VALENCY, OXIDATION NUMBER, IONS AND RADICALS

 

FIRST TERM E-LEARNING NOTES

SUBJECT: CHEMISTRY

CLASS: SS1

SCHEME OF WORK

PLAN LESSON NOTE 

WEEK: 4

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.[mediator_tech]

Therefore, in Cu2O: Cu2O

2y + (-2) = 0

2y = +2

y = +2/2 = +1

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

  1. Determine the ON of X in X2O72-

Solution

ON of each O atom = -2

The net charge on the ion = -2

Therefore, in X2O72-

2X + 7(-2) = -2

2X – 14 = -2

2X = -2 + 14 = + 12

X = + 12/2 = +6

Therefore, ON of each X in X2O72- is +6

EVALUATION:

  1. What do you understand by the term “oxidation number”.
  2. States four rules guiding the derivation of oxidation number of elements.
  3. Determine the oxidation number of the boldened atoms in the following: (a) H2SO4 (b) Cr2O7 (c) Fe2O3 (d) MnO4

PERIOD 3: IONS AND RADICALS

DEFINITION:

An ion is any atom or group of atoms which possess an electric charge.

Some substances are not built of atoms or molecules but are made up of charged particles called ions. Ions are formed when an atom losses or gains electron.

There are two types of ions. The positively charged ions or cations e.g. K+, Ca2+, Zn2+, Al3+ etc. they are formed when atoms loss electrons and negatively charged ions or anions which are formed when atom gain electrons e.g. Cl, N3-, O2-, S2- etc.

RADICALS: These are groups of atoms of different elements that come together and react as a unit. These radicals are charged, that is, they either carry a positive or a negative charge. An acid radical is thus a small group or cluster of atoms carrying a negative charge that keeps its identity. These groups of atoms originate from the acids which have formed the salts. For examples

Radical Symbol Valency Oxidation No
Ammonium ion NH4+ 1 +1
Hydroxyl ion OH 1 -1
Trioxocarbonate (iv) CO32- 2 -2
Tetraoxosulphate(vi) SO42- 2 -2
Trioxonitrate(v) NO3 1 -1

EVALUATION:

  1. Differentiate between ions and radicals.
  2. List four examples each of ions and radicals.
  3. State the valencies of the following radiclas: (a) PO43-(b) MnO4(c) C2O42-
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