An atomic number of Tritium is ________ .

Question

UnlockIAS · Accepted Answer

Correct — option (4), 01. Tritium is an isotope of hydrogen, and the atomic number of an element is fixed by the number of protons in the nucleus of its atoms. Hydrogen has one proton, so its atomic number is one, and that number cannot change without the atom ceasing to be hydrogen: the atomic number is what defines the element and fixes its place in the periodic table. Tritium therefore has an atomic number of one, exactly like ordinary hydrogen, and the answer is option (4). The question is built on the distinction between atomic number and mass number, and that distinction is the whole of it. The mass number is the total of the protons and the neutrons in the nucleus, and it is the mass number, not the atomic number, that varies between the isotopes of an element. Hydrogen has three isotopes and they differ only in their neutrons. Protium, the ordinary form that makes up almost all naturally occurring hydrogen, has one proton and no neutron at all, so its mass number is one. Deuterium has one proton and one neutron, so its mass number is two. Tritium has one proton and two neutrons, so its mass number is three. All three have a single proton and therefore an atomic number of one; all three have a single electron in the neutral atom and therefore behave in essentially the same way chemically; and all three occupy the same square of the periodic table. The name is itself the clue a candidate can use: tri- refers to the three particles in the nucleus, that is to the mass number, and not to any count of protons. Tritium is the only one of the three isotopes that is radioactive. It decays by emitting a beta particle, with a half-life of about twelve years, and it occurs naturally only in traces, formed when cosmic rays strike the upper atmosphere; the quantities used in research and in industry are made artificially. Deuterium is the isotope of the greatest practical importance in India, because deuterium oxide, or heavy water, is the moderator used in the country's pressurised heavy water reactors, and the heavy water plants that supply them are among the more distinctive parts of India's nuclear infrastructure. Option (4) is the answer.

Why the other options are wrong — (a) This is the trap the question exists to set, and it will collect every candidate who reaches for the number three because the name begins with tri-. Three is the mass number of tritium, not its atomic number: the nucleus contains one proton and two neutrons, and one plus two is three. The two quantities answer different questions. The atomic number counts protons alone and identifies the element; the mass number counts protons and neutrons together and identifies the isotope. Written in the conventional way, tritium is hydrogen-3, with the mass number three appearing as a superscript before the symbol and the atomic number one as a subscript. The number three is also the atomic number of lithium, which is a genuinely different element with three protons, and keeping tritium and lithium apart is a further reason to be clear that the three in tritium refers to the particles in the nucleus and not to the protons in it.  (b) Four corresponds to nothing in the description of tritium: the nucleus contains one proton and two neutrons, three particles in all, and there is no count associated with the isotope that comes to four. As an atomic number, four belongs to beryllium, a light metal of the second group, which has nothing to do with hydrogen or with any of its isotopes. The option is in the set as a pure distractor, to absorb a candidate who is guessing among small numbers, and it can be eliminated the moment the composition of the tritium nucleus is written down. That is the general method for this family of questions and it takes only a few seconds: write the number of protons, the number of neutrons and the number of electrons for the species named, and read the required quantity off from what has been written rather than trying to recall it directly.  (c) Two is the number of neutrons in a tritium nucleus, and the option is placed to catch a candidate who has correctly recalled the composition of the isotope but has then read off the wrong quantity from it. The atomic number counts protons and only protons, so the two neutrons are irrelevant to it. Two is also the atomic number of helium, and the confusion has a further trap attached to it, because the decay of tritium does in fact produce helium-3: the beta emission converts a neutron into a proton, so the nucleus that had one proton and two neutrons becomes one with two protons and one neutron, and the atomic number rises from one to two. That is a change of element, however, and it describes what tritium becomes rather than what tritium is.

Concept: Every atom is described by two numbers. The atomic number is the number of protons in the nucleus; it is equal to the number of electrons in a neutral atom, it fixes the chemical identity of the element, and it is the quantity by which the periodic table is ordered. The mass number is the sum of the protons and the neutrons, the two particles that carry effectively all the mass of the atom. Atoms of the same element with different numbers of neutrons are isotopes: they have the same atomic number but different mass numbers, and because chemistry is governed by electrons and therefore by protons, isotopes of an element are chemically almost identical while differing in mass and sometimes in nuclear stability. Hydrogen provides the clearest illustration because its three isotopes differ so greatly in relative mass — protium with no neutron, deuterium with one, tritium with two — and it is the only element whose isotopes are given separate names. The related terms are worth keeping distinct: isobars are atoms of different elements with the same mass number, isotones are atoms with the same number of neutrons, and isoelectronic species have the same number of electrons. Isotopes have wide practical uses that the Commission asks about directly: carbon-14 for dating archaeological material, cobalt-60 in the treatment of cancer, iodine-131 in the diagnosis and treatment of thyroid disease, uranium-235 as the fissile material of nuclear reactors, and deuterium as heavy water, the moderator of India's pressurised heavy water reactors.

State PCS 2022 Q51 — Science & Technology PYQ with Answer

State PCS 2022 Q51 — Science & Technology PYQ with Answer