Formulas

Compounds

A compound is a substance composed of atoms of two or more elements.   Each compound will always contain the same ratio of elements.  For example, water (H2O) will always contain two hydrogen atoms and one oxygen atom.

Rules for Writing Formulas

1. Represent each atom in the compound by its element symbol.

2. The number of each type of atom is indicated by a subscript immediately following the element symbol.

3. When there is only one atom of a certain type in the compound, you need not use a subscript. 

Chemical Formulas

Each molecule of a given compound is written in terms of a chemical formula.  

Examples

Each molecule of water will have two hydrogen atoms and one oxygen atom.

1. Each molecule of water will have two hydrogen atoms and one oxygen atom

Each molecule of carbon monoxide will have one atom of carbon and one atom of oxygen.

2. Each molecule of carbon monoxide will have one atom of carbon and one atom of oxygen.

Each molecule of methane has one atom of carbon and four atoms of hydrogen.

3. Each molecule of methane has one atom of carbon and four atoms of hydrogen.

4. Each molecule of a certain compound has six atoms of carbon, twelve atoms of hydrogen, and six atoms of oxygen.   The chemical formula for this compound is C6H12O6.

A coefficient tells how many molecules of the compound there are.  For example, 2H2O tells us there are two molecules of water.  The 2 in front of H2O is the coefficient.

The coefficient 2 tells us that there are two separate molecules of H2O.

Two molecules of H2O would be 2H2O

The coefficient 2 tells us that there are two separate molecules of H2O.

Compounds that Contain Ions

The ionic compound NaCl, also known as common table salt

Chemists believe that many compounds, including table salt (NaCl) contain ions.  NaCl, for example, does not melt until 800oC or boil until 1500oC.  Compare that to water, which melts at 0oC and boils at 100oC and you see a huge difference!  Water does not conduct electricity, but when NaCl is dissolved in water, it suddenly becomes very conductive.  These observations are what led chemists to believe that NaCl is actually made up of the ions Na+ and Cl-.  Since opposite charges attract, the positive charge on the sodium ion and the negative charge on the chlorine atom are held tightly together, and therefore it take a lot of heat to melt the solid and pull the Na+ and Cl- apart.

Ionic compounds are those substances which contain ions.  These are compounds formed from a metal and a nonmetal.  They are understood to be ions because of characteristic traits such as high melting points and good electrical conductivity.

The figure above is the ionic compound NaCl, also known as common table salt.  In this compound, the sodium and chlorine are held together because of the attraction of the positive charges of the sodium cations and the negative charges of the chlorine anions.

Formulas for Ions

A chemical compound must have a net charge of zero.  If the compound is an ionic compound, then the number of positively charged cations must equal the number of negatively charged anions.

Consider common table salt, or NaCl:

Table Salt=NaCl

Periodic Table highlighting Na and Cl

Formulas for Ions with Unbalanced Charges

 Calcium Chloride

What about when the charges on each element are not equal?  Using the formula to the left, it is easy to see that a compound with one calcium cation and one chloride ion will not balance.

2 negatively charged Cl's are needed

Calcium and Chlorine on Periodic Table

In order to get a net charge of zero, two negative charges are needed. Therefore, a compound with calcium and chloride must have one calcium atom (Ca2+) and 2 chlorine atoms (Cl-).  Referring to the chemical formulas section above, you can deduce that the formula for this compound is CaCl2, meaning there is one calcium atom and two chlorine atoms in each molecule of calcium chloride.

Example 1: A formula of a compound which contains the ions Li+ and N3-:

A formula of a compound which contains the ions Li+ and N3-:

 

 

 

 

Lithium and Nitrogen on Periodic Table

Therefore, 3 Li+ ions are needed so that the equation equals zero.

 

 

 

 

Therefore, 3 Li+ ions are needed so that the equation equals zero.

The formula for this compound is Li3N.

Example 2: A formula for a compound which contains the ions Ba2+ and O2-:

A formula for a compound which contains the ions Ba2+ and O2-:

 

 

 

 

Barium and Oxygen in the Periodic Table

Since the equation holds true when one atom of each element is used, the formula for this compound is BaO.