Question

Assertion:
The mass number of hydrogen is equal to its atomic number
Reason:
$H - $ atom has no neutrons.
A.Both assertion and reason are correct and reason is the correct explanation of the assertion.
B.Both assertion and reason are correct, but reason is not the correct explanation for assertion.
C.Assertion is correct but reason is not correct.
D.Assertion is incorrect, but reason is correct.

Answer 1

Hint: The number of protons present in the nucleus of an atom is considered as atomic number and the sum of number of protons and the number of neutrons is known as mass number. The mass of electrons is not considered while calculating the mass of an atom.

Complete step by step answer:
For a chemical element the number of protons found in its nucleus is known as its atomic number. Atomic number is unique for each and every element so it is considered as the basis of the periodic table formation. Earlier mass numbers were considered but due to some irregularities it was dismissed.
The nucleus of an atom consists of neutrons and protons. And the whole mass of the atom is decided on the basis of the mass of the nucleus because the mass of electrons present outside the nucleus is very negligible as compared to mass of protons and neutrons.
So the formula to calculate mass number is as follows:
${\text{Mass number = Number of protons + Number of neutrons}}$
And for a hydrogen atom which is represented as $_1^1H$ the atomic number is one and the mass number is also one therefore both are equal. And we know that the number of protons is equal to the atomic number, therefore the number of protons is also one.
By putting the values in the above formula we get:
${\text{Number of neutrons = 1 - 1 = 0}}$
Hence the number of neutrons present in the hydrogen atom is also zero.
So from the above discussions we can say that Assertion i.e. the mass number of hydrogen is equal to the atomic number is correct. And the Reason i.e. hydrogen atoms have no neutrons is also correct and reason is the correct explanation for assertion.

So, the correct answer is Option A .

Note:
The well studied arrangement of elements in rows and columns according to the properties of elements is known as periodic table. Basically it was invented or discovered to find out the relation between elements and to study the behavior and properties of elements.