Understanding Chemical Notation (Molecular Formulas)

General Chemistry Index

Where are we going with this? This page will give the ability to demonstrate an understanding of the law of conservation of mass through the use of particle diagrams and mathematical models.

Understanding Chemical Notation (Molecular Formulas)
This is where it all begins! 

The basis for understanding how to describe chemical reactions is a shorthand system we'll call chemical notation. It's very picky, but not hard.

1. The Atomic Symbol of each element is the first aspect of chemical notation. A few remarks…

The atomic symbol of an element MUST begin with a capital letter.

If it is a two (or more) letter symbol the following letter is lower case. This is absolutely required! Co is not the same as CO. Co is an element. CO is a compound made up of C and O.

The letters L and I present some issues, depending on the font used. It is possible that a lowercase l and an uppercase I look a lot alike. You should be careful with these! 

2. The next thing in chemical notation is the subscript attached to the atomic symbol. 

• The subscript FOLLOWS the atomic symbol. 
  
  
• The subscript tells how many of that atom are present. 
  
  
• H2 means there are two atoms of hydrogen present.
  
  
• If there is NO subscript, the number of that atom in the molecule is 1. For instance, in CO2, there is 1 carbon (C) because there is no subscript.
  
  
• Sometimes, the "subscript" will not be depicted in a smaller font. (Because the person who wrote/typed it was lazy). But the number AFTER the atomic symbol should be considered the subscript.

3. For a compound the molecule is written by placing the atomic symbols and their subscripts in a string.  

So, H2O or (H2O if someone is lazy) means there are 2 H atoms and 1 O atom in the compound (water).

Generally, the element appearing on the left of the periodic table is listed first.

4. A number in front of the compound (or element) is called the coefficient, and it indicates how many molecules of that element are present.

2H2O means that there are 2 things—hold this thought—of water present.

The coefficient is a number, so 2 molecules. Or, you can apply any multiplier to the coefficient and the meaning carries through. It can be 2 molecules, 2 dozen molecules, 2 thousands of molecules… or 2 moles of atoms… 

In chemistry, it is moles that are used. So, the coefficient establishes a ratio of moles of the molecules. This becomes the basis for the stoichiometry process.

 

5. Sometimes it is convenient to keep groups of atoms together. Doing this brings in polyatomic notation, which brings in parenthesis and more subscripts!

Atoms inside parenthesis are considered to be polyatomic "chunks" (i.e. polyatomic ions).

The subscript FOLLOWING the parenthesis tells how many of the chunks are present.

The subscript multiplies all of the atoms inside the parenthesis, including those already having subscripts.

EXAMPLES:

Ca(OH)2
1 Ca and 2 (OH)

1 Ca, 2 O, and 2 H

2H2(SO4)
2 X 2 H and 2 (SO4)
4H and  2 S, and  2 X 4 O
4 H and 2 S and 8 O

4Ca(NO3)2 
4 Ca and 4 (NO3)2
4 Ca and 4 X 2 (NO3)
4 Ca and 8(NO3)
4 Ca and 8 N and 8 X 3 O
4 Ca and 8 N and 24O

That escalated quickly!