FIRST TERM E-LEARNING NOTES
SCHEME OF WORK
PLAN LESSON NOTE
TOPIC: VALENCY, OXIDATION NUMBER, IONS AND RADICALS
- Valency of elements
- Oxidation numbers of elements in compounds
- Ions and radicals
PERIOD 1: VALENCY OF ELEMENTS
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.
||2 or 4
||3 or 5
||2 or 4
||3 or 5
||2, 4 or 6
- What do you understand by the word chemical symbol?
- Define valency of an element.
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
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-
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
- What do you understand by the term “oxidation number”.
- States four rules guiding the derivation of oxidation number of elements.
- Determine the oxidation number of the boldened atoms in the following: (a) H2SO4 (b) Cr2O7 (c) Fe2O3 (d) MnO4–
PERIOD 3: IONS AND RADICALS
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
- Differentiate between ions and radicals.
- List four examples each of ions and radicals.
- State the valencies of the following radiclas: (a) PO43-(b) MnO4–(c) C2O42-