Consider the following statements :
(a) An atom is electrically neutral.
(b) The negative charge on nucleus is equal to the total negative charge of all the orbiting electrons.

Question

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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).

Why the other options are wrong — (a) 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.  (b) 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.  (d) 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.

Concept: 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.

State PCS 2021 Q51 — Science & Technology PYQ with Answer

State PCS 2021 Q51 — Science & Technology PYQ with Answer