Nuclear & Radioactivity — Set 1

Correct Answer: D. Spontaneous emission of radiation from an unstable nucleus

• **Spontaneous emission of radiation from an unstable nucleus** = Radioactivity is the phenomenon where an unstable nucleus emits radiation on its own without any external trigger. • **1896** — Henri Becquerel discovered radioactivity in 1896 when uranium salts darkened a photographic plate; the process is nuclear, not chemical, and unaffected by temperature or pressure. • 💡 Wrong-option analysis: Heating due to friction only: friction is a mechanical process having nothing to do with nuclear instability; Flow of electrons in a conductor: that describes electric current, not nuclear emission; Chemical reaction of atoms with oxygen: combustion or oxidation is a chemical process, whereas radioactivity involves the nucleus itself.

Correct Answer: B. Protons

• **Protons** = The atomic number (Z) equals the number of protons in the nucleus and uniquely identifies an element in the periodic table. • **Z = 1 to 118** — Elements from hydrogen (Z=1) to oganesson (Z=118) are arranged by atomic number; in a neutral atom, the number of electrons equals Z. • 💡 Wrong-option analysis: Neutrons: the number of neutrons is the neutron number N, not the atomic number; Nucleons: total nucleons (protons + neutrons) give the mass number A, not Z; Electrons plus neutrons: this sum has no standard name and does not define an element.

Correct Answer: C. Same atomic number but different mass numbers

• **Same atomic number but different mass numbers** = Isotopes of an element have the same number of protons (same Z) but different numbers of neutrons, giving different mass numbers A. • **Carbon-12 and Carbon-14** — Both have Z=6 (carbon), but C-12 has 6 neutrons and C-14 has 8 neutrons; C-14 is radioactive while C-12 is stable. • 💡 Wrong-option analysis: Same mass number but different atomic numbers: atoms with the same A but different Z are called isobars, not isotopes; Different atomic numbers but same mass number: again describes isobars; Same number of neutrons only: atoms with the same neutron number but different Z are called isotones, not isotopes.

Correct Answer: D. Nuclear binding energy

• **Nuclear binding energy** = The mass defect is the difference between the sum of masses of individual nucleons and the actual nuclear mass; this 'missing' mass is converted into binding energy via E = mc². • **E = mc²** — Even a tiny mass defect (∼0.1% of nuclear mass) converts to enormous energy; for iron-56, binding energy per nucleon is ≈8.8 MeV, the highest among all elements. • 💡 Wrong-option analysis: Color of metals: metal color arises from electron band structure, unrelated to nuclear mass; Ohm's law: Ohm's law relates voltage, current, and resistance — a purely electrical concept; Formation of chemical bonds: chemical bonds involve valence electron interactions, not nuclear mass differences.

Correct Answer: C. Becquerel (Bq)

• **Becquerel (Bq)** = The becquerel is the SI unit of radioactive activity; 1 Bq = 1 nuclear disintegration per second. • **3.7 × 10¹⁰ Bq = 1 Ci** — The older curie unit equals 3.7 × 10¹⁰ Bq, based on the activity of 1 gram of radium-226; the becquerel replaced it as the SI standard. • 💡 Wrong-option analysis: Tesla (T): the tesla is the SI unit of magnetic flux density, not radioactivity; Sievert (Sv): the sievert measures equivalent radiation dose (biological effect), not activity; Gray (Gy): the gray measures absorbed dose (energy per kg of tissue), not disintegration rate.

Correct Answer: B. 3.7 × 10^10 Bq

• **3.7 × 10¹⁰ Bq** = One curie was historically defined as the activity of 1 gram of radium-226, which equals exactly 3.7 × 10¹⁰ disintegrations per second. • **Marie Curie** — The unit was named in honour of Marie Curie; modern lab sources are typically described in MBq or GBq since 1 Ci is a very large activity. • 💡 Wrong-option analysis: 3.7 × 10⁶ Bq: that is only 10⁻⁴ Ci — four orders of magnitude too small; 3.7 × 10³ Bq: that is 0.1 microcurie — far too small; 3.7 × 10¹² Bq: that would be 100 Ci — two orders of magnitude too large.

Correct Answer: D. A helium nucleus (2 protons and 2 neutrons)

• **A helium nucleus (2 protons and 2 neutrons)** = An alpha particle is identical to the nucleus of helium-4, carrying a charge of +2e and a mass number of 4. • **+2e charge, mass ≈ 4 u** — Alpha decay reduces the parent nucleus by 2 protons and 2 neutrons (Z−2, A−4); alpha particles are the least penetrating but most strongly ionising common nuclear radiation. • 💡 Wrong-option analysis: A photon: a photon is a quantum of electromagnetic radiation with zero rest mass — that describes gamma radiation; A neutron: a neutron has zero charge and mass ≈1 u — very different from an alpha particle; An electron orbiting the nucleus: that describes a bound atomic electron, not a nuclear particle emitted in radioactive decay.

Correct Answer: B. An electron emitted from the nucleus

• **An electron emitted from the nucleus** = In beta-minus decay, a neutron inside the nucleus converts to a proton, emitting an electron (β−) and an antineutrino. • **n → p + e⁻ + ν̄ₑ** — The daughter nucleus has Z+1 and the same mass number A; beta particles have intermediate penetrating power (stopped by a few mm of aluminium). • 💡 Wrong-option analysis: A gamma photon: gamma rays are high-energy electromagnetic photons, not particles with mass; A helium nucleus: that is an alpha particle (2p + 2n), not a beta particle; A proton: a proton is a stable positively charged particle; beta-minus particles are negatively charged electrons.

Correct Answer: C. High-energy electromagnetic waves

• **High-energy electromagnetic waves** = Gamma rays are photons of electromagnetic radiation with very short wavelengths (< 10 pm) and very high energy (typically >100 keV), emitted by excited nuclei. • **λ < 10 pm, E > 100 keV** — Gamma emission usually follows alpha or beta decay when the daughter nucleus is in an excited state; gamma rays have no charge and no rest mass, travelling at the speed of light. • 💡 Wrong-option analysis: Neutral atoms: neutral atoms are complete atoms with electrons — they are not a type of nuclear radiation; Streams of protons: proton beams are distinct high-energy particles, not the same as gamma rays; Helium nuclei: helium nuclei describe alpha particles, which are charged and massive, unlike massless gamma photons.

Correct Answer: D. Gamma radiation

• **Gamma radiation** = Gamma rays have the greatest penetrating power among common nuclear radiations because they are uncharged photons and interact weakly with matter, requiring thick lead or concrete to reduce intensity significantly. • **Half-value layer of lead ≈ 1–2 cm** — Even centimetres of lead only halve gamma intensity; alpha particles are stopped by a sheet of paper and beta by a few mm of aluminium. • 💡 Wrong-option analysis: Alpha radiation: alpha particles are stopped by just a few cm of air or a sheet of paper due to strong ionisation; Sound waves: sound is a mechanical pressure wave, not ionising radiation at all; Beta radiation: beta particles penetrate more than alpha but are stopped by thin aluminium — far less than gamma rays.