Basic chemistry help for students and teachers

 

Interactive Chemistry Worksheets for Students

 

Periodic table

Naming compounds

Basic formula

Simple compounds

Ionic compounds 1

Ionic compounds 2

Chemical suffixes

Old chemical names

Hydrocarbons - Alkanes

Alkanes, alkenes & alkynes

Alkanols to alkanoic acids

 

Common compounds

Common formula quiz

Acids, bases and salts

Covalent compounds

Printable worksheets
 

 

How do you write chemical formula?

The following method uses the concept of ionic charges to help students write the chemical formula of ionic compounds. Some students however, become confused with charges. For a simpler method of writing chemical formula first use the arms and link method.

When writing chemical formula for compounds it is important to know the combining powers or valencies of the elements. This allows you to determine the ratio the elements combine in. All compounds are made of elements that are chemically combined in whole number ratios. eg. Water is made up of 2 parts hydrogen to 1 part oxygen. Its chemical formula is therefore H2O

The elements in the periodic table are grouped together in columns with similar combining powers. These valencies also contain a charge. Eg. Mg is in group 2. It has a valency of 2+, O is in group 6 and has a valency of 2-

Group

1

2

 

3

4

5

6

7

8

Valency

1+

2+

 

3+

4

3-

2-

1-

0

 

 

 

 

 

 

 

 

 

 

 Periodic table

H

 

 

 

 

 

 

 

 

 

Li

Be

Transition block

B

C

N

O

F

Ne

 First 20 elements

Na

Mg

variable

Al

Si

P

S

Cl

Ar

 

K

Ca

valencies

 

 

 

 

 

 

A metal combines with a non-metal to produce an ionic compound. During the reaction ions or charge particles are formed. Refer to the valencies in the table above to help you determine the ionic charge. Eg. The sodium ion Na+ has a charge of +1, the calcium ion, Ca2+ is +2, the oxide ion, O2- is -2 and the chloride ion, Cl- is -1. Note: The 1 is ignored when determining the ionic charge in group I and group 7 elements.

When writing chemical formula determine the simplest ratio of positive and negative ions that are needed to produce a neutral compound. These numbers are written as subscripts. Eg. Al2O3
 

Look at the following worked examples on how to determine the chemical formula of compounds.
 

Writing Chemical formula
 

Chemical compound

Make the simplest neutral compound from the ions

Chemical formula (ignore ions)

Sodium chloride

Na+ Cl-

Check: + 1 + -1 = 0

NaCl

Magnesium chloride

Mg2+ Cl-
          Cl-

Check: 2+ + 2- = 0

MgCl2

Aluminum chloride

Al3+  Cl-
         Cl-
         Cl-

Check: 3+ + 3- = 0

AlCl3

Calcium oxide

Ca2+ O2-

Check: 2+ + 2- = 0

CaO

Sodium oxide

Na+ O2-
Na+

Check: 2+ + 2- = 0

Na2O

Lithium phosphide

Li+ P3-
Li+
Li+

Check: 3+ + 3- = 0

Li3P

Aluminum oxide

Al3+ O2-
Al3+ O2-
       O2-

Check: 6+ + 6- = 0

Al2O3