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FCS Physics and Chemistry

Formulas and Equations...cont
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Types of Formulas
There are two general types of chemical formulas:
 
Empirical Formulas - represents the simplest integer ratio in which atoms combine to form a compound
          ex: CH2O
 
Molecular Formulas - represents some ratio of an empirical formula.  
          ex: C2H4O2 

***Notice that the molecular formula is simpy twice the number of each of the elements in the empirical formula

Atoms, Ions and Compounds
It is easy to interpret a formula for an element or a compound, but it's a bit more complicated to write the formula for a compound.  How do you know what elements form the compound and in what proportion?  To understand how elements form compounds, an understanding of atoms and ions is essential.
 
Key Points to Remember:
 
1. All atoms are electrically neutral, except if they are ions
 
2. Ions are atoms that have gained or lost electrons
 
3. When writing formulas for compounds, the overall net charge must be zero, or if it's a polyatomic ion, equal to the charge on the ion
 
4. If the compound contains a metal to a nonmetal, it is called an ionic bond and uses specific rules for writing and naming the compounds
 
5. If the compound contains two nonmetals together, it is called a covalent bond and uses different rules for writing and naming the compounds
 
Writing Formulas AND Naming Ionic Compounds
 
1. Use the first oxidation state listed on the periodic table for the nonmetal and metal
 
2. Bring them together placing the metal first and the nonmetal second
 
3. Equalize the charges by adding subscripts to the each of the ions
 
4. Polyatomic Ions are two or more covalently bonded atoms together that overall posess a net charge.  See the front of your reference table to find these.  Use the overall net charge on the polyatomic ion when writing formulas
 
5. To name, use the complete name of the metal ion; remove the -in ending from the nonmetal ion and add -ide
 
6. If a metal has more than one oxidation state, use a roman numeral to indicate which ion is to be used.  For example, if the copper uses the +2 ion in a compound with Bromine, it would be called copper (II) bromide
 
For Example

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Writing Formulas AND Naming Covalent Compounds
 
1. Two nonmetals form molecules and these molecules can be in different ratios (see molecular formulas).  Knowing this, we need to know the name of the molecule before we can write the formula
 
2. A prefix system is used to identify the quantity of each of the elements in the compound.  Prefixes will be included before the names of the elements in a compound with one exception:
 
mono- is not included before the first element, however, other prefixes can be if necessary
 
3. The least electronegative element is always listed first
 
 
 

Number of Atoms
Prefix
1
mono-
2
di-
3
tri-
4
tetra-
5
penta-
6
hexa-
7
hepta-
8
octo-

covalentexample.jpeg

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PRACTICE QUESTIONS!

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Forestville, NY 14062