We have already learnt that elements are found in three states: solid, liquid and gas at room temperature and standard atmospheric pressure. The elements are classified according to their properties- metals and non-metals. But it is very difficult to study all the properties of all elements at the same time.
Therefore, it took long time to study the properties of elements. Finally, scientists made a table in which properties of different elements are summarized. This is called Periodic Table. Scientific Periodic table was first introduced by Mendeleev a Russian scientist. He is known as father of Modern Periodic table. In 1869 he suggested that “there is a periodic recurrence of elements with similar chemical properties when these are written in order of increasing atomic weights”.
Working on this principle he arranged the various elements known in the form of a table and showed that elements having similar properties fall one below the other in the same column. He was also able to predict the existence and properties of some elements unknown at that time. Although this table best system of classification of elements. This periodic table is given above.
Periodic Table of the Elements
Modern Periodic Table consists of seven periods and eight groups and the elements placed in the same period or group show certain regularities. In group IA, Li, Na, K, Rb, Cs and Fr are placed. The chemical properties of Na resemble to Lithium or Potassium. For example, all these three elements are soft metals. They are reactive with water. Therefore, they are called reactive elements. The characteristics of groups and period are given below:
A period is a horizontal row in which the elements are written until there is a recurrence of elements having similar properties. The first period consists of two elements, Hydrogen and Helium. The second and third period contain eight elements each. These are known as short periods. But fourth and fifth periods contain eighteen elements each and are known as long periods. The sixth period is again a long period and contains thirty- two elements. The seventh periods consists of six naturally occurring and thirteen recently discovered Transuranium elements (discovered by nuclear researches) and is thus, a long and incomplete period.
Characteristics of Periods:
1.Change in valency: in short periods, the valency with respects to oxygen increases from 1 to 7 while that with respect to Hydrogen rises from 1 to 4 and then falls back from 1.
2.Change in properties: elements in the same period show gradual change in their physical properties. There is a gradual change from metallic to non-metallic character and from electropositive to electronegative behavior.
3. Atomic radii: the distance from the centre of the nucleus to the outer shell is called Atomic Radius. The larger the radius the more metallic is the atom. There is progressive decrease in atomic radius from left to right.
4. Electro-negativity: it is the tendency of the atom to attract electron to itself. It is found to increase progressively from left to right.
These are the vertical columns. These are eight groups numbered as Group I to Group VIII. Each of the group I to VIII are further been divided into two sub-groups, i.e., the elements placed to left constituting sub-group “A” and those placed to the right constituting sub-group “B”.
Characteristics of Group
1.Valency: The valency of the elements in any group with respect to oxygen is given by the group number.
2. Regular gradation in properties: the elements of the same sub-group show a regular gradation of physical and chemical properties with the rise of atomic weight.