Question

Among the following which are not given a proper place in the periodic table.
A. Alkali metals
B. Lanthanides
C. Actinides
D. Both B and C

Answer 1

The modern periodic table consists of various elements. Some of them are metals while some are non-metals. According to the trend in physical properties these all elements are well organized in the modern periodic table. But in case of some elements due to variable physical properties are not properly placed in the modern periodic table.

Complete Answer:
The modern periodic table is given by Mendeleev for the elements which are present on earth. Till now we have discovered $118$ elements and have successfully placed them in the modern periodic table. The principle behind the arrangement of elements in this table is that physical and chemical properties of an element are related to their atomic mass. Thus the modern periodic table classified the elements on the order of their atomic mass. The elements which are discovered till now are placed in some similar rows and columns. These are known as periods and groups. Some groups get their names as well. The first group of the modern periodic table is known as the alkali metals group. But some elements do not get successfully placed in the modern periodic table. This is because of their variable physical and chemical properties. These elements are called lanthanides and actinides. Both these groups contain fourteen elements which show different physical and chemical properties from the rest of the elements of the periodic table.

Thus on the basis of our observation the correct answer is option D.

Note:
The reason behind variable physical and chemical properties is their variable valency. Also they are somewhere radio-active in nature. Thus they are continuously emitting neutrons. Thus these elements cannot be placed in any group in the periodic table. If they got placed, then there will be discontinuity of the trend in physical and chemical properties of groups and periods.