Overview
This lecture covers the concept of atomic number, how it led to the synthesis of new elements, and explains nuclear reactions and the creation of transuranium elements.
Atomic Number and Moseley's Work
- The atomic number is the number of protons in an atom, determining its properties.
- Henry Moseley used X-ray spectroscopy to show atomic number, not atomic mass, organizes the periodic table.
- Moseley found frequency of X-rays from an element is proportional to its atomic number.
- Four gaps in the periodic table corresponded to atomic numbers 43, 61, 85, and 87.
Nuclear Transmutation and the Synthesis of New Elements
- Nuclear transmutation changes one element into another via nuclear reactions.
- Rutherford first demonstrated nuclear transmutation by converting nitrogen to oxygen using alpha particles in 1919.
- Alpha particles and target nuclei repel each other; neutrons are used instead to synthesize new elements.
- Particle accelerators use magnetic and electric fields to boost particle energies for synthesis.
Discovery of the Missing Elements
- Elements 43 (technetium), 61 (promethium), 85 (astatine), and 87 (francium) were missing from the table.
- Technetium was made by bombarding molybdenum with neutrons; astatine by bombarding bismuth with alpha particles.
- Promethium and francium were discovered as decay products of uranium.
Transuranium Elements and Particle Accelerators
- Elements with atomic numbers >92 are called transuranium elements; all are unstable and radioactive.
- Neptunium (93) and plutonium (94) were synthesized by bombarding uranium with neutrons or deuterons.
- Heavier transuranium elements were produced using high-energy ions.
- Glen Seaborg and colleagues synthesized elements 93–101 by various bombardment techniques.
Types of Nuclear Reactions
- Nuclear transmutation: a nucleus reacts with a particle to form a new element or isotope.
- Nuclear decay (radioactive decay): unstable nuclei emit radiation and transform into more stable nuclei.
- Alpha decay: emits a helium-4 nucleus, reducing atomic number by 2 and mass number by 4.
- Beta decay: a neutron turns into a proton, increasing atomic number by 1; mass number unchanged.
- Spontaneous fission: nucleus splits into smaller nuclei with atomic/mass numbers summing to the original.
Key Terms & Definitions
- Atomic Number — Number of protons in an atom’s nucleus.
- Nuclear Transmutation — Changing one element/isotope into another via nuclear reactions.
- Particle Accelerator — Device that increases particle energy for nuclear synthesis.
- Transuranium Elements — Elements with atomic numbers greater than 92.
- Alpha Decay — Emission of a helium-4 nucleus, lowering atomic and mass numbers.
- Beta Decay — Neutron converts to proton, atomic number increases by one.
- Spontaneous Fission — Nucleus splits into two or more nuclei.
Action Items / Next Steps
- Review the periodic table and locate elements discussed (43, 61, 85, 87, 92–101).
- Practice writing nuclear equations for transmutation and decay reactions.
- Prepare for the next lesson on nuclear energy or properties of radioactive elements.