Which of the following sentence is wrong ?
(I) Atomic nucleus is made up of protons and neutrons.
(II) Proton has a positive charge.
(III) Electron has a negative charge.
(IV) Neutron has a positive c

Question

Which of the following sentence is wrong ?
(I) Atomic nucleus is made up of protons and neutrons.
(II) Proton has a positive charge.
(III) Electron has a negative charge.
(IV) Neutron has a positive charge.
Answer options :

UnlockIAS · Accepted Answer

Correct — option (4), Only (IV). This is a negative stem — the word 'wrong' is printed in bold in the paper — so the task is to find the statement that is false, not the ones that are true. Note also that this question labels its statements with Roman numerals (I) to (IV), unlike most of the paper, so the labels in the options refer to those numerals. Statement (IV) says the neutron has a positive charge, and it does not: the neutron is electrically neutral, which is exactly what its name records. That neutrality is why it was the last of the three basic particles to be found — an uncharged particle leaves no track in a cloud chamber and is not deflected by electric or magnetic fields — and James Chadwick identified it only in 1932, decades after the electron and the proton. At a deeper level the neutron is not structureless but is made of three quarks whose fractional charges, one of plus two-thirds and two of minus one-third, add exactly to zero. The other three statements are all correct. Statement (I) describes the nucleus as made up of protons and neutrons, which is the nuclear model established after Rutherford's scattering experiment showed the atom's mass and positive charge to be concentrated in a tiny central volume; the two particles together are called nucleons, the number of protons fixes the atomic number and the chemical identity of the element, and protons plus neutrons give the mass number. Statement (II) gives the proton its positive charge, equal in size to the electron's and opposite in sign, and statement (III) gives the electron its negative charge. Since exactly one statement fails, and it is statement (IV), the answer is the choice that names it alone.

Why the other options are wrong — (a) This names statements (III) and (II) as the wrong ones, but both are correct: the electron carries a negative charge and the proton a positive charge of the same magnitude, about 1.6 times ten to the minus nineteen coulomb each. The equality of those magnitudes is what allows an atom with equal numbers of protons and electrons to be electrically neutral overall, and an imbalance between them is precisely what makes an ion. Choosing this pair marks two of the most secure facts in atomic structure as errors while leaving the one genuinely false statement unselected.  (b) This names statements (II) and (I) as wrong. Statement (II), the positive charge of the proton, is correct, and statement (I) is the standard description of the nucleus as a body of protons and neutrons — the model that followed Rutherford's discovery of the nucleus and was completed once Chadwick identified the neutron in 1932. Before that identification the composition of the nucleus was genuinely unsettled, since protons alone could not account for the mass of any element beyond hydrogen; but the statement as printed reflects the settled position and is not the error the stem is asking for.  (c) This selects statement (I) alone as the wrong one, that is, it rejects the description of the nucleus as made up of protons and neutrons. That description is correct, and it is the foundation of everything else in the question: the atomic number is the count of protons in the nucleus, the mass number is the count of protons and neutrons together, and isotopes of an element differ only in the number of neutrons. Rejecting it would leave no account of where the atom's mass resides, and it also leaves statement (IV), the only genuinely false statement in the list, standing unchallenged.

Concept: The modern picture of the atom rests on three particles and two regions. The electron, identified by J. J. Thomson in 1897, carries a negative charge and occupies the space around the nucleus; the proton, positively charged and about eighteen hundred times heavier than the electron, sits in the nucleus; the neutron, of very nearly the same mass as the proton but with no charge at all, sits there with it. Rutherford's scattering experiment established that the positive charge and almost all the mass of an atom occupy a nucleus tiny in comparison with the atom, and Chadwick's discovery of the neutron in 1932 explained why atomic masses exceed what the protons alone can account for. Protons and neutrons together are called nucleons: the number of protons is the atomic number, which fixes the element and, in a neutral atom, the number of electrons; the total number of nucleons is the mass number; and atoms of one element differing only in neutron count are isotopes. The neutron's electrical neutrality has consequences beyond bookkeeping. It allows a neutron to approach a nucleus without being repelled, which is why neutrons are the particles used to initiate nuclear fission, and it means a free neutron outside a nucleus is unstable, decaying with a half-life of about ten minutes into a proton, an electron and an antineutrino.

State PCS 2024 Q54 — Science & Technology PYQ with Answer

State PCS 2024 Q54 — Science & Technology PYQ with Answer