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 :
- (1)(III) and (II)
- (2)(II) and (I)
- (3)Only (I)
- (4)Only (IV)
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.
- (1)(III) and (II) — 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.
- (2)(II) and (I) — 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.
- (3)Only (I) — 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.
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.
MPSC's general science section returns to atomic structure every year because it is the foundation of both physics and chemistry at school level, and the questions are usually true-or-false statement lists of exactly this kind. Two procedural points decide such items more often than knowledge does. The first is the negation: the emphasised word asking for the wrong statement is easy to skim past, and a candidate who reads the question positively will find three true statements and no way to choose among them. The second is the labelling: this question and the one after it use Roman numerals where the rest of the paper uses letters, so a candidate working quickly can misread which statement an option refers to. Reading the label system before reading the options costs a couple of seconds and removes both risks.
- The neutron carries no net electric charge, which is why it was the last of the three basic atomic particles to be identified; James Chadwick discovered it in 1932.
- The proton carries a positive charge and the electron an equal and opposite negative charge, about 1.6 times ten to the minus nineteen coulomb in magnitude, so an atom with equal numbers of each is electrically neutral.
- The nucleus consists of protons and neutrons, together called nucleons; the number of protons is the atomic number and fixes the element, while protons and neutrons together give the mass number.
- Because the neutron is uncharged it is not repelled by a nucleus, which is why neutrons are used to initiate and sustain nuclear fission chain reactions.
- A free neutron outside a nucleus is unstable and decays with a half-life of about ten minutes into a proton, an electron and an antineutrino, whereas a neutron bound in a stable nucleus does not decay.
That neutrality is why the neutron was found last — an uncharged particle leaves no cloud-chamber track and is not deflected by fields, so Chadwick identified it only in 1932 — and why neutrons, being unrepelled by a nucleus, are what start and sustain a fission chain reaction. Note this question labels its statements (I)–(IV), unlike the rest of the paper.
- Reading a negative stem positively, which on a list of otherwise true statements leaves no basis for choosing an answer
- Misreading which statement an option refers to when the labels are Roman numerals rather than the letters used elsewhere in the same paper
- Assuming that because the neutron sits in a positively charged nucleus it must itself carry positive charge
- Confusing electrical neutrality with the absence of internal structure, when the neutron is made of charged quarks whose charges cancel
Atomic structure appears in MPSC papers as short statement lists asking which statement is correct or, as here, which is wrong, and occasionally as a matching item pairing particles with their discoverers or their properties. The content is narrow and completely predictable — charge, mass, location and discoverer for each of the three particles, plus atomic number, mass number and isotopes — so it is worth learning as a single compact table. The difficulty the Commission adds is procedural rather than factual: an emphasised negation in the stem, an unusual labelling system, and option combinations that pair two true statements together.
No directly related past PYQ was found.
- practice — not a real PYQ
The neutron was the last of the three basic constituents of the atom to be discovered chiefly because of which of the following properties ?
- (a)Its very small mass
- (b)Its lack of electric charge
- (c)Its very short lifetime inside the nucleus
- (d)Its location outside the nucleus
Answer(b) Its lack of electric charge — an uncharged particle is not deflected by electric or magnetic fields and leaves no ionisation track, so the techniques that revealed the electron and the proton could not detect it. Chadwick identified it in 1932 by the recoil it produced in other nuclei. Its mass is close to the proton's, it is stable while bound in a nucleus, and it sits inside the nucleus.
- practice — not a real PYQ
In a neutral atom, the atomic number is equal to which of the following ?
- (a)The number of neutrons in the nucleus
- (b)The number of protons in the nucleus
- (c)The sum of protons and neutrons
- (d)The difference between neutrons and protons
Answer(b) The number of protons in the nucleus — the atomic number fixes the identity of the element, and in a neutral atom it also equals the number of electrons. The sum of protons and neutrons is the mass number, and atoms of one element that differ only in neutron count, and therefore in mass number, are isotopes.