Prepare Periodic Table elements with atomic number and their atomic mass with Electronic configuration
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(a) The modern periodic law states that the properties of elements are a periodic function of their atomic numbers.
(b) When elements are arranged according to increasing atomic numbers, there is a periodicity in the electronic configurations of the elements. The elements in a period have consecutive atomic numbers. The elements having same number of valence electrons in their atoms are placed in a group. All the elements in a group have similar electronic configurations and show similar properties.
(c) When the elements are arranged according to their atomic numbers on the basis of modern periodic law, then all the anomalies (or defects) of Mendeleev's classification disappear. This is discussed below.
(i) Explanation for the position of isotopes: All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table. For example, both the isotopes of chlorine, Cl-35 and Cl-37, have the same atomic number of 17, so both of them can be put at one place in the same group of the periodic table.
(ii) Explanation for the position of cobalt and Nickel: The atomic number of cobalt is 27 and that of nickel is 28. Now, according to modern periodic law, the elements with lower atomic number (Co) are arranged in the first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.
(iii) Explanation for the Position of Hydrogen: Hydrogen element has been placed at the top of group 1, above the alkali metals becuase the electronic configuration of hydrogen is similar to those of alkali metals. Both, hydrogen as well as alkali metals have 1 valence electron each.
(d) Atomic number is always a simple whole number. It can either be 1 or 2 . There can be no element with atomic number 1.5
(e) The modern periodic table was prepared by Niels Bohr.
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