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1. Anomalous position of hydrogen. Hydrogen has one electron in its only shell, i.e., K shel1.Alkali metals also have only one electron in their respective outermost shells. Therefore, many properties of hydrogen are similar to those of alkali metals. For example, like alkali metals, hydrogen combines with halogens, oxygen and sulphur to form compounds having similar formulae as shown below.
Thus, hydrogen can be placed along with alkali metals of group IA of the Mendeleev‘s periodic table.As stated above, hydrogen has one electron in its only shell. In other words, it has one electron less than the nearest noble gas, helium (which has two electrons). Halogens, on the other hand, have seven electrons in their respective outermost shells. In other words, like hydrogen, halogens also have one electron less than their respective nearest noble gases (which have eight electrons). Therefore, many properties of hydrogen are similar to those of halogens. For example, just like halogens (F2, Cl2, Br2, I2, etc.) Hydrogen (H2) also exists as a diatomic molecule. Similarly, like halogens, hydrogen also combines with alkali metals to form ionic compounds and with non-metals to form covalent compounds as shown below:
In other words, position of hydrogen in the periodic table is controversial, i.e. , no fixed position can be assigned to hydrogen in the periodic table. Even then placement of hydrogen along with alkali metals seems to be more appropriate because it is the first element in the periodic table and has the lowest atomic mass.
2.Position of isotopes. Isotopes were discovered long after Mendeleev gave his periodic table.
isotopes are the atoms of the same element having similar chemical properties but different atomic masses.
For example, hydrogen has three isotopes called protium (H), deuterium (D) and tritium (T) having atomic masses 1, 2 and 3 respectively. Similarly, carbon has three isotopes, i.e. , C-12. C- 13 and C- 14 and chlorine has two isotopes, C1-35 and Cl-37.
Since in the Mendeleev’s Periodic Table, elements are arranged in increasing order of their atomic masses and isotopes of an element have different atomic masses, therefore, they should be placed at different positions in the periodic table. Further since isotopes of an element have the same chemical properties, therefore, they should be placed in the same position in the periodic table. Thus, the position of isotopes in the Mendeleev‘s periodic table is anomalous. In other words, isotopes of all elements posed a challenge to Mendeleev‘s Periodic Law.
3. Anomalous position of some pairs of elements. Although the elements in the Mendeleev’s Periodic Table have been arranged in order of increasing atomic masses, yet in some cases, an element with higher atomic mass has been placed before an element with lower atomic mass. For example, cobalt (Co) with higher atomic mass (58•93 u) has been placed before nickel (Ni) with lower atomic mass (58•7 1 u). Similarly, tellurium (Te) with higher atomic mass (127•6 u) has been placed before iodine (1) with lower atomic mass (126.9 u). This is contrary to the Mendeleev’s periodic law.
4.Uncertainty in prediction of new elements. In Mendeleev’s Periodic Table, the elements are arranged in increasing order of their atomic masses. But the atomic masses do not increase in a regular manner going from one element to the next. Further, the difference in atomic masses between two successive elements is small among higher elements but large among heavier elements. Therefore, it is not possible to predict how many new elements could be discovered between two known elements especially among heavier elements.Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah.