Which of the following is not an isotope of hydrogen? 

EXPLANATION:

Isotopes of Hydrogen

• Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number.
• Hydrogen has three main isotopes:
  • Protium (1^H): It has 1 proton and 0 neutrons.
  • Deuterium (2^H or D): It has 1 proton and 1 neutron.
  • Tritium (3^H or T): It has 1 proton and 2 neutrons.
• Among these, the correct answer is:
  • Dubnium (2): It is not an isotope of hydrogen.
  • Dubnium is a chemical element with the symbol Db and atomic number 105, which is entirely different from hydrogen.

Therefore, Dubnium is not an isotope of hydrogen.