An element is a pure substance that cannot be decomposed into any pure substance and which forms basic building blocks of any matter.

The Periodic table is an arrangement of elements in the order of their increasing atomic numbers to show that elements have related properties. Earlier tables, such as those of Dmitry Mandeleev (photograph) (1869) and Lother Meyer (1869-70) were based on atomic weights which are measured as bulk properties and valency relationships. At that time the concept of atomic numbers was unknown.

The main purpose of the periodic table was:

The modern form of the periodic law states that properties of the elements are the periodic function of their atomic numbers and the properties of the elements depend on their electronic configuration.

The modern periodic table consists of arrangements of elements in three broad categories.

The periodic table (Figure 2) arranges 109 known elements in order of their increasing atomic number. All these elements are arranged in Groups and Periods.

From the view point of electronic configuration the elements can be subdivided into four major sub groups. They can be classified as s,p,d and f block elements.

1. s-block elements : These have two atoms and include groups I A & II A. They contain one or two electrons in their outer S-orbital. Strongly electropositive metals. The group I A elements are alkali metals. Elements with two electrons in the outer orbit are called alkaline earth metals.

2. p-block elements : The elements from groups III A to VII A are included in this type. These are generally non-metals. These elements are generally non-conductors with the exception being Silicon and Germanium which are semi-conductors.

3. d-block elements : These are the elements in which the last electron is placed in d-orbital of penultimate energy level. These elements are also known as transitional elements. All are metallic and have high melting and boiling points. Good conductors of heat and energy.

4. f-block elements : The transition elements in which pre-penultimate f-sub orbital is being filled up are called as inner-transition elements.

5. zero-group elements : Earlier these elements were known as inert or Noble gases. They are chemically unreactive. They are incapable of forming any chemical compounds.

Figure 2 shows that each box in periodic table contains the following data:

Note the difference between the atomic number and the atomic weight. Thus element 12 is Magnesium (atomic number is 12) and not Carbon (atomic weight is 12).