Actinides are elements 90-103, including uranium and plutonium, which are all radioactive and have varying oxidation states. They have been present on Earth since its formation and drive plate tectonics and volcanism. Some actinides have practical uses, such as thorium in gas cloaks and americium in smoke detectors.
Actinides is the collective name given to elements 90-103 in the periodic table, including thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrentium. The element actinium, atomic number 89, from which the group takes its name, is not – strictly speaking – itself one of the actinides, but is often included in them. As with all elements heavier than lead, none of the actinide series have stable isotopes and all are therefore radioactive, generally undergoing alpha decay to other elements. Uranium and thorium occur naturally, along with trace amounts of actinium, protactinium, plutonium and neptunium. The remaining elements have never been observed in nature, but have been produced in extremely small quantities in particle accelerators.
Uranium and thorium have long half-lives and have been present in the Earth in significant quantities since its formation. Much of the heat in the Earth’s core, which drives plate tectonics and volcanism, is thought to be due to the radioactive decay of these elements. The plutonium-244 isotope has a relatively long half-life, and traces of Earth’s native plutonium still survive; however, most of the plutonium in the environment comes from nuclear reactors and nuclear weapon tests. Naturally occurring actinium, protactinium, and neptunium have much shorter half-lives, so any amount of these elements that were present when the Earth formed would have long ago decayed into other elements. Actinium, protactinium and neptunium are formed through nuclear processes associated with the decay of uranium isotopes.
Like the elements lanthanides, the actinides occupy a separate block from the main periodic table, as it is usually represented, due to their electron configurations. In both of these blocks, the outermost electron subshell was occupied before an earlier subshell, since the latter has a higher energy level, and it is the number of electrons in this subshell that differentiates elements from each other. other. For lanthanides, the 4f subshell is important, and for actinides, the 5f subshell. These elements are also known as f-block elements. The outermost subshell is the same for all elements within each block, except for lawrencium, which differs from the previous element not in the 5f subshell, but in having an additional 7p subshell containing an electron.
Actinide chemistry is governed by the fact that the valence electrons, which can bond with other atoms, are not confined to the outermost subshell, giving a varying number of oxidation states between these elements. For example, plutonium can have oxidation states from +3 to +7. All elements are chemically reactive and oxidize rapidly in the air, becoming coated with an oxide layer. Reactivity increases with atomic weight within the group; however, investigation of the chemical properties of some of the heavier members is difficult due to their intense radioactivity and very short half-lives.
The longest-lived actinide isotopes have found a variety of uses. Thorium has been used since the end of the 19th century in the manufacture of gas cloaks. The ability of some isotopes of uranium and plutonium to undergo nuclear fission has led to their use in nuclear reactors and nuclear weapons, and plutonium has also been used as a long-lasting energy source for space probes. Americium is used in smoke detectors.
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