Page 64 - Chemistry_Form_2

Chemistry
for Secondary Schools

Note: Ammonium radical (NH ) has a valency of 1 and can react like metals. Its
+
4
compounds are similar to those of Group I elements.
Oxidation state

Oxidation state (also called oxidation number) is the total number of electrons
FOR ONLINE READING ONLY
that an atom either gains or loses in order to form a chemical bond with another
atom. It is the measure of the electron control that an atom has in a compound
compared to the atom in the pure element. The neutral atom has no charge.

Rules for assigning oxidation states

The following are the rules used to assign oxidation states of the elements:
1. The oxidation number of free elements is zero. For example, all elements
in the periodic table have the oxidation number of zero.

2. The sum of the oxidation states of all atoms forming a molecule or ion is
the net charge of that species. For example, nitrogen (N ), hydrogen (H )
2
2
and oxygen (O ) molecules have the oxidation number of zero.
2
3. In simple ions that consist of only one atom, the oxidation number is equal
to the charge on the ion. For example, the oxidation number of a sodium
ion (Na ) is +1, aluminium (Al ) is +3, iron(II) (Fe ) is +2, and iron(III)
3+
+
2+
(Fe ) is +3. In an oxide ion (O ), the oxidation number of oxygen is –2.
3+
2–
4. In their compounds, Group I metals have an oxidation number of +1.
Group II metals have an oxidation number of +2, while, Group III metals
have an oxidation number of +3.

5. In their compounds, halogens always have an oxidation number of –1.

6. Hydrogen has an oxidation state of +1 in most compounds. The exception
is in hydrides of active metals where the oxidation number is –1. For
example, the hydrogen atom gains an electron from the lithium atom in
lithium hydride (LiH).

7. Oxygen has an oxidation state of –2 when present in most compounds,
except:
(a) in peroxides, for example H O , where the oxidation number is –1.
2 2
(b) when bonded with fluorine to form F O, the oxidation number is +2.
2

Student’s Book Form Two 57

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 57 01/08/2025 11:20:55
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 57