When α-particle emits from a radioactive element then

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Correct — option (1), 'atomic number decreases by 2 and mass number decreases by 4'. The answer follows directly from what an alpha particle is. An alpha particle is a helium nucleus of mass number four: two protons and two neutrons bound together, carrying a charge of plus two and no electrons. When a heavy radioactive nucleus ejects one, it loses precisely those four nucleons and no others. The atomic number is by definition the number of protons in the nucleus, and two protons have gone, so it falls by two. The mass number is the total number of protons and neutrons, and four nucleons have gone, so it falls by four. Nothing else in this arithmetic is open to interpretation, which is why the question is a definitional one dressed as a physical one. The consequence is that the daughter is not merely a lighter version of the parent but a different element altogether, standing two places to the left of the parent in the periodic table — the alpha half of what is known as the Soddy and Fajans displacement law. The standard illustrations are worth carrying because they let the rule be checked in a moment. Uranium-238 has atomic number 92; emitting an alpha particle it becomes thorium-234, of atomic number 90, the mass number having dropped by four and the atomic number by two. Radium-226, of atomic number 88, becomes radon-222, of atomic number 86, by the same arithmetic — which is also why radon, a gas, seeps out of rocks containing radium. Set the alpha rule beside the other two modes of decay and each becomes easier to hold: beta-minus emission raises the atomic number by one and leaves the mass number alone, because a neutron has turned into a proton; gamma emission changes neither, because only energy has been released. Option (1) is the only one that matches the composition of the particle actually emitted.

Why the other options are wrong — (b) This option halves both of the correct figures, and it is arithmetically consistent only with the emission of a particle carrying a charge of plus one and a mass number of two — that is, a deuteron, the nucleus of heavy hydrogen. Deuteron emission is not a mode of natural radioactive decay, and it is certainly not what the stem describes, since the stem names the alpha particle explicitly. The option is included to catch a candidate who remembers that an alpha decay reduces both numbers but has not fixed the amounts, and who then guesses at the smallest plausible pair. The remedy is to reason from the particle rather than from a memorised pair of numbers: once the alpha particle is identified as a helium nucleus with two protons and two neutrons, the two figures are read straight off its composition and there is nothing left to remember.  (c) An increase of one in the atomic number is the signature of beta-minus decay, not of alpha decay. In that process a neutron inside the nucleus converts into a proton, with an electron and an antineutrino emitted; the total number of nucleons is unchanged, so the mass number stays the same while the atomic number rises by one and the daughter moves one place to the right in the periodic table. Choosing this option means the two modes of decay have been swapped, which is the commonest confusion in the topic. There is a second warning built into the option: it mentions the atomic number only and says nothing at all about the mass number, and an incomplete description of a decay is worth distrusting on its own account, since any genuine decay rule has to account for both numbers.  (d) Leaving both numbers unchanged describes gamma emission. A nucleus left in an excited state after an alpha or beta decay sheds its surplus energy as a high-energy photon, and since a photon carries neither charge nor nucleons, neither the atomic number nor the mass number is altered and the element remains the same element. That is a real process, and it commonly follows the other two, which is why all three radiations are often observed together from a single sample — but it is not what happens when an alpha particle is emitted, and the stem specifies the alpha. The option also fails a simple consistency test: a particle with mass and charge has left the nucleus, so the nucleus cannot possibly have the same composition afterwards as before.

Concept: Radioactivity is the spontaneous disintegration of an unstable nucleus, and it proceeds by three distinct routes that are distinguished by what leaves the nucleus. Alpha emission expels a helium nucleus, two protons and two neutrons together, and is characteristic of the heaviest elements, in which the electrical repulsion between so many protons strains the nucleus; because the alpha is massive and doubly charged it interacts strongly with matter, which makes it the most ionising of the three radiations and simultaneously the least penetrating, stopped by a sheet of paper or a few centimetres of air. Beta emission expels an electron created at the moment a neutron converts into a proton; being far lighter and singly charged, it penetrates further and is stopped by a few millimetres of aluminium. Gamma emission expels no particle at all, only a photon of very short wavelength released as an excited nucleus settles to a lower energy state, and it is the most penetrating of the three, requiring thick lead or concrete to attenuate it. The bookkeeping for all three is captured in the displacement laws formulated by Soddy and Fajans: alpha emission moves the element two places to the left in the periodic table, beta-minus emission one place to the right, and gamma emission not at all. Alongside these sits the idea of half-life, the time in which half the nuclei in a sample decay, a quantity fixed for each nuclide and unaffected by temperature, pressure or chemical combination.

State PCS 2023 Q39 — Science & Technology PYQ with Answer

State PCS 2023 Q39 — Science & Technology PYQ with Answer