Q.What are isotopes ?

Protium has no neutrons.

Protium is the lightest isotope of hydrogen, symbol 1/1 H. It has 1 proton, 1 electron and no neutrons (mass number 1 = 1 proton + 0 neutrons). Deuterium has 1 neutron and tritium has 2 neutrons.

Tritium is the radioactive isotope of hydrogen with 1 proton and 2 neutrons.

Hydrogen has three isotopes: protium (¹H, no neutrons, the most abundant), deuterium (²H or D, 1 neutron), and tritium (³H or T, 2 neutrons, radioactive with a half-life of about 12.3 years). All three have the same atomic number (1) but different mass numbers, so tritium is an isotope of hydrogen.

Among hydrogen's three isotopes (protium, deuterium, tritium), only tritium is radioactive.

Hydrogen has three naturally occurring isotopes, all with 1 proton but differing numbers of neutrons:

• Protium (1-H): 0 neutrons, the most abundant (about 99.98%), stable.

• Deuterium (2-H or D): 1 neutron, stable, used in heavy water (D2O).

• Tritium (3-H or T): 2 neutrons, present in only trace amounts, and it is radioactive — it undergoes beta decay, emitting a low-energy beta particle (electron) and converting to helium-3 (3-He), with a half-life of approximately 12.3 years.

Deuterium differs from hydrogen in physical properties.

Deuterium (2H, D) and protium (1H) are isotopes: they have the same atomic number (1) and the same electronic configuration (one electron), so their chemical properties are essentially identical. They differ in mass (deuterium is about twice as heavy), which affects mass-dependent physical properties such as density, boiling point, freezing point and reaction rate. Deuterium is not radioactive. Hence the difference is in physical properties.

Hydrogen isotopes are covered in the NCERT Class 11 Chemistry chapter Hydrogen.