MPSC 2022 Prelims Paper-I · Question 51 of 100

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.

• (1)03 — 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.
• (2)04 — 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.
• (3)02 — 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.

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.

Questions on atomic structure are among the most dependable marks in the MPSC science section, because they turn on definitions that do not change and can be answered in seconds by writing down the composition of the nucleus. What makes this particular item worth attention is the way the trap is built: the name of the isotope contains a numeral, that numeral is the mass number, and the question asks for the atomic number. The same construction recurs with carbon-14, uranium-235 and cobalt-60, and the defence is always the same — the number carried in the name of an isotope is its mass number, and the atomic number has to come from the identity of the element. The paper also prints the numerals here with a leading zero, so that the options read 03, 04, 02 and 01, and prints them out of their natural order; neither the padding nor the order carries any meaning, and this paper reorders option sets often enough that no inference should ever be drawn from where a value appears in the list. A candidate who reads the four values as numbers rather than as positions loses nothing to either device.

• The atomic number of an element is the number of protons in the nucleus of its atoms; it equals the number of electrons in a neutral atom and determines the element's identity and its position in the periodic table.
• Tritium is an isotope of hydrogen with one proton and two neutrons, so its atomic number is one and its mass number is three; the three in its name refers to the mass number.
• Hydrogen has three isotopes: protium with no neutron, deuterium with one neutron and tritium with two, all sharing the atomic number one and differing only in mass number.
• Tritium is the only radioactive isotope of hydrogen, decaying by beta emission with a half-life of about twelve years, and it occurs naturally only in traces formed by cosmic rays in the upper atmosphere.
• Deuterium oxide, known as heavy water, is used as the moderator in India's pressurised heavy water reactors, which makes deuterium the hydrogen isotope of the greatest practical importance in the Indian nuclear programme.

• Reading the numeral in the name of an isotope as its atomic number, when the number carried in a name such as tritium, carbon-14 or uranium-235 is always the mass number
• Counting neutrons when the atomic number is asked for, which produces the answer two for tritium and is exactly what one of the options is placed to collect
• Assuming that different isotopes of an element occupy different places in the periodic table, when they share an atomic number and therefore a single position
• Inferring anything from the order in which numerical options are printed, since this paper lists values out of their natural sequence in several questions

Atomic structure appears in MPSC papers as short definitional questions — the atomic number or mass number of a named species, the number of neutrons in an isotope, the meaning of isotope, isobar or isotone — and as applied questions on radioisotopes, where the use of a particular isotope in medicine, agriculture or dating is wanted. Hydrogen's isotopes are a favourite because they carry names of their own and because heavy water connects them to the nuclear programme, which the Commission examines separately as current affairs and as science policy. The reliable method for the whole family is mechanical: write the number of protons, neutrons and electrons for the species named before looking at the options, and then read the required quantity off. Doing so also protects against option sets that offer a correct number for the wrong quantity, which is how nearly every question in this family is made difficult.

No directly related past PYQ was found.

• practice — not a real PYQ

  Deuterium and tritium are both isotopes of hydrogen. In which of the following do they differ from each other ?

  • (a)In the number of protons in the nucleus
  • (b)In the number of electrons in the neutral atom
  • (c)In the number of neutrons in the nucleus
  • (d)In their position in the periodic table

  Answer(c) In the number of neutrons in the nucleus — deuterium has one neutron and tritium has two, which gives them mass numbers of two and three respectively. Both have a single proton and therefore the atomic number one, both have a single electron in the neutral atom, and both occupy the same position in the periodic table, since it is the atomic number that fixes an element's place there. The difference between isotopes is always a difference of neutrons and never of protons.

• practice — not a real PYQ

  Heavy water, used as a moderator in Indian nuclear reactors, is an oxide of which of the following ?

  • (a)Protium
  • (b)Deuterium
  • (c)Tritium
  • (d)Helium

  Answer(b) Deuterium — heavy water is deuterium oxide, in which both hydrogen atoms of the water molecule are the isotope with one proton and one neutron, making the molecule measurably denser than ordinary water. It is used in India's pressurised heavy water reactors to slow down neutrons so that they can sustain a chain reaction, and its manufacture is carried out at dedicated heavy water plants. Ordinary water is the oxide of protium, and tritium oxide is radioactive and has no such use.