Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. Ions may consist of a single charged atom or a group of atoms that have a net charge on them. An ion can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–). A group of atoms carrying a charge is known as a polyatomic ion.
|Ionic Compound||Constituting Elements||Ratio By Mass|
|Calcium Oxide||Calcium and Oxygen||5:2|
|Magnesium Sulphide||Magnesium and Sulphur||3:4|
|Sodium Chloride||Sodium and Chlorine||23:35:5|
Writing Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition. The chemical formulae of different compounds can be written easily. For this exercise, we need to learn the symbols and combining capacity of the elements. The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.
|Valency||Name Of Ion||Symbol||Non-Metallic Element||Symbol|
The valency of the atom of an element can be thought of as hands or arms of that atom. Human beings have two arms and an octopus has eight. If one octopus has to catch hold of a few people in such a manner that all the eight arms of the octopus and both arms of all the humans are locked, how many humans do you think the octopus can hold? Represent the octopus with O and humans with H. Can you write a formula for this combination? Do you get OH4 as the formula? The subscript 4 indicates the number of humans held by the octopus. The valencies of some common ions are given in above Table.
The rules that you have to follow while writing a chemical formula are as follows:
• The valencies or charges on the ion must balance.
• When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are nonmetals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.
• in compounds formed with polyatomic ions, the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket. For example, Mg (OH)2. In case the number of polyatomic ion is one, the bracket is not required. For example, NaOH.
FORMULAE OF SIMPLE COMPOUNDS
The simplest compounds, which are made up of two different elements are called binary compounds. Valencies of some ions. You can use these to write formulae for compounds.
While writing the chemical formulae for compounds, we write the constituent elements and their valencies as shown below. Then we must crossover the valencies of the combining atoms.
1. Formula of hydrogen chloride
Formula of the compound would be HCl.
2. Formula of hydrogen sulphide
Some more examples
(a) Formula for aluminium oxide:
Formula : Al2O3
(b) Formula for calcium oxide:
Here, the valencies of the two elements are the same. You may arrive at the formula Ca2O2. But we simplify the formula as CaO.