77. How many blocks are present in the long form of periodic table? How will you classify the elements of the periodic table into s, p, d and f blocks? 

The long form of periodic table has been divided into four blocks. These are called:
(i) s-block (ii) p-block
(iii) d-block , (iv) f-block.
Division of elements into s, p, d and f-blocks: This division is based upon the name of the orbital which receives the last electron.

1. s-block elements. The elements in which the last electron enters the s-orbital of the outermost enthalpy level are called .s-block elements. The general electronic configuration of s-block elements is ns^1-2 where n stands for the outermost shell. In s-block, the alkali metals of group 1 and alkaline earth metals of group 2 are included.

2. p-block elements: The elements in which the last electron enters the p-orbital of the outermost enthalpy level are called p-block elements. The general electronic configuration of p-block elements in ns²np^1-6 where n stands for the outermost shell. The elements of Groups 13, 14, 15, 16, 17, 18 having 3, 4, 5, 6, 7 and 8 electrons respectively in the outermost enthalpy levels constitute p-block elements.

3. d-block elements:The elements in which the last electron enters the d-orbital of the penultimate enthalpy level (last but one shell) are called d-block elements. General electronic configuration of d-block elements is (n - 1) d^{1-10 n}s^1-2 where n represents the valence shell, d-block elements have three complete series of ten elements in each whereas the fourth series is incomplete.

Note, Exception is ₄₆Pd whose configuration is 4d¹⁰ 55⁰:

4. f-block elements:The elements in which the last electron enters the f-orbital of the anti-penultimate (third to the outermost shell) enthalpy level are called f-block elements. The general electronic configuration of f-block elements is (n -2) f^1-14 (n-1) d^0-1 ns²where n represents the outermost shell. Such type of elements have two series. First is 4f series which is also called lanthanoides series whereas second is 5f series which is also described as actinoide series. These two series are placed separately at the bottom of the periodic table. The elements included in these two series are called inner transition elements.