University:
Carleton University
Course:
CHEM 1001 | General Chemistry I
Academic year:

2024
Views:

205

Pages:

3
Author:

ali

NUCLEAR CHEM 23 FEB 22 Lecture 4 Nuclear Stability Predicting Radioactive Modes of Decay RADIOACTIVITY The stability of a nucleus involves the competition between two forces: Electrostatic (Coulomb) repulsion between protons Strong nuclear force which is a short-range attraction between all nucleons How do the electrostatic & the strong nuclear force influence decay? In nuclei with too few neutrons, the electrostatic repulsion overwhelms the strong nuclear attraction As the nucleus gets larger, the long-range electrostatic repulsion between protons accumulates and eventually overwhelms the strong nuclear attraction. Nuclides with M > 208 (e.g. Uranium) are unstable When there are too many neutrons, the nucleus is also unstable DECAY MECHANISMS In all nuclear reactions, both mass number and charge are balanced α decay: the α particle is simply a helium nucleus ²¹²Bi → ²⁰⁸Tl + ⁴He β decay: β is an electron ejected from the nucleus. One neutron is changed into a proton in this nuclear reaction to balance the charge ¹⁴C → ¹⁴N + e⁻ 3. Positron (β+) decay: A positron is ejected from the nucleus. One proton is changed into a neutron to balance the charge. The ejected positron usually collides with its anti-particle (electron) in the surrounding environment. ¹¹C → ¹¹B + e⁺ 4. Electron capture: Electron is captured into lower energy states to fill the vacancy left by the captured electron. A proton combines with the electron, forming a neutron. ⁵⁵Fe + e⁻ → ⁵⁵Mn 5. Neutron emission: Simple emission of a neutron, which changes A but leaves Z unchanged. ²¹He → ²¹He + ¹n 6. Gamma emission: A high-frequency radiation that often accompanies other forms of decay. No change in N or Z is associated with γ emission on its own. ⁴³Tc* → ⁴³Tc + γ NUCLEAR STABILITY Size of the nucleus Composition of the nucleus (neutrons & protons), also termed N/Z ratio There are no stable nuclei heavier than Pb Tc has no stable isotopes Heavier elements undergo alpha decay Zone of stability: all brown stable nuclides fall inside the zone of stability This zone has a N/Z ratio of ~1 for Z < 20, but "bends" towards more neutrons per proton as the nucleus gets larger Unstable isotopes must decay towards the zone of stability, finally falling below Pb N/Z ratio If N/Z < zone of stability: N↑ and/or Z↓ β⁺ decay, electron capture If N/Z > zone of stability: N↓ and/or Z↑ β⁻ decay, neutron emission If A > zone of stability: N↑ and/or Z↑ α decay LEARNING OUTCOMES Explain the factors that govern nuclear stability Balance nuclear equations Predict the mode of radioactive decay for a given unstable nucleus