Summary
Highlights
The video introduces physical science and the topic of atomic number and the synthesis of new elements. It outlines the learning objectives: explaining how atomic number led to element synthesis, identifying new elements, and realizing the importance of atomic number.
Henry Moseley, an English physicist, demonstrated that an element's atomic number (number of protons) determines most of its properties. In 1913, he published a paper on the arrangement of elements in the periodic table based on atomic numbers, using X-ray spectroscopy to determine them. His findings showed that the frequency of X-rays emitted by an element was mathematically related to its position in the periodic table and proportional to the nuclear charge or atomic number. This led to the identification of four gaps in the periodic table corresponding to atomic numbers 43, 61, 85, and 87, which were later synthesized.
In 1919, Ernest Rutherford carried out the first successful nuclear transmutation, transforming one element into another. He bombarded nitrogen nuclei with alpha particles to form oxygen nuclei. Due to the repulsion between positively charged alpha particles and atomic nuclei, scientists later began bombarding atomic nuclei with neutrons or neutral particles in particle accelerators to synthesize new elements.
In 1925, four vacancies existed in the periodic table for elements with atomic numbers 43, 61, 85, and 87. Particle accelerators were used to synthesize elements 43 and 85. In 1937, Ernest Lawrence synthesized element 43, Technetium (Tc), by bombarding molybdenum with fast-moving neutrons, making it the first man-made element. In 1940, researchers discovered element 85, Astatine (At), by bombarding bismuth atoms with alpha particles in a cyclotron. Elements 61 (Promethium) and 87 (Francium) were discovered through studies in radioactivity as decay products of uranium.
In the 1930s, uranium (atomic number 92) was the heaviest known element. In 1940, Edwin McMillan created element 93, Neptunium, by bombarding uranium with neutrons. Later, element 94, Plutonium, was synthesized by bombarding uranium with deuterons in a cyclotron. Elements with atomic numbers greater than 92 are called transuranium elements; they are unstable and undergo radioactive decay. These elements are artificially generated in nuclear reactors or particle accelerators.
Nuclear transmutation is a reaction that transforms one element into another, occurring when a nucleus reacts with a subatomic particle to produce a more massive nucleus. This usually happens under special conditions, such as the collision of target nuclei with high-energy particle beams. Early transmutation experiments used alpha particles, but particle accelerators, invented in 1932, enabled faster progress by using high-energy projectiles.
Transuranium elements, with atomic numbers greater than 92, are all unstable and undergo radioactive decay. Many were prepared using particle accelerators, largely by scientists led by Glenn Theodore Seaborg and Albert Ghiorso. Lighter transuranium elements (93-95) were often prepared through nitrogen bombardment, while heavier ones (96-101) typically used high-energy positive ions like deuterons, carbon nuclei, and other ions. Neptunium (93) was first identified in 1940 by bombarding uranium oxide with slow neutrons, and its decay led to the discovery of Plutonium (94).
Nuclear decay, also known as radioactive decay, involves a nucleus emitting radiation and transforming into a new, more stable nucleus with lower mass and energy. Transuranium elements undergo radioactive decay. Key nuclear decay reactions in transuranium element synthesis include alpha decay, beta decay, and spontaneous fission.
Alpha decay emits a helium-4 nucleus, reducing the atomic number by 2 and mass number by 4. Beta decay converts a neutron into a proton, emitting an electron; the atomic number increases by 1, and the mass number remains the same. Spontaneous fission causes very massive nuclei to break into pieces with different atomic and mass numbers. In all nuclear reactions, the parent nucleus is on the left, and the daughter nucleus and emitted particle are on the right, with the numbers of nuclei always conserved.
A recap of the lesson: atomic number is the number of protons; Moseley demonstrated its importance; Rutherford performed the first nuclear transmutation; particle accelerators synthesize new elements like technetium and astatine; transuranium elements (atomic number > 92) are unstable and artificially generated; nuclear reactions include transmutation and decay; and three main decay types are alpha, beta, and spontaneous fission, each affecting atomic and mass numbers differently.