A model entrance exam being conducted at CEOA Matriculation Higher Secondary School, Kosakulam branch. Representative image.
| Photo Credit: The Hindu
The Supreme Court has asked IIT Delhi to put together a team of three experts to check whether a physics question in the 2024 NEET-UG paper could have two possible correct answers. The check comes on the back of a challenge from some students who gave the exam, alleging that the list of top-ranked students could change depending on how the students were marked on this question.
The question is in the form of two statements followed by four options. The student is required to pick one option from the four, with the assumption that only one of the four options is correct. The question is as follows:
1. Atoms are electrically neutral as they contain equal numbers of positive and negative charges.
2. Atoms of each element are stable and emit their characteristic spectrum.
The options are:
(i) 1 is correct and 2 is incorrect
(ii) 1 is incorrect and 2 is correct
(iii) 1 and 2 are both correct
(iv) 1 and 2 are both incorrect
The clear part
The first statement is clear: it refers to the numbers of protons and electrons in an atom. In fact, atoms are electrically neutral by definition; charged atoms are called ions.
The second statement is more ambiguous. It can be split into two, before and after the “and”. The statement after is that the atoms of each element emit a characteristic spectrum. ‘Spectrum’ in this context refers to the range of energy levels an atom can have. It depends on the gaps between the energy levels electrons in the atom can occupy. This in turn depends on the total charge.
Every element has a characteristic spectrum, and this part of statement 2 is correct.
Source of confusion
The ambiguity arises in the first part: that “atoms of each element are stable”. This is not always true because of the existence of isotopes, IIT Mandi assistant professor Nirmalya Kajuri said. An isotope of an atom has the same number of protons (and electrons) but a different number of neutrons. Many isotopes are unstable as a result; some isotopes are also more unstable than others.
Unstable atoms can undergo radioactive decay and turn into an isotope of a different element. One way to measure the extent of stability is by the amount of time required for the number of an element’s atoms to halve by radioactive decay, called the half-life.
For example, carbon-8 and carbon-14 are isotopes of the element carbon. The number in each case is the mass number — the sum of the numbers of protons and neutrons in its nucleus. Carbon-8 has a half-life of around 10-21 seconds whereas carbon-14 has a half-life of around 5,700 years. Thus, carbon-14 atoms are a lot more stable than carbon-8 atoms.
Stability is arguably also subjective: i.e. no atom is perfectly stable and each atom is only more or less stable than another. This may not have been the meaning of the question in the NEET-UG paper, but if it was, statement no. 2 would be correct only for isotopes like tellurium-128, which has a life of around 1024 years — 100-trillion times longer than the current age of the universe itself!
Indeed, physicists have noticed that when an atom’s nucleus has a ‘magic number’ of protons or neutrons, it is much more stable. This explains the stability of the atoms of helium, oxygen, calcium, tin, and lead, for example: they have a magic number of protons. Physicists have also predicted that there could be an as yet undiscovered element with a super-heavy atom that doesn’t rapidly decay like other super-heavy atoms by virtue of having a magic number of protons or neutrons.
The answer is…
Thus, of the four options available to students, those that say statement no. 1 is correct are definitely correct whereas statement no. 2 could be right or wrong depending on the isotope in question. That is, both options (i) and (iii) could be correct.
But for the students at the Supreme Court, the ‘official answer’ appears to turn on what the NCERT chemistry textbook for class XII states: that “all atoms of an element are stable” according to an old version and that “most” atoms are stable according to the new one.
