The filling of the table defined above leads to a natural division of the periodic table into blocks according to the outer electron configurations of atoms (see Fig. 2). All s block elements have configurations (ns)1 or (ns)2. In periods 2 and 3 these are followed by the p block with configurations (ns)2(np)x. Lower p block elements are same as the (n-1)d orbitals are too tightly bound to be chemically important. The p and s blocks are collectively known as

Fig. 1. Showing the order of filling of orbitals in the periodic table.

Fig. 2. Structure of the periodic table, showing the s, p, d and f blocks.

Main groups: d-block elements of periods 4, 5 and 6 have (n-1)d and ns outer electrons, and are known as transition elements. Their configurations give some complexities as the s and d orbitals are similar in energy. The f-block elements are known as the actinides (5f) and lanthanides (4f). For ease of presentation they are usually shown as separate blocks below the main table. In the case of the lanthanides, this procedure is justified as the elements have very similar properties.