Login

Articles

How do you write Chemical Formulas?

Print

Chemical Formula


A chemical formula of a compound is defined as a symbolic representation or notation of atoms constituting an element. The chemical or molecular formula identifies the number of particular atoms of each element present in a compound.


chm_a5_3


The molecular formula depends on the combining capacity or valency of the elements. Therefore valency is used to find how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.








Sometimes in chemical
formula subscripts are also used to represent the ratio of elements in the ionic or non molecular compounds. In case of ionic compounds, chemical formulas are termed as Empirical Formulas.


Following steps are taken to write chemical formula of any compound:

  • Write down the symbol of an element
  • Write down the valency of each of the element
  • Cross over the valency

For example:
Formula of Hydrogen chloride

Symbol          H          Cl
                    chm_a5_4
Valency         1           1

Formula        HCl



chm_a5_2
The valency of Hydrogen and chlorine is one, therefore molecular formula for hydrochloric acid is written as Hcl



Beside the above steps, following three rules must be followed while writing a chemical formula





Rules for writing Chemical Formula

Rule 1:

The valency or charges present on the ion must balance each other.

For example

1. Formula for water: The valency of hydrogen is 1 and that of oxygen is 2. Therefore 2 H atom is required to balance single oxygen atom. Therefore chemical formula for water is written as H2O

Symbol          H          O
                            chm_a5_4
Valency         1           2

Formula        H2O

2.  Formula for Magnesium Chloride: As the charges on magnesium and chloride ion is 2+ and 1- respectively. Therefore 2 chloride ions are required to balance single magnesium ion.

Symbol              Mg        Cl
                           chm_a5_4
Charge             2+           1-

Formula            MgCl2


Rule 2:

When a metal and non metal constitute a chemical compound, then the symbol of metal is written before the symbol of non metal.


For example:chm_a5_1
1. Formula for Sodium chloride: As sodium is a metal and oxygen is a non metal therefore sodium is written first while oxygen is written after it.

Symbol              Na        Cl
                          chm_a5_4
Charge             1+            1-


Formula            NaCl

 


Rule 3:

When polyatomic ions form a chemical compound, the number of atoms is written after enclosing the polyatomic ion in brackets.


For example: 
1.
Formula of calcium hydroxide: The formula of calcium hydroxide is written as Ca(OH)2 .Here, the bracket around OH with a subscript 2 indicates that there are two hydroxyl (OH) groups joined to one calcium atom.

Symbol          Ca        OH                     
                      chm_a5_4
Charge          2+        1-

Formula           Ca(OH)2

2. Formula of calcium carbonate: In this example, brackets are not required as it contains only one ion.

Symbol          Ca        CO3

                      chm_a5_4
Charge          2+          2-

Formula           CaCO3


Chemical Formula for other Compounds


Other examples:


Formula for Aluminum Chloride:

Symbol          Al        Cl
                           chm_a5_4
Charge         3+          1-

Formula           AlCl3

Formula for Copper Nitrate


Symbol          Cu        NO3
                      chm_a5_4                    
Charge         2+            1-

Formula           CuNO3

Formula for Aluminum Sulphate

Symbol          Al        SO4

                     chm_a5_4

Charge         3+          2-

Formula           Al2 (SO4)3

Formula for Calcium Oxide: As the valency of calcium and oxygen is same but to simplify the formula we write CaO instead of Ca2O2.

Symbol          Ca        O

                     chm_a5_4

Charge         2+            2-

Formula          CaO

Write the chemical formula of Magnesium carbonate, Phosphorus penta chloride, and Hydrogen sulphate.



Try to answer. Still need help? Want to know more about writing chemical formulas? Click here to schedule live help from a certified tutor!


About eAge Tutoring:

eAgeTutor.com is the premium online tutoring provider.  Using materials developed by highly qualified educators and leading content developers, a team of top-notch software experts, and a group of passionate educators, eAgeTutor works to ensure the success and satisfaction of all of its students.  

Contact us today to learn more about our guaranteed results and discuss how we can help make the dreams of the student in your life come true!

Reference Links:

    

Archives

Blog Subscription