How could the modern periodic law remove various anomalies of Mendeleev's periodic table? Explain with examples.
Answers
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.
Explanation:
1. In the Modern Periodic Table atomic number of an elements is a more fundamental property than its atomic mass.
2. The anomalous position of hydrogen can be discussed after we see what are the basis on which the position of an elements in the Modern Periodic Table depends.
3. The elements present in any one group have the same number of valence electrons.
4. Atoms of different elements with the same number of occupied shells are placed in the same period.
5. In the Modern Periodic Table, a zig-zag line separated metals from non-metals.