Valency is the combining power of an element or radical. In other words it is the measure of the power of an element to combine with others. The power of an element to combine with others is determined by the structure of its atoms. This therefore implies that the valency of an element can be determined from the electron structure of its atoms.

Elements with atoms having 1-4 electrons in the outer most shell, their valencies are equal to the number of electrons in the outer most shell.i.e. Group I elements have a valency of 1; group II elements have a valency of 2; group III elements have a valency of 3 and group IV elements have a valency of 4.

Elements in group V-VII, their valencies are equal to the number of electrons gained to acquire a stable electronic configuration i.e. Elements in group V gain three electrons each to make them stable, they therefore have a valency of 3; Elements in group VI gain two electrons each to make them stable, they therefore have a valency of 2 and elements in group VII gain one electron each to make them stable therefore have a valency of 1.

NB. Therefore, valency can also be defined as the number of electrons lost or gained by an element in order to attain a stable electronic structure.

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NB. For metals with variable valencies, Roman numerals are included in their name to indicate the valencies of the metal. E.g. Iron (II) has a valency of 2,Lead (IV) has a valency of 4 etc.
A radical is an atom or group of atoms that exist in several compounds but does not exist on its own.

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A chemical formula is a representation showing the proportions of elements present in a chemical compound using symbols.

Important concepts about chemical formulae

  1. In a chemical formula, the number written as a subscript after the symbol of an element indicates the number of atoms of that element chemically combined.
    Consider examples of the chemical formulae below.
    In water, H2O-the two (2) represents the number of hydrogen atoms chemically combined. There fore, water consist of two (2) hydrogen atoms and one (1) oxygen atom chemically combined together.
  1. For groups of atoms (radicals), a bracket is used showing that they are being considered under the same valency.
    In calcium nitrate, Ca(NO3)2,the 2 indicates that there are 2 nitrate radicals (NO3). Both nitrogen and oxygen in nitrate are being considered under the same valency.
    In aluminium sulphate, Al2(SO4)3,-the 3 shows that there are 2 sulphate radical (SO4), both the sulphur and oxygen in the sulphate are being considered under the same valency. There fore, Al2(SO4)3 consists of 2 atoms of aluminium and 3 sulphate radicals.
  2. A number put infront of the formula of a compound indicates the number of molecules of the compound. For example
    2H2SO4 this means two molecules of sulphuric acid
    8HNO3 means eight molecules of nitric acid
    3CO2 means three molecules of carbondioxide
    H2O means one molecule of water
  3. To be able to write a chemical formula, one has to know the symbol and valency of the atoms or radicals.
    Steps taken in writing chemical formulae
  4. Identify from the name of the compound, the elements and radicals present e.g. in sodium chloride, there is sodium and chloride radical.
  5. Write the symbol of the element or the formula of the radical separately, beginning with the one of a metal or ammonium radial followed by a non metal or radical.

In sulphuric acid (H2SO4)- there are 2 atoms of hydrogen, 1 atom of sulphur and 4 atoms of oxygen chemically combined together.
CaCO3 is the chemical formula of calcium carbonate (chalk), which consists of 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen chemically combined.

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Write the valencies as superscripts to the right of the symbols written separately.

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Write the formulae of the following compounds
a) Sodium chloride b) sodium sulpate c) Iron(III) oxide

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Write the formula of the following compounds
a) Potassium hydroxide b) Zinc carbonate c) magnesium sulphate d) ammonium phosphate e) copper(II) oxide f)potassium permanganate g) calcium hydroxide.


This is the representation of a chemical change (chemical reaction) by means of symbols and formulae. So, when chemical equations are written, symbols and formulae are normally used. Word equations are not significant and not commonly used. A chemical reaction consists of reactants and products.
Consider the example below for illustration
Hydrogen reacts with oxygen to produce water vapor

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Components of a chemical equation
a) Formulae of reactants and products. These represent compounds reacting and compounds being formed during a chemical reaction respectively.
b) State symbols. A state symbol is one small letter or two small letters that represent the physical state of a compound. They are written after the formulae and enclosed in brackets. There are four state symbols used. These are aqueous solution (aq); liquid (l); solid state(s) and gaseous state (g).
c) The plus (+) sign. The plus sign on the left hand side of the equation means ‗react with‘ and the one at the right hand side of the equation means ‗and‘ .
d) The arrow. This means to produce and the arrow head points to the products.

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Balancing is the process of making each kind of atoms on both the reactant and product sides equal. A balanced equation there fore has equal numbers of each kind of atoms on both the reactant and product sides.
N.B. When balancing a chemical equation, the formula of the compound is not altered but rather it is the number of molecules that is adjusted.

Steps in writing chemical equations

  1. Write the correct formulae of the reactants on the left hand side and the correct formulae of products on the right hand side of the equation. The reactants and products are separated by an arrow pointing to the produncts.
    e.g. consider the reaction calcium and hydrochloric acid producing calcium chloride and hydrogen gas.
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Atomicity is the number of atoms in a molecule or compound.
A molecule is the smallest particle of an element or compound that can exist on its own(i.e. in a free and separate state).
A molecule with one atom is referred to as a monoatomic molecule e.g. Helium(He); argon(Ar);sodium(Na) etc.
A molecule containing two atoms is referred to as a diatomic molecule e.g. Oxygen(O2); hydrogen(H2); Chlorine(Cl2) etc.
A molecule containing three atoms is said to be triatomic e.g. Ozone (O3)
A molecule containing four atoms is said to be tetraatomic e.g. Phosphorus (P4)
A molecule containing many atoms is said to be polyatomic e.g. Sulphr (S8).

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Sample questions on the atomic structure and the periodic table
Atomic structure and periodic table

  1. With an aid of a labeled drawing, describe the structure of an atom showing the distribution of the different particles that make it up.
  2. What are protons, neutrons and electrons? Give a brief detail of each.
  3. Differentiate between mass number and atomic number, isotope and isotopy. Give three elements that show isotopy, write down and name the isotopes.
  4. The figure below shows part of the periodic table. The letters used are not the correct symbols of the elements.
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  1. Silver is an element which exists naturally as a mixture of two isotopic forms. A and B represent atoms of these isotopes. They occur in equal numbers. A is and B is . State the number of (a) protons, (b) electrons and (c) neutrons in atoms A and B. What is the relative atomic mass of the naturally occurring silver (Ans. 108(Atomic mass).
  2. Elements X, Y, and Z have the following atomic number, 11, 15 and 19 respectively. Write the electronic configuration of the elements. State each element is a metal or non metal.
  3. Metals lose electrons to become stable like the noble gases while non metals gain electrons, in the process they form ions. Write down the electronic configuration of the ions formed by magnesium, chlorine and calcium.