Question:

For the element $O$ or $F$, the electron gain enthalpy is less negative than that of the succeeding element. Electron gain enthalpy becomes less negative as we go down a group.

Updated On: Jul 6, 2022
  • If both assertion and reason are true and reason is the correct explanation of assertion.
  • If both assertion and reason are true but reason is not the correct explanation of assertion.
  • If assertion is true but reason is false.
  • If both assertion and reason are false.
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The Correct Option is B

Solution and Explanation

Generally, electron gain enthalpy becomes less negative (decreases) as we go down the group because size of the atom increases and the added electron will be farther from the nucleus. But in case of $O$ or $F$, the added electron goes to the smaller quantum level i.e., $n = 2$ and suffers significant repulsion from other electrons present in this level.
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Notes on Classification of elements and periodicity in properties

Concepts Used:

Classification of Elements & Periodicity in Properties

Since many elements were being discovered in the 19th century and the study of these elements individually was proving difficult, classification of elements was made necessary.

Classification by Johann Dobereiner - German chemist Johann Dobereiner classified certain elements on the basis of their similar properties in the groups of continuing - three elements each. These groups were called ‘triads’. In every triad, the atomic weight of the middle element was equal to the average of the atomic weights of the first and third elements. 

Newlands Law of Octaves - The elements were arranged in increasing order of their atomic weights and found that every 8th element shows similarity with the 1st element. 

Mendeleev’s Periodic Table - The arrangement of all 63 elements in rows or columns in order of their atomic weight was made by Mendeleev. He left some space for corresponding elements in his periodic table which were not even discovered till then. Although he predicted the properties of those elements through his periodic classification of elements. 

Modern Periodic Law - The properties of the elements of the modern periodic law are periodic functions of their atomic numbers.