Question

Number of elements present in period $ 1 $ are ______.
(A) $ 2 $
(B) $ 4 $
(C) $ 6 $
(D) $ 8 $

Answer 1

Hint :Note down the number of subshells present in the atom for principal quantum number $ n = 1 $ and then to get the total number of elements in the given period, add the number of electrons that can be filled in the subshells.

Complete Step By Step Answer:
In the modern periodic table, the chemical and physical properties of the elements are the functions of their respective atomic numbers. The elements are subdivided into various categories as Noble gas elements, representative elements, transition metal elements and inner transition elements which are also known as lanthanides and actinides.
The modern periodic table arranges the elements in groups and periods and elements are placed according to the increasing value of their atomic number. The modern periodic table consists of $ 7 $ periods and $ 18 $ groups.
For principal quantum number $ n = 1 $ :
The value of an azimuthal quantum number will be $ 0 $ to $ n - 1 $ i.e., will have only one value which is equal to zero. This indicates that there is only one subshell present which is $ 1s $ subshell.
As we know, the total number of electrons that can be filled in the $ 1s $ subshell is two. Therefore, we can conclude that there are only two elements present in the period $ 1 $ of the modern periodic table.
Hence, option (A) is the correct answer.

Note :
While writing the number of subshells for principal quantum number $ n > 4 $ , make sure there are only four subshells available that are s, p, d and f subshells and a total of $ 120 $ electrons can be filled in these subshells.