The correct option is (a).
Explanation
Isotopes are atoms of the same element having the same atomic number (number of protons) but different mass numbers (sum of protons and neutrons). Hydrogen has three main isotopes: Protium, Deuterium, and Tritium, which differ in the number of neutrons present in their nuclei.
Statement-wise Analysis:
- Statement 1 is Correct: Protium ($^{1}_{1}\text{H}$) is the most common isotope of hydrogen. Its nucleus consists of only one proton and zero neutrons. It is the only stable isotope with no neutrons.
- Statement 2 is Incorrect: Deuterium ($^{2}_{1}\text{H}$ or D), also known as heavy hydrogen, has a mass number of 2. Its nucleus contains one proton and one neutron, not two.
- Statement 3 is Incorrect: Tritium ($^{3}_{1}\text{H}$ or T) is a radioactive isotope with a mass number of 3. Its nucleus contains one proton and two neutrons, not one.
Key Takeaway:
The neutron count for hydrogen isotopes is: Protium = 0, Deuterium = 1, Tritium = 2. All possess exactly one proton and one electron (in a neutral state).