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TOPIC: VALENCY, OXIDATION NUMBER, IONS AND RADICALS
CONTENT
- Valency of elements
- Oxidation numbers of elements in compounds
- 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:
- What do you understand by the word chemical symbol?
- 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
- 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)
- 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.
- The ON of hydrogen atom is +1 in its compounds, except in hydrides of metals (e.g. NaH), where it is -1
- For any neutral compound the sum of the ONs of all the atoms adds to zero.
- The ON of oxygen in a compound is always -2 except in peroxides, H2O2, Na2O2, where it is -1
- 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
- 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
- Determine the ON of X in X2O72-
Solution
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