Uranium: atomic number 106 nucleus

Atomic number. The number of protons in the nucleus- of an atom. The hundred or so known elements are usually arranged in the order of increasing atomic numbers for categorization purposes, with hydrogen (atomic number of one) the lightest, and uranium (atomic number of 92) one of the heaviest. 

The uranium atom s nucleus loses an alpha particle its atomic number goes down from 92 to 90, so it becomes an atom of thorium the mass number goes down from 238 to 234 because it has lost 2 neutrons and 2 protons. 

In the mid-1930s Enrico Fermi (1901-1954), an Italian physicist, tried to synthesize a new element by bombarding uranium—the heaviest known element at that time— with neutrons. Fermi hypothesized that if a neutron were incorporated into the nucleus of a uranium atom, the nucleus might undergo beta decay, converting a neutron into a proton. If that happened, a new element, with atomic number 93, would be synthesized for the first time. The nuclear equation for the process is ... 

In 1938 Niels Bohr had brought the astounding news from Europe that the radiochemists Otto Hahn and Fritz Strassmann in Berlin had conclusively demonstrated that one of the products of the bom-bardmeiit of uranium by neutrons was barium, with atomic number 56, in the middle of the periodic table of elements. He also announced that in Stockholm Lise Meitner and her nephew Otto Frisch had proposed a theory to explain what they called nuclear fission, the splitting of a uranium nucleus under neutron bombardment into two pieces, each with a mass roughly equal to half the mass of the uranium nucleus. The products of Fermi s neutron bombardment of uranium back in Rome had therefore not been transuranic elements, but radioactive isotopes of known elements from the middle of the periodic table. 

In a neutral atom, the number of protons in the nucleus is exactly equal to the number of electrons outside the nucleus. Consider, for example, the elements hydrogen (Z = 1) and uranium (Z = 92). All hydrogen atoms have one proton in the nucleus all uranium atoms have 92. In a neutral hydrogen atom there is one electron outside the nucleus in a uranium atom there are 92. 

Instead of producing new kinds of substances by combination of atoms, the element uranium has combined with a neutron and as a result has split into two other elements—barium and krypton—plus three more neutrons. Atoms of a given element are characterized by their atomic number, the number of units of positive charge on the nucleus. For one element to change into another element the nucleus must be altered. In our example the uranium nucleus, as a result of reacting with a neutron, splits or fissions into two other nuclei and releases, in addition, neutrons. ... 

Very few nuclides with Z < 60 emit a particles. All nuclei with Z > 82 are unstable and decay mainly by a-particle emission. They must discard protons to reduce their atomic number and generally need to lose neutrons, too. These nuclei decay in a step-by-step manner and give rise to a radioactive series, a characteristic sequence of nuclides (Fig. 17.16). First, one a particle is ejected, then another a particle or a (3-particle is ejected, and so on, until a stable nucleus, such as an iso tope of lead (with the magic atomic number 82) is formed. For example, the uranium-238 series ends at lead-206, the uranium-235 series ends at lead-207, and the thorium-232 series ends at lead-208. 

Problem 8 The next step in the uranium-238 decay scheme is the emission of an alpha particle from thorium-230. Describe the mass number, atomic number, and element name for the resulting nucleus. 

Despite their instability, some unstable atoms may last a long time the half-life of uranium 238, for example, is about 4.5 billion years. Other unstable atoms decay in a few seconds. Radioactive decay is one of the topics of nuclear chemistry, and it involves nuclear forces, as governed by advanced concepts in chemistry and physics, such as quantum mechanics. Researchers do not fully understand why some atoms are stable and others are not, but most radioactive nuclei have an unusually large (or small) number of neutrons, which makes the nucleus unstable. And all heavy nuclei found so far are radioactive—nuclides with an atomic number of 83 or greater decay. 

Conservation of mass and charge are used when writing nuclear reactions. For example, let s consider what happens when uranium-238 undergoes alpha decay. Uranium-238 has 92 protons and 146 neutrons and is symbolized as After it emits an alpha particle, the nucleus now has a mass number of 234 and an atomic number of 90. 

III hen a radioactive nucleus emits an alpha or beta particle, the identity UU of the nucleus is changed because there is a change in atomic number. The changing of one element to another is called transmutation. Consider a uranium-238 nucleus, which contains 92 protons and 146 neutrons. When an alpha particle is ejected, the nucleus loses 2 protons and 2 neutrons. Because an element is defined by the number of protons in its nucleus, the 90 protons and 144 neutrons left behind are no longer identified as being uranium. What we have now is a nucleus of a different element—thorium. 

Because the emission of an a particle from a nucleus results in a loss of two protons and two neutrons, it reduces the mass number of the nucleus by 4 and reduces the atomic number by 2. Alpha emission is particularly common for heavy radioactive isotopes, or radioisotopes Uranium-238, for example, spontaneously emits an a particle and forms thorium-234. 

The number of protons in the nucleus is different for each element this number is called the atomic number and often is given the symbol Z (the physicist uses z to represent a number of unit charges). Thus Z for hydrogen is 1 and for uranium is 92. 

As you know, the nucleus of an atom also contains neutrons. In fact, the mass of an atom is due to the combined masses of its protons and neutrons. Therefore, an element s mass number (symbol A) is the total number of protons and neutrons in the nucleus of one of its atoms. Each proton or neutron is counted as one unit of the mass number. For example, an oxygen atom, which has 8 protons and 8 neutrons in its nucleus, has a mass number of 16. A uranium atom, which has 92 protons and 146 neutrons, has a mass number of 238. 

Quasiatoms containing two nuclei of uranium and a common electron shell may be produced by bombarding uranium with uranium ions. In such a quasiatom an intermediate apparent atomic number Z = 91 + 92= 184 may be obtained. At these high atomic numbers, the K electrons arc near the nuclei and at Z — 184 their mean distance from the nucleus is of the order of its diameter. Therefore, both nuclei must approach each other to distances of the order of the diameter of a nucleus in order that K electrons observe both nuclei as only one. 

Nuclear Fission. Ordinary uranium contains two isotopes, (99.3%) and ( 7%). When a neutron collides with a nucleus it combines wdth it, forming a U- nucleus. This nucleus is unstable, and it immediately decomposes spontaneously by splitting into two large fragments, plus several neutrons. Each of the two fragments is itself an atomic nucleus, the sum of their atomic numbers being 92, the atomic number of uranium. 

An unstable nucleus that has an N/Z number that is much larger than 1 can decay by emitting an alpha particle. In addition, none of the elements that have atomic numbers greater than 83 and mass numbers greater than 209 have stable isotopes. So, many of these unstable isotopes decay by emitting alpha particles, as well as by electron capture or beta decay. Uranium-238 is one example. 

All the nuclear reactions that have been described thus far are examples of radioactive decay, where one element is converted into another element by the spontaneous emission of radiation. This conversion of an atom of one element to an atom of another element is called transmutation. Except for gamma emission, which does not alter an atom s atomic number, all nuclear reactions are transmutation reactions. Some unstable nuclei, such as the uranium salts used by Henri Becquerel, undergo transmutation naturally. However, transmutation may also be forced, or induced, by bombarding a stable nucleus with high-energy alpha, beta, or gamma radiation. 

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