SS1 Chemistry Lesson Note on Valency and Oxidation Number

The website has the complete lesson note for all the subjects in secondary school but this piece showcases the SS1 Chemistry Lesson Note on Valency and Oxidation Number. You can use the website search button to filter out the subject of interest to you.

CLICK HERE to download the complete Document: DOWNLOAD HERE

TOPIC: VALENCY, OXIDATION NUMBER, IONS AND RADICALS

CONTENT

1. Valency of elements
2. Oxidation numbers of elements in compounds
3. Ions and radicals

PERIOD 1: VALENCY OF ELEMENTS

VALENCY

The valency of an element is the combining power of the element. It is defined as the number of atoms of hydrogen that will combine with or displace one atom of the element in chemical reactions. Some elements have more than one valency. For example.

First twenty elements with their valencies show this character.

Atomic Number	Element	Symbol	Valency
1	Hydrogen	H	1
2	Helium	He	Nil
3	Lithium	Li	2
4	Beryllium	Be	2
5	Boron	B	3
6	Carbon	C	2 or 4
7	Nitrogen	N	3 or 5
8	Oxygen	O	2
9	Fluorine	F	1
10	Neon	Ne	Nil
11	Sodium	Na	1
12	Magnesium	Mg	2
13	Aluminum	Al	3
14	Silicon	Si	2 or 4
15	Phosphorus	P	3 or 5
16	Sulphur	S	2, 4 or 6
17	Chlorine	Cl	1
18	Argon	Ar	Nil
19	Potassium	Ic	1
20	Calcium	Ca	2

EVALUATION:

1. What do you understand by the word chemical symbol?
2. Define valency of an element.

READING ASSIGNMENT:

Find out the valency of the following element.

(i) Gold (ii) Silver (iii) Zinc and copper

PERIOD 2: OXIDATION NUMBER OF ELEMENTS

To be able to write correctly chemical formulae for compounds, a system of small whole numbers, related to the combining ratio of element has been developed on the basis of arbitrary rules. Such numbers are called oxidation numbers or oxidation states.

Rules for assigning oxidation number

1. The ON of an uncombined free element, whether monoatomic or polyatomic is zero; e.g. Noble gases (He, Ne, etc), metals (Na, Zn, etc),solid non-metals (O₃, N₂, F₂,etc)
2. The oxidation number of a monoatomic ion is equal in magnitude and sign to its ionic charge; e.g., the ON of bromide ion, Br^-1, is -1; that of F⁺³, is +3.
3. The ON of hydrogen atom is +1 in its compounds, except in hydrides of metals (e.g. NaH), where it is -1
4. For any neutral compound the sum of the ONs of all the atoms adds to zero.
5. The ON of oxygen in a compound is always -2 except in peroxides, H₂O₂, Na₂O₂, where it is -1
6. In any radical, the sum of the ON of all the atoms is equal to the charge on its ion.

    Worked examples on oxidation numbers

1. Calculate the ON of copper in Cu₂O

Solution

Let y represents the ON of each copper atom.

ON of one hydrogen atom, O is -2.

In a neutral compound, sum of all ON is zero.

Therefore, in Cu₂O:        Cu₂O

2y + (-2) = 0

2y = +2

y = +2/2 = +1

Thus, ON of a copper atom in Cu₂O = +1

 

2. Determine the ON of X in X₂O₇^2-

Solution

Click on the Downloadable Button to get the FULL NOTE