Question:

The spectrum of helium is expected to be similar to that of

Updated On: May 4, 2024

H
Na
$Li^+$
$He^+$

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The Correct Option is C

Solution and Explanation

The spectrum of an atom depends on the number of electrons present in it. Here, helium has two electrons, so the spectrum of $Li^+$ (Z = 3) is similar to that of helium because both He and $Li^+$ have two electrons.

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Concepts Used:

Classification of Elements & Periodicity in Properties

Since many elements were being discovered in the 19^th 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 8^th element shows similarity with the 1^st 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.