Atomic Theory

PROPERTIES OF IONIC/ELECTROVALENT COMPOUNDS

1.  Ionic compounds are made up of two or more oppositely charged ions.

2.  These oppositely charged ions attract one another and form a large three-dimentional lattice, called a giant structure, which is held together by inter-ionic electrostatic attraction. So, ionic compounds are usually crystalline solids.

3.  Because of the great attraction between the ions, a large amount of energy has to be used to separate them. So ionic compounds have :

·         High boiling points

·         High melting points

4.   When molten (melted) or in aqueous solution, ionic compounds conduct an    

      electric current.

5.  Ionic compounds are usually soluble in water. But they do not dissolve in organic                    

      solvents, such as, ethanol, benzene or tetrachloromethane.

COVALENT BONDING

Elements in groups 4 and 5, having 4 and 5 electrons in their outermost electron shells are not stable. But unlike in the case of the elements in groups 1, 2 and 3, the elements in these two groups could not become stable by loss or gain of electrons. This is because while a group 4 element could not gain 4 or lose four electrons, a group 5 element could not gain 3 of lose 5 electrons. Elements in this group could therefore only get a stable outer electronic configuration by the sharing of electrons with other elements.

EXAMPLE 1

Consider an atom of carbon which has an atomic number of 6 and an electronic configuration of  (2 4). This element is found in group 4 of the Periodic Table. In order to become stable this has to share 4 electrons with other atoms.

Now suppose the element carbon is made to react with the element hydrogen whose atomic number is 1 and which therefore has an electronic configuration of (1). For hydrogen to become stable, it has share 1 electron with carbon. So, the compound formed between these two elements, could be represented by the following dot and cross diagram. In this case the valency electrons, that is, outer electrons only are shown.