Page 67 - Chemistry_Form_2

Chemistry
for Secondary Schools

1. Positively charged ions (cations) are written before negatively charged ions
(anions).
2. A radical must be treated as a unit.

3. Brackets are not used for single elements.
FOR ONLINE READING ONLY
4. The valency 1 is simply assumed and not written in the formula.

The symbols and valencies of the atoms and radicals are important in writing a
chemical formula. For example, for arbitrary elements W and X with valencies m
and n, respectively, and where X can be a radical or an atom, the following steps
can be used to come up with a chemical formula of their compounds:
Step 1: Write the symbols of the elements and radicals, in this case W and X.

Step 2: Write down the ions used, with their valencies as superscripts, that is
W X .
m
n
Step 3: Interchange the valencies of W and X and write them as subscripts.

The formula of the chemical compound is W X .
n m
Note: When m and n are equal, there is no need for the exchange and, therefore,
are not written since they are in a ratio of 1:1.

Example 3.4

1. Give the formula of the compound of calcium and chlorine atoms.
Step 1: Write down the symbols for calcium or chlorine atoms.
Ca Cl

Step 2: Write down the ions used in the compound with their charges.
Ca Cl –
2+
Step 3: Write down the ions used with their valencies as superscripts.
Ca Cl 1
2
Step 4: Interchange the valencies and write them as subscripts.

60 Student’s Book Form Two

01/08/2025 11:20:56
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 60
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 60 01/08/2025 11:20:56