Class 11 Chemistry — Chapter 307: Nuclear Chemistry and Radioactivity

Key concepts: Nuclear Chemistry and Radioactivity (Class 11 Chemistry)

Ordinary chemistry rearranges electrons; nuclear chemistry changes the nucleus itself. Beginning with Becquerel's accidental discovery in 1896 that uranium salts fog a wrapped photographic plate, this chapter looks at the three natural radiations α, β and γ and how they differ in charge, mass, ionising power and penetrating power. It then explains why some nuclei are unstable in terms of the neutron-to-proton ratio and the band of stability, works through the decay modes (α, β⁻, β⁺ and electron capture) and the group displacement law, treats decay as a first order rate process with a decay constant, half-life and average life, and finally covers artificial transmutation, fission, fusion, mass defect and binding energy, nuclear reactors and the everyday uses of radioisotopes in dating, medicine and agriculture.

Radioactivity

The spontaneous emission of α, β or γ radiation by an unstable nucleus. It was discovered by Henri Becquerel in 1896 with uranium salts, and Marie and Pierre Curie then isolated polonium and radium from pitchblende. The rate is unaffected by temperature, pressure or chemical combination.

Alpha, beta and gamma radiation

An α-particle is a helium nucleus ⁴₂He²⁺, a β-particle is a fast electron ⁰₋₁e, and a γ-ray is a high energy photon with no charge and no rest mass. Ionising power falls α > β > γ while penetrating power rises in the reverse order α < β < γ; α is stopped by paper, β by thin aluminium and γ needs thick lead.

n/p ratio and the band of stability

Plotting neutrons against protons for stable nuclides gives a narrow band. Light stable nuclei have n/p about 1, and the ratio rises to roughly 1.5 near the heavy end. Nuclides outside the band are radioactive and every nuclide with atomic number greater than 83 is unstable.

Modes of decay

A nucleus above the band (too many neutrons) emits a β⁻ particle, turning a neutron into a proton. A nucleus below the band (too many protons) emits a positron or captures a K-shell electron, turning a proton into a neutron. Very heavy nuclides shed mass by α emission, and γ emission simply removes excess energy.

Group displacement law

Stated by Soddy, Fajans and Russell: loss of an α-particle lowers the atomic number by 2 and moves the daughter two groups to the left in the periodic table, while loss of a β⁻ particle raises the atomic number by 1 and moves the daughter one group to the right, with the mass number unchanged.

Decay constant and half-life

Decay obeys first order kinetics, −dN/dt = λN, so N = N₀e^(−λt). The decay constant λ is the fraction of nuclei disintegrating per unit time. The half-life t½ = 0.693/λ is the time for half the sample to decay and is a fixed property of a nuclide, independent of the amount taken.

Average life and activity

Average (mean) life τ = 1/λ = 1.44 t½ is the average survival time of a nucleus. Activity A = λN is the number of disintegrations per unit time; its SI unit is the becquerel (1 Bq = 1 disintegration per second) and the older unit is the curie, with 1 Ci = 3.7 × 10¹⁰ Bq.

Radioactive series and artificial transmutation

Heavy natural nuclides decay in chains, such as the ²³⁸U series that ends at stable ²⁰⁶Pb and the ²³²Th series that ends at ²⁰⁸Pb. Artificial transmutation is the deliberate conversion of one element into another by bombarding it with particles, first done by Rutherford using α-particles on nitrogen to give oxygen and a proton.

Mass defect and binding energy

The actual mass of a nucleus is less than the sum of the masses of its nucleons; this difference Δm is the mass defect and it appears as the binding energy E = Δmc² that holds the nucleus together. Binding energy per nucleon measures stability and peaks near mass number 56, which is why both fission of heavy nuclei and fusion of light nuclei release energy.

Fission, fusion and reactors

In fission a heavy nucleus such as ²³⁵U splits into two medium fragments plus 2 to 3 neutrons, which can sustain a chain reaction. In fusion light nuclei combine at temperatures near 10⁷ K, as in the sun. A reactor uses fuel rods, a moderator (graphite or heavy water) to slow neutrons, cadmium or boron control rods to absorb them, and a coolant.

Applications of radioisotopes

Carbon dating with ¹⁴C (t½ = 5730 years) fixes the age of once-living material, and ²³⁸U/²⁰⁶Pb ratios date rocks. In medicine ⁶⁰Co gives γ-rays for cancer therapy and ¹³¹I is used for thyroid disorders. In agriculture ³²P traces fertiliser uptake and γ-irradiation preserves food.