Chemistry · Structure Of Atom · NEET
| Number of neutrons | Protium 0, Deuterium 1, Tritium 2 | Atomic number stays 1 for all |
| Mass number (A) | 1, 2, 3 respectively | Because A = protons + neutrons |
| Symbol | 1/1 H, 2/1 H (D), 3/1 H (T) | Same element symbol H, different superscript |
| Stability | Protium and deuterium stable | Tritium is radioactive (beta emitter) |
| Abundance | Protium about 99.985% | Deuterium and tritium are very rare |
All three are isotopes of hydrogen, so all have exactly 1 proton and atomic number Z = 1. The only difference is the number of neutrons in the nucleus. Protium has 0 neutrons, deuterium has 1 neutron, and tritium has 2 neutrons. This makes their mass numbers 1, 2 and 3. Their symbols are protium = 1 over 1 H, deuterium = 2 over 1 H (also written D), and tritium = 3 over 1 H (also written T).
Atomic number (Z) counts only protons. All hydrogen isotopes have 1 proton, so Z is always 1 for each of them. Mass number (A) counts protons plus neutrons. Since protium, deuterium and tritium have different numbers of neutrons (0, 1, 2), their mass numbers are different (1, 2, 3). So the neutron is the only thing that changes between the three.
Use neutrons = mass number A minus atomic number Z, where Z = 1 for all hydrogen isotopes. Protium: A = 1, so neutrons = 1 - 1 = 0. Deuterium: A = 2, so neutrons = 2 - 1 = 1. Tritium: A = 3, so neutrons = 3 - 1 = 2. Easy pattern: 0, 1, 2 neutrons.
Tritium ( 3 over 1 H) is the only radioactive isotope of hydrogen. It is unstable and undergoes beta-minus decay: it turns into helium-3 and emits a beta particle (an electron). Protium and deuterium are both stable and are not radioactive. NEET has directly asked which particle tritium emits, and the answer is a beta (beta-minus) particle.
Yes. Deuterium ( 2 over 1 H, symbol D) is called heavy hydrogen because it has 1 extra neutron, so it is about twice as heavy as normal protium. When deuterium bonds with oxygen it forms heavy water, D2O. This is a common exam fact you should remember.
Isotopes are atoms of the SAME element with the same atomic number Z but different mass numbers A. Protium, deuterium and tritium all have Z = 1, so they are isotopes. Isobars are different elements with the same mass number but different Z, like 14 over 6 C and 14 over 7 N. Do not mix these up: iso-TOPE = same protons, iso-BAR = same mass.
Tritium, a radioactive isotope of hydrogen, emits which of the following particles?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Protium ( 1 over 1 H), deuterium ( 2 over 1 H or D), and tritium ( 3 over 1 H or T). All have 1 proton but 0, 1 and 2 neutrons respectively.
Protium is by far the most common. About 99.985 percent of natural hydrogen atoms are protium, which has just 1 proton and no neutron.
Yes, almost the same. Chemical properties depend on the number of electrons, and all three have 1 electron. Neutrons barely affect chemistry, so all hydrogen isotopes show nearly the same chemical behaviour.
Heavy water is D2O, water made using deuterium instead of ordinary protium. It is heavier than normal water because deuterium carries an extra neutron.
Use neutrons = A - Z (mass number minus atomic number). For hydrogen Z = 1 always, so neutrons = A - 1: giving 0, 1 and 2 for protium, deuterium and tritium.
