Understanding Chemical Notation

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
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 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 or typesetting issues were present.). But the number AFTER the atomic symbol should be considered the subscript.

3. For a compound the chemical formula 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).

As a general rule of thumb, the element appearing on the left of the periodic table is listed first. Specifically, the IUPAC'S standards say that cations always written before anions. (There's a rule, of course, dealing with electropositive/electronegative characteristics of the elements.) 

4. A number in front of the compound (or element) is called the coefficient, and it indicates how many how many particles (e.g. molecules) or moles of the substance are represented. It multiplies the entire formula.

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 whatever you're talking about… 

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

 

5. Sometimes it is convenient to keep groups of atoms together. This introduces parentheses and additional subscripts—which is used with something called polyatomic ions.

Atoms inside parenthesis are considered to be a group that is being treated as a unit "chunks" (often 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)  -->NOTE: officially, this would be written as 2H2SO4
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!

Summary

Understanding Chemical Notation — Summary

• Atomic symbols
  • Every element has a chemical symbol.
  • The first letter is always capitalized.
  • If there is a second letter, it is lowercase.
  • Example: Co = cobalt, while CO = carbon monoxide.
  • Capitalization matters.
• Subscripts
  • A subscript appears after an element symbol.
  • It tells how many atoms of that element are present.
  • Example: H₂ means 2 hydrogen atoms.
  • If there is no subscript, assume the number is 1.
  • Example: CO₂ contains 1 C atom and 2 O atoms.
  • In plain text, subscripts may appear as normal-sized numbers, such as H2O.
• Chemical formulas
  • A compound is represented by combining element symbols and subscripts.
  • Example: H₂O contains:
    • 2 H atoms
    • 1 O atom
  • For ionic compounds, the cation is written before the anion.
• Coefficients
  • A coefficient is a number placed in front of a chemical formula.
  • It multiplies the entire formula.
  • It can represent a number of particles or a number of moles.
  • Example: 2H₂O represents 2 units of H₂O.
  • Coefficients establish mole ratios, which are used in stoichiometry.
• Parentheses
  • Parentheses keep a group of atoms together as a unit or “chunk.”
  • These groups are often polyatomic ions.
  • A subscript outside parentheses multiplies everything inside the parentheses.
• Example: Ca(OH)₂
  • 1 Ca
  • 2 O
  • 2 H
• Example: 2H₂(SO₄)
  • Used to demonstrate the multiplication process.
  • The actual formula would normally be written 2H₂SO₄.
  • Total atoms:
    • 4 H
    • 2 S
    • 8 O
• Example: 4Ca(NO₃)₂
  • The coefficient 4 multiplies the entire formula.
  • The subscript 2 multiplies everything inside (NO₃).
  • Total atoms:
    • 4 Ca
    • 8 N
    • 24 O
• Key idea
  • Subscripts multiply individual atoms or grouped atoms.
  • Coefficients multiply the entire chemical formula.
  • Work from the inside outward when parentheses and coefficients are both present.