Answer: Option (c) is the correct answer.
Explanation:
As it is known that mass number means the sum of total number of protons and neutrons present in an atom.
Whereas atomic number means only total number of protons.
As it is given that mass number of element X is 7 and neutrons present in it are 4. Hence, its atomic number will be calculated as follows.
Atomic number = Mass number - no. of neutrons
= 7 - 4
= 3
Now, it is given that mass number of element Y is 9 and neutrons present in it are 5. Hence, its atomic number will be calculated as follows.
Atomic number = Mass number - no. of neutrons
= 9 - 5
= 4
Therefore, atomic number of element X is 3 and atomic number of element Y is 4. So, it means both these elements are present in same period.
Thus, we can conclude that it is true about the two atoms that atom X and Atom Y are in the same row on the periodic table.
