MPSC 2021 Prelims Paper-I · Question 51 of 100

Correct — option (3): statement (a) holds and statement (b) does not. Statement (a) is one of the basic facts of atomic structure. An atom contains a nucleus made of protons and neutrons, with electrons outside it. A proton carries a charge of +e and an electron a charge of −e, where e is the elementary charge, about 1.6 × 10⁻¹⁹ coulomb; a neutron carries no charge at all. In an atom the number of protons and the number of electrons are equal — that number is the atomic number Z — so the positive and negative charges cancel exactly and the atom as a whole is electrically neutral. When the two numbers stop being equal the particle is no longer a neutral atom but an ion, positive if it has lost electrons and negative if it has gained them. Statement (b) fails on a single word, and it is the word that carries the physics. The sentence as printed says 'The negative charge on nucleus is equal to the total negative charge of all the orbiting electrons.' The nucleus does not carry a negative charge. It carries a POSITIVE charge, of magnitude +Ze, contributed by its Z protons; the neutrons add mass but no charge; and the negative charge lies entirely outside the nucleus, on the Z orbiting electrons, totalling −Ze. That the nucleus is positive is not a definitional convention but an experimental result — it comes from Rutherford's gold-foil experiment of 1911, in which a small fraction of positively charged alpha particles fired at a thin gold foil were deflected through very large angles and a few came almost straight back, which can only happen if the atom's positive charge and nearly all its mass are concentrated in a minute central region rather than spread through the atom as Thomson's model had supposed. So the sentence describes an atom that does not exist: if both the nucleus and the electrons were negative, the atom would carry a large net negative charge and statement (a) would collapse with it, and the two would fly apart rather than bind. It is worth being exact about what the correct version of this sentence would say. The POSITIVE charge on the nucleus is equal in MAGNITUDE, and opposite in sign, to the total negative charge of all the orbiting electrons — and it is that equality of magnitudes with opposite signs which makes statement (a) true. As printed, statement (b) is wrong, and the row that accepts the first statement and rejects the second is option (3).

• (1)Both are incorrect — This option rejects statement (a) along with statement (b), and statement (a) is not rejectable. An atom is electrically neutral by construction: it holds equal numbers of protons and electrons, whose charges are equal in magnitude and opposite in sign, so the total charge is zero. A candidate who arrives here has usually reasoned backwards from the second statement — noticing that something is wrong with the charge description and concluding that the whole account of the atom in the question must be unsound. The two statements have to be judged separately, which is the discipline every statement-verification item in this paper rewards. There is also a genuine confusion worth clearing: a particle with unequal numbers of protons and electrons is an ion, not an atom, so pointing to sodium ions or chloride ions does not make the neutrality of an atom false. Neutrality is part of what the word 'atom' means in this context.
• (2)Both are correct — This is the option that the question exists to catch, and it catches a reader who recognises the shape of the sentence without reading its adjective. The familiar true statement is that the charge on the nucleus is equal in magnitude and opposite in sign to the total charge of the electrons, and statement (b) has the rhythm of that sentence exactly — the same clause order, the same comparison, the same idea of an internal balance. What it does not have is the right sign: it puts a negative charge on the nucleus. A nucleus made of protons and neutrons cannot be negative, because protons are positive and neutrons carry no charge, and there is nothing else in it. Accepting this option also makes the two statements contradict each other, since two negative charges cannot cancel to zero — so a candidate choosing it is asserting that an atom is neutral and simultaneously that everything in it is negatively charged. Reading the adjectives in a physics statement is not a nicety; here it is the entire question.
• (4)(a) incorrect, (b) correct — This option inverts both judgments and is therefore wrong twice. It denies that an atom is electrically neutral, which the equality of proton and electron numbers guarantees, and it accepts a sentence placing a negative charge on the nucleus, which the composition of the nucleus forbids. The route to it is usually mechanical rather than conceptual: in a two-statement item with four rows, a candidate who has decided the first statement is true and the second false, and who then reads the rows in a hurry, can select the row that says the opposite. That is a real and avoidable loss, and the defence is to write a tick and a cross beside the two statements on the question paper and then match the row to the marks rather than to memory. On a paper that deducts a quarter of the marks for a wrong answer, a transcription error of this kind costs exactly as much as not knowing the physics at all.

The modern picture of the atom was assembled experiment by experiment. J. J. Thomson's identification of the electron in 1897 showed that atoms have parts and that one of those parts is negatively charged, and his model distributed the compensating positive charge through the whole atom. Rutherford's gold-foil experiment of 1911, performed by Geiger and Marsden, destroyed that picture: most alpha particles passed through the foil undeflected, a few were deflected sharply and a very small number rebounded, which is only possible if the positive charge and almost all the mass sit in a tiny central nucleus. Chadwick's discovery of the neutron in 1932 completed the composition of that nucleus. The bookkeeping that follows is simple and is examined constantly. The atomic number Z is the number of protons, and it fixes the element; the mass number A is the number of protons plus neutrons; atoms of the same element with different neutron numbers are isotopes; the nuclear charge is +Ze; and in a neutral atom the electron count equals Z so the electron charge totals −Ze. The electrostatic attraction between the positive nucleus and the negative electrons is what binds the atom together, which is the deepest reason a negatively charged nucleus is impossible: like charges repel, and an atom whose parts were all negative would not hold together at all. Bohr's model of 1913 then quantised the electron orbits to explain why the orbiting electrons do not spiral into the nucleus and why atomic spectra consist of sharp lines.

The general science section of MPSC papers reaches for atomic structure regularly, because it can be tested with no calculation and no diagram: the composition of the nucleus, the sign and magnitude of the charges, the meaning of atomic number and mass number, isotopes, and the experiments behind the models. This item is a reading test as much as a physics test. Statement (b) is built to look like the standard true sentence about atomic neutrality with a single adjective changed, and a candidate who reads for the shape of a sentence rather than for its content will accept it. That habit — reading physics statements adjective by adjective — is worth practising deliberately, because examiners change signs, directions and comparatives far more often than they change nouns. One further point belongs on the record. The word 'negative' appears in statement (b) in both language columns of this paper: the English prints 'negative charge on nucleus' and the Marathi prints 'केंद्रकावरील ऋण भार'. The sentence is therefore wrong on the physics in the paper itself and not through any slip in translation, and the Commission's final key treats statement (b) as incorrect, which is how the sentence reads. The card reproduces the stem exactly as printed and does not repair it.

• An atom is electrically neutral because it contains equal numbers of protons and electrons: the proton carries +e and the electron −e, where e is about 1.6 × 10⁻¹⁹ coulomb, so the two totals cancel exactly.
• The nucleus carries a POSITIVE charge of magnitude +Ze, contributed by its Z protons; neutrons contribute mass but no charge, and the atom's entire negative charge, −Ze, lies on the electrons outside the nucleus.
• A particle in which the number of electrons differs from the number of protons is an ion — positive if electrons have been lost, negative if they have been gained — and is no longer a neutral atom.
• Rutherford's gold-foil experiment of 1911 established that the positive charge and nearly all the mass of an atom are concentrated in a minute nucleus, because a small fraction of alpha particles were deflected through very large angles and a few almost straight back.
• The electrostatic attraction between the positive nucleus and the negative electrons is what binds the atom; an atom whose nucleus and electrons were both negative would repel itself apart and could not be neutral either.
• Statement (b) in this question prints 'negative charge on nucleus' in the English column and 'केंद्रकावरील ऋण भार' in the Marathi, so the error stands in the printed paper itself rather than in the translation between the two columns.

• Reading a physics statement for its shape rather than its adjectives, so that a sentence with the sign reversed passes as the familiar true one
• Letting a wrong statement contaminate the judgment of a correct one, and rejecting both when only one fails
• Confusing an ion with an atom, and concluding that atoms need not be electrically neutral
• Attributing charge to the neutron, or forgetting that the nucleus's charge comes only from its protons
• Deciding the two statements correctly and then selecting the row that reverses them, which costs exactly as much as not knowing the physics

Atomic structure reaches MPSC papers in three shapes. The first is a direct recall item — which particle carries no charge, where the mass of the atom is concentrated, what the atomic number counts — and it is the cheapest kind of mark in the science section. The second is a statement-verification item of this kind, in which a true sentence about charge, mass or composition is printed alongside one that has been altered by a single word; the alteration is nearly always a sign, a comparative or a location rather than a name. The third attaches the physics to an experiment or a scientist, asking who discovered the neutron or what the gold-foil experiment established. Preparing the particles as a small table of charge and mass, and the four experiments as a short chronology, answers all three shapes without any calculation.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following statements about the nucleus of an atom is correct ?

  • (a)It carries a negative charge equal in magnitude to the total charge of the electrons
  • (b)It carries a positive charge equal in magnitude to the total charge of the electrons
  • (c)It carries no charge, since protons and neutrons cancel each other
  • (d)Its charge depends on the number of neutrons it contains

  Answer(b) It carries a positive charge equal in magnitude to the total charge of the electrons — the nucleus holds Z protons, each of charge +e, giving it a charge of +Ze, while the Z electrons outside carry a total of −Ze. The two are equal in magnitude and opposite in sign, which is exactly why a neutral atom has no net charge. Neutrons carry no charge at all, so they add mass without altering the nuclear charge, and the number of neutrons distinguishes isotopes rather than changing the element's charge.

• practice — not a real PYQ

  Rutherford's gold-foil experiment led to the conclusion that

  • (a)the positive charge of an atom is spread uniformly through its volume
  • (b)electrons are embedded in a sphere of positive charge
  • (c)the positive charge and almost all the mass of an atom are concentrated in a tiny nucleus
  • (d)the atom is indivisible and has no internal structure

  Answer(c) The positive charge and almost all the mass of an atom are concentrated in a tiny nucleus — most alpha particles passed through the foil undeflected, showing the atom is largely empty; a few were deflected through very large angles and a very small number rebounded, which requires a dense concentration of positive charge to repel them. The first two options describe Thomson's earlier model, which the experiment overturned, and the last describes the pre-electron view of the atom that Thomson himself had already disproved.