Extended or long form of periodic table

Modern periodic table is also called long form of the periodic table or Bohr’s table. In this table, the elements are arranged in order of their increasing atomic number. It consists of 4 blocks (s, p, d and f), 18 groups numbered from 1 to 18 and 7 periods numbered from 1 to 7.

Blocks : The periodic table is divided into four main blocks (s, p, d and f) depending upon the subshell to which the valence electron enters into.

(1) Elements of group 1 and 2 constitute s-Block.

(2) Elements of group 13, 14, 15, 16, 17, 18 constitute p-Block

(3) Elements of group 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 constitute d-Block

(4) The f-Block elements comprise two horizontal rows placed at the bottom of the periodic table to avoid its unnecessary expansion.

Elements of s- and p-blocks are called normal or representative elements, those of d-block are called transition elements while the f-block elements are called inner transition elements.

Groups : The 18 vertical columns are called groups. The elements belonging to a particular group is known as a family and is usually named after the first number. Apart from this some of the groups are given typical names as examplified beneath,

(1) Elements of group 1 are called Alkali-Metals.

(2) Elements of group 2 are called Alkaline Earths.

(3) Elements of group 3 are called Pnicogens.

(4) Elements of group 16 are called Chalcogens.

(5) Elements of group 17 are called Halogens.

(6) Elements of group 18 are called Noble Gases or Aerogens.

Periods : The horizontal rows are called periods. There are seven periods in the long form of the periodic table,

(1) Ist period ₁H—>₂ He) contains 2 elements. It is the shortest period.

(2) 2nd period (₃Li —>₁₀ Ne) and 3rd period ₁₁Na —>₁₈ Ar) contains 8 elements each. These are short periods.

(3) 4th period (₁₉K—>₃₆ Kr) and 5th period (₃₇R_b —>₅₄ Xe) contains 18 elements each. These are long periods.

(4) 6th period (₅₅Cs —>₈₆ Ra) consists of 32 elements and is the longest period.

(5) 7th period starting with ₈₇Fr is incomplete and consists of 19 elements.

Periodicity in properties

''The repetition of similar electronic configuration after a definite period is the cause of periodicity of the properties of elements.''

It can be explained with the help of electronic arrangement of elements. According to the modern views, the valency of an element is indicated by the number of electrons present in the outermost orbit. The chemical properties of elements are dependent on valency electrons. Variation in electronic arrangement leads to the variation in properties. After a definite interval, recurrence of similar electronic arrangement takes place when the number of valency electrons is the same. Thus, there is a regular gradation and repetition in the properties of elements.

Periodic properties are directly or indirectly related to their electronic configuration and show a regular gradation on moving from left to right in a period or from top to bottom in a group. Some period or from top to bottom in a group. Some important periodic properties are : oxidation number, shielding effect, atomic radii, ionization energy, electron affinity, electro-negativity, valency, density, m.pt. and b.pt.