The number of neutrons present in the deuterium isotope of hydrogen is
A. $2$
B. $3$
C. $5$
D. $1$

Hint: Deuterium isotope is an isotope of hydrogen with relatively low abundance. It has a mass number of two. We can get the number of neutrons by subtracting the atomic number from the mass number, since the sum of neutrons and protons is equal to the mass number.

 Formulas used: Number of neutrons $ = A - Z$
Where $A$ is the mass number and $Z$ is the atomic number.

Complete step by step answer:
Isotopes are atoms of the same element which have different mass numbers. That is, they have the same atomic number but different atomic mass.
As we know, mass number is the sum of neutrons and protons in an atom. And since the number of protons is also equal to the atomic number, we can say that the sum of atomic number and the number of neutrons is equal to the mass number. In other words, the number of neutrons in an atom is the difference between the mass number and the atomic number.
Number of neutrons $ = A - Z$
Where $A$ is the mass number and $Z$ is the atomic number.
As deuterium is the second isotope of hydrogen, it has a mass number of $A = 2$. And since it is an isotope of hydrogen, its atomic number is the atomic number of hydrogen, which is equal to $Z = 1$.
Hence, the number of neutrons in deuterium $ = 2 - 1 = 1$

So, the correct answer is Option D.

Note: Hydrogen has three isotopes, namely protium, with a mass number of $1$, deuterium, with a mass number of $2$ and tritium, which has a mass number of $3$. Out of these, protium is the most abundant and is what we actually refer to when we say hydrogen. Note that two atoms of deuterium can combine with an oxygen molecule to form a compound known as heavy water, having the molecular formula ${D_2}O$.