Long form of the Periodic Table or Moseley’s Periodic Table :
(i) Moseley (1909) studied the frequency of X-rays produced by the bombardment of a strong beam of electrons on a metal target. He found that the square root of the frequency of X-rays √ν is directly proportional to the total nuclear charge (z) of metal.
√ν = a(Z-b) , where a and b are constants. Nuclear charge of metal is equal to the atomic number. So Moseley related the properties of elements with their atomic number and gave the new periodic law.
(ii) Moseley’s Periodic Law or Modern Periodic Law: Physical and chemical properties of elements are the periodic functions of their atomic number. If the elements are arranged in order of their increasing atomic number, after a regular interval, element with similar properties are repeated.
Long form of the Periodic Table and Electronic Configuration of elements
Many different forms of a periodic classification of the elements have appeared since the 1871 table by Mandeleef. Each table was designed to point up the various trends and relationship which its author considered most significant. From the literally hundreds of tables which have been proposed,perhaps the most popular and easily reproduced periodic table is the conventional extended or long form, which is shown in table.
(i) Each period starts with an alkali metal whose outermost electronic configuration is ns1.
(ii) Each period ends with a noble gas of outermost electronic configuration ns2np6 except He. The electronic configuration of He is 1s2.
(iii) The number of elements in a period is equal to the number of necessary electrons to acquire ns2np6 configuration in the outermost shell of first element (alkali metal) of the period. First period contains two elements.
(iv) The number of elements in each period may be determined by the number of electrons in a stable configuration as under
Periods Stable electronic configuration Number of electrons
First 1s2 2
Second 2s22p6 8
Third 3s23p6 8
Fourth 4s23d104p6 18
Fifth 5s24d105p6 18
Sixth 6s24f145d106p6 32