71.

What is long form of the periodic table? Discuss in brief its salient structural features.

Long form of the periodic table (also called Bohr's table) is constructed on the basis of repeating configuration of the atoms when they are arranged in order of increasing atomic numbers.

Salient structural features:  This table has the following features:

(i) All the elements have been arranged in the increasing order of atomic numbers.

(ii) Elements having similar electronic configurations have been placed together at one place and those having different electronic configurations have been placed at different places in the periodic table.

(iii) Table consists of seven horizontal rows called periods and each period starts with different principal quantum number. The first, second and third periods are called short periods. They contain 2.8, 8 elements respectively. Fourth, fifth and sixth periods are called long periods. They contain 18, 18 and 32 elements respectively. Seventh period is incomplete and contains 19 elements (2+14+3).

(iv) The series of 14 elements just after lanthanum (₅₇La) i.e. from cerium (₅₈Ce) to lutetium (₇₁Lu) is collectively called lanthanoides and are placed at the bottom of the periodic table to avoid undue side-wise expansion of the periodic table.

(v) The series of 14 elements just after actinium (₈₉Ac) i.e. from thorium (₉₀Th) to lawrencium (₁₀₃Lr) is collectively called actinoides and are placed at the bottom of the periodic table to avoid undue side-wise expansion of the periodic table.

(vi) Lanthanoids and actinoids are also called Inner Transition elements.

(vii) The elements beyond uranium (atomic number 93 to 105) which have been prepared artificially by nuclear reactions are also called Transuranic elements or man made elements.

(viii) Table consits of eighteen vertical columns called groups or families.

(ix) Elements in each column have the greatest electronic similarities on the basis of their electronic configuration, the elements have been divided into s-block, p-block, d-block and f-block.

(x) According to 1UPAC system, groups are numbered as 1-18:
(i) Alkali metals (Group 1)
(ii) Alkaline earth metals (Group 2)
(iii) In between groups 2 to 13 there are ten columns which constitute group 3 to 12. The elements present in these groups are known as Transition elements.
(iv) Pincogens (group 15) (v) Chalcogens (Group 16) (vii) Halogens (group 17) (viii) Aerogens, Noble gases, inert or rare gases (group 18).

(ix) The elements constituting groups 1, 2 and group 13 to 18 are called normal or representative elements.