
Example: the chemical formula for water is H2O
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 ions
NEVER change the subscripts in a polyatomic ion!
Formulas with polyatomic ions (ions made up of more than one atom)
Chemical Formulas 2