Page 66 - Chemistry_Form_2

Chemistry
for Secondary Schools

Example 3.3
Give the oxidation number of Cr in Cr O 2–
2 7
Solution

Total charge on the dichromate ion is –2
3READING ONLY
For oxygen, – 2 × 7 = –14

Therefore, 2Cr – 14 = –2
2Cr = +12
Cr = +6

Therefore, the oxidation number of Cr is +6.

Exercise 3.2

Calculate the oxidation number of each underlined element in the following
chemical substances:

1. (a) Na PO (b) SO 3 2– (c) NO 2 –
4
3
FOR ONLINE
2. Explain why ammonia (NH ) is a covalent compound, while ammonium
chloride (NH Cl) is an ionic compound.
4

Concept of chemical formulas

Task 3.5
Obtain containers such as bottles containing chemicals in the laboratory. Examine
them and identify the chemical formulas of the substances on their labels.

A chemical formula is a representation that uses symbols to show the proportions
of the elements present in a chemical compound. The number of atoms or groups
of atoms are shown by number subscripts. For example, the chemical formula
for sodium sulfide is Na S, which shows that two atoms of sodium combine with
2
one atom of sulfur to form the molecule of sodium sulfide. For groups of atoms
such as radicals, a bracket is used to show that they are being considered as a unit
under one valency. For example, in calcium nitrate, Ca(NO ) the NO radical is
–
3 2 ,
3
in brackets. There are some points to remember when writing chemical formulas.

Student’s Book Form Two 59

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 59