The total number of neutrons present in one molecule of ${\mathrm{D}}_2\mathrm{O}$¹⁶ is ______.

D or deuterium (${}_1{}^2\mathrm{D}$) is an isotope of hydrogen.

• It is an isotope of hydrogen.
• Atomic number is the number of protons of an atom. Here the subscript represents the atomic number and is given as 1. Hence it has 1 proton.
• Mass number is the sum of the number of protons and neutrons of an atom. Here the superscript represents the mass number and is given as 2.
• Mass number =number of protons+number of neutrons; number of neutrons=mass number -number of protons=(2-1)=1.

O-16 (${}_8{}^{16}\mathrm{O}$)

• it is an isotope of oxygen.
• Here the subscript represents the atomic number and is given as 8. Hence it has 8 protons.
• Here the superscript represents the mass number and is given as 16.
• Mass number =number of protons+number of neutrons; number of neutrons=mass number -number of protons=(16-8)=8.

${\mathrm{D}}_2{\mathrm{O}}^{16}$

• Hence total number of neutrons= (2×1) + 8 =10
• Therefore, the total number of neutrons present in one molecule of ${\mathrm{D}}_2{\mathrm{O}}^{16}$ is 10.