Q. Which of the following isotope of Hydrogen has only one neutron? Answer:
Deuterium
Notes: 99.985% of hydrogen atoms contain only one proton. This isotope is called protium( 1H1). Rest of the percentage of hydrogen atom contains two other isotopes, the one containing 1 proton and 1 neutron is called deuterium (1D2, 0.015%) and the other one possessing 1 proton and 2 neutrons is called tritium (1T3 ). The latter isotope is found in trace amounts on the earth.