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Time Allotment
Time Allotment
The suggested time for exploring this discussion about the topic of the concept of atomic number that led to the synthesis of new elements in the laboratory is 45 minutes.
Discussion
Discussion
An explanation of the superscripts and subscripts seen in atomic number notation. Atomic number is the number of protons, and therefore also the total positive charge, in the atomic nucleus.
Atomic Number
Atomic Number
The atomic number or nuclear charge number (symbol Z) of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (np) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons.
Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes.
The periodic table and a natural number for each element
The periodic table and a natural number for each element
Dmitri Mendeleev
Dmitri Mendeleev
Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, and so they can be numbered in order.
Dmitri Mendeleev created a classification of elements based on their atomic weight (first published on March 6, 1869). He found that organizing the elements at the time by their calculated weight demonstrated a periodic pattern of both physical and chemical properties, such as luster, physical state, reactivity to water, and others.
A simple numbering based on periodic table position was never entirely satisfactory, however. Besides the case of iodine and tellurium, later several other pairs of elements (such as argon and potassium, cobalt and nickel) were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However, the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium (element 71) onward (hafnium was not known at this time).
Ernest Rutherford
Ernest Rutherford
In 1911, Ernest Rutherford proposed a model of the atom in which a central nucleus had the majority of the atom's mass and a positive charge that was about equivalent to half of the atom's atomic weight, expressed in numbers of hydrogen atoms when measured in units of the electron's charge. This central charge weighs about half the atomic weight (although it differs about 25% from the atomic number of gold (Z = 79, A = 197), the only element for which Rutherford made his predictions. ). However, despite Rutherford's estimate that gold had a central charge of about 100 (but element Z = 79 in the periodic table), a month after Rutherford's paper appeared, Antonius van den first officially proposed to Broek that the central charge and number of electrons in an atom correspond exactly to its position in the periodic table (also known as the element number, atomic number, and symbol Z). This ultimately turned out to be the case.
Henry Moseley's 1913 experiment
Henry Moseley's 1913 experiment
The experimental perspective was greatly improved following the work of Henry Moseley in 1913. Moseley, after discussions with Bohr, who was in the same laboratory (and who used Van den Broek's hypothesis in the de Bohr), decided to check on Van den Broek's theory and Bohr's hypothesis directly, by seeing whether the spectral lines emitted by excited atoms match the postulate in Bohr's theory that the frequency of the spectral lines has proportional to the square of Z or not.
To do this, Moseley measured the wavelengths of the innermost photonic transitions (K and L lines) produced by elements ranging from aluminium (Z = 13) to gold (Z = 79) used as a series of moving anode targets inside the x-ray tube. The square root of the frequency of these photons (X-rays) increases from target to target arithmetic. This leads to the conclusion (Moseley's law) that the atomic number corresponds closely (with a displacement of one unit to the K current, in Moseley's work) to the calculated charge of the nucleus, i.e. Z element. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium) must have 15 members – no fewer and no more – something that was not obvious in the chemistry known at the time.
Discovery of the Missing Elements
Discovery of the Missing Elements
Recall that in 1925, there were four vacancies in the periodic table corresponding to the atomic numbers 43, 61, 85, and 87. Two of these elements were synthesized in the laboratory using particle accelerators. A particle accelerator is a device that is used to speed up the protons to overcome the repulsion between the protons and the target atomic nuclei by using magnetic and electrical fields. It is used to synthesize new elements. In 1937, American physicist Ernest Lawrence was also able to synthesize element with atomic number 43 using a linear particle accelerator. He bombarded molybdenum (Z=42) with fast-moving neutrons. The newly synthesized element was named Technetium (Tc) after the Greek word "technêtos" meaning “artificial.” Tc was the first man-made element.
Ernest Lawrence
The bombarding of Mo with deuteron formed technicium which is the first artificially made element.
In 1940, Dale Corson, K. Mackenzie, and Emilio Segre discovered an element with atomic number 85. They bombarded atoms of bismuth (Z=83) with fast-moving alpha particles in a cyclotron. A cyclotron is a particle accelerator that uses an alternating electric field to accelerate particles that move in a spiral path in the presence of a magnetic field. Element-85 was named astatine from the Greek word “astatos” meaning unstable.
Dale Corson
Dale Corson
K. Mackenzie
K. Mackenzie
Emilio Segre
Emilio Segre
The two other elements with atomic numbers 61 and 87 were discovered through studies in radioactivity. Element-61 (Promethium) was discovered as a decay product of the fission of uranium while element-87 (Francium) was discovered as a breakdown product of uranium.
The Synthesis of the Element
The Synthesis of the Element
The invention of the cyclotron paved the way for artificial transmutation of one element into another. The high-energy particles produced by the cyclotron produce heavier nuclei upon hitting heavy target nuclei.
The Universe ran into the Be problem. Red giant cores get past this via the Triple-Alpha process, but the Universe expands right through this possibility and the density/temperature is quickly too low to synthesize any additional elements.
The Transuranic Elements
The Transuranic Elements
In the 1930s, the heaviest element known was uranium, with an atomic number 92. Early in 1940, Edwin McMillan proved that an element having the atomic number 93 could be created. He used a particle accelerator to bombard uranium with neutrons and created an element with an atomic number 93 which he named neptunium.
Transuranic elements are synthetic elements with atomic numbers higher than that of Uranium (Z = 92).
At the end of 1940, element-94 was synthesized by Seaborg, McMillan, Kennedy, and Wahl. They bombarded uranium with deuterons (particles composed of a proton and a neutron) in a cyclotron. Element-94 was named plutonium.
Elements with atomic numbers greater than 92 (atomic number of uranium) are called transuranium elements. Hence, neptunium and plutonium are both transuranium elements. They are unstable and decay radioactively into other elements. All these elements were discovered in the laboratory as artificially generated synthetic elements. They are prepared using nuclear reactors or particle accelerators. In the next lesson, you will learn the nuclear reactions involved in the synthesis of these transuranium elements.
Stellar nucleosynthesis
Stellar nucleosynthesis
This process creates elements within stars by combining the protons and neutrons together from the nuclei of lighter elements. Fusion inside stars transforms hydrogen into helium, heat, and radiation. Heavier elements are created in different types of stars as they die or explode.
Let's learn further!
Let's learn further!
The Atomic Number and the Synthesis of New Elements.
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