Chemistry X | Periodic Classification of Elements | THE MODERN PERIODIC TABLE


In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass as described below. Accordingly, Mendeléev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table and the Modern Periodic Law can be stated as follows:

‘Properties of elements are a periodic function of their atomic number.’ Let us recall that the atomic number gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next. Elements, when arranged in order of increasing atomic number Z, lead us to the classification known as the Modern Periodic Table (Table 5.6). Prediction of properties of elements could be made with more precision when elements were arranged on the basis of increasing atomic number.

Activity 5.3

  1. How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
  2. How were the positions of isotopes of various elements decided in the Modern Periodic Table?
  3. Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
  4. Where do you think should hydrogen be placed in the Modern Periodic Table?

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Chemistry X | Periodic Classification of Elements | Limitations of Mendeléev’s Classification

Limitations of Mendeléev’s Classification

Electronic configuration of hydrogen resembles that of alkali metals. Like alkali metals, hydrogen combines with halogens, oxygen and sulphur to form compounds having similar formulae, as shown in the examples here.

On the other hand, just like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to form covalent compounds.


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