The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. Speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are included in any general discussion of the chemistry of the actinide elements. Actinium is the more omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, but IUPAC acknowledges its inclusion based on common usage; the actinide series derives its name from the first element in actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium; the series corresponds to the filling of the 5f electron shell, although actinium and thorium lack any 5f electrons, curium and lawrencium have the same number as the preceding element.
In comparison with the lanthanides mostly f-block elements, the actinides show much more variable valence. They all have large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides behave to the lanthanides, the elements thorium and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position. All actinides are release energy upon radioactive decay; these are used in nuclear weapons. Uranium and thorium have diverse current or historical uses, americium is used in the ionization chambers of most modern smoke detectors. Of the actinides, primordial thorium and uranium occur in substantial quantities; the radioactive decay of uranium produces transient amounts of actinium and protactinium, atoms of neptunium and plutonium are produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment.
In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series shown in a single cell of the main table, between barium and hafnium, radium and rutherfordium, respectively. This convention is a matter of aesthetics and formatting practicality. Like the lanthanides, the actinides form a family of elements with similar properties. Within the actinides, there are two overlapping groups: transuranium elements, which follow uranium in the periodic table—and transplutonium elements, which follow plutonium. Compared to the lanthanides, which are found in nature in appreciable quantities, most actinides are rare; the majority of them do not occur in nature, of those that do, only thorium and uranium do so in more than trace quantities. The most abundant or synthesized actinides are uranium and thorium, followed by plutonium, actinium, protactinium and curium.
The existence of transuranium elements was suggested by Enrico Fermi based on his experiments in 1934. However though four actinides were known by that time, it was not yet understood that they formed a family similar to lanthanides; the prevailing view that dominated early research into transuranics was that they were regular elements in the 7th period, with thorium and uranium corresponding to 6th-period hafnium and tungsten, respectively. Synthesis of transuranics undermined this point of view. By 1944, an observation that curium failed to exhibit oxidation states above 4 prompted Glenn Seaborg to formulate an "actinide hypothesis". Studies of known actinides and discoveries of further transuranic elements provided more data in support of this point of view, but the phrase "actinide hypothesis" remained in active use by scientists through the late 1950s. At present, there are two major methods of producing isotopes of transplutonium elements: irradiation of the lighter elements with either neutrons or accelerated charged particles.
The first method is most important for applications, as only neutron irradiation using nuclear reactors allows the production of sizeable amounts of synthetic actinides. The advantage of the second method is that elements heavier than plutonium, as well as neutron-deficient isotopes, can be obtained, which are not formed during neutron irradiation. In 1962–1966, there were attempts in the United States to produce transplutonium isotopes using a series of six underground nuclear explosions. Small samples of rock were extracted from the blast area after the test to study the explosion products, but no isotopes with mass number greater than 257 could b
Point Lookout Light is an active lighthouse on Point Lookout, a headland on North Stradbroke Island, Australia. A proposal for a lighthouse and a signal station on Point Lookout were made as early as 1825. A pilot station was built in 1825 elsewhere on the island, on Amity Point, lighting the South Passage into Moreton Bay. In 1848 this pilot station was moved. A lighthouse on Point Lookout was constructed in 1932, the first settlement at the point; the light source was a carbide lamp operated by acetylene gas, a hut for storage of the gas cylinders was built at the close by beach, to be named Cylinder Beach for this reason. The lighthouse is 16-foot high, painted white; the current display is three white flashes every 15 seconds. List of lighthouses in Australia Searle, Garry. "List of Lighthouses - Queensland". Lighthouses of Australia. SeaSide Lights
The occupation of the Baltic states involved the military occupation of the three Baltic states—Estonia and Lithuania—by the Soviet Union under the auspices of the 1939 Molotov–Ribbentrop Pact in June 1940. They were annexed into the Soviet Union as constituent republics in August 1940, though most Western powers and nations never recognised their incorporation. On 22 June 1941, Nazi Germany attacked the Soviet Union and within weeks occupied the Baltic territories. In July 1941, the Third Reich incorporated the Baltic territory into its Reichskommissariat Ostland; as a result of the Red Army's Baltic Offensive of 1944, the Soviet Union recaptured most of the Baltic states and trapped the remaining German forces in the Courland pocket until their formal surrender in May 1945. The Soviet "annexation occupation" or occupation sui generis of the Baltic states lasted until August 1991, when the three countries regained their independence; the Baltic states themselves, the United States and its courts of law, the European Parliament, the European Court of Human Rights and the United Nations Human Rights Council have all stated that these three countries were invaded and illegally incorporated into the Soviet Union under provisions of the 1939 Molotov–Ribbentrop Pact.
There followed occupation by Nazi Germany from 1941 to 1944 and again occupation by the Soviet Union from 1944 to 1991. This policy of non-recognition has given rise to the principle of legal continuity of the Baltic states, which holds that de jure, or as a matter of law, the Baltic states had remained independent states under illegal occupation throughout the period from 1940 to 1991. In its reassessment of Soviet history that began during perestroika in 1989, the Soviet Union condemned the 1939 secret protocol between Germany and itself. However, the Soviet Union never formally acknowledged its presence in the Baltics as an occupation or that it annexed these states and considered the Estonian and Lithuanian Soviet Socialist Republics as three of its constituent republics. On the other hand, the Russian Soviet Federative Socialist Republic recognized in 1991 the events of 1940 as "annexation". Revisionist Russian historiography and school textbooks continue to maintain that the Baltic states voluntarily joined the Soviet Union after their peoples all carried out socialist revolutions independent of Soviet influence.
The post-Soviet government of the Russian Federation and its state officials insist that incorporation of the Baltic states was in accordance with international law and gained de jure recognition by the agreements made in the February 1945 Yalta and the July–August 1945 Potsdam conferences and by the 1975 Helsinki Accords, which declared the inviolability of existing frontiers. However, Russia agreed to Europe's demand to "assist persons deported from the occupied Baltic states" upon joining the Council of Europe in 1996. Additionally, when the Russian Soviet Federative Socialist Republic signed a separate treaty with Lithuania in 1991, it acknowledged that the 1940 annexation as a violation of Lithuanian sovereignty and recognised the de jure continuity of the Lithuanian state. Most Western governments maintained that Baltic sovereignty had not been legitimately overridden and thus continued to recognise the Baltic states as sovereign political entities represented by the legations—appointed by the pre-1940 Baltic states—which functioned in Washington and elsewhere.
The Baltic states recovered de facto independence in 1991 during the dissolution of the Soviet Union. Russia started to withdraw its troops from the Baltics in August 1993; the full withdrawal of troops deployed by Moscow ended in August 1994. Russia ended its military presence in the Baltics in August 1998 by decommissioning the Skrunda-1 radar station in Latvia; the dismantled installations were repatriated to Russia and the site returned to Latvian control, with the last Russian soldier leaving Baltic soil in October 1999. Early in the morning of August 24, 1939, the Soviet Union and Germany signed a ten-year non-aggression pact, called the Molotov–Ribbentrop pact; the pact contained a secret protocol by which the states of Northern and Eastern Europe were divided into German and Soviet "spheres of influence". In the north, Finland and Latvia were assigned to the Soviet sphere. Poland was to be partitioned in the event of its "political rearrangement"—the areas east of the Narev and San Rivers going to the Soviet Union while Germany would occupy the west.
Lithuania, adjacent to East Prussia, would be in the German sphere of influence, although a second secret protocol agreed in September 1939 assigned the majority of Lithuanian territory to the Soviet Union. According to this secret protocol, Lithuania would regain its historical capital Vilnius subjugated during the inter-war period by Poland. Following the end of Soviet invasion of Poland on 6 October, the Soviets pressured Finland and the Baltic states to conclude mutual assistance treaties; the Soviets questioned the neutrality of Estonia after the escape of an interned Polish submarine on 18 September. A week on 24 September, the Estonian foreign minister was given an ultimatum in Moscow; the Soviets demanded the conclusion of a treaty of mutual assistance to establish military bases in Estonia. The Estonians had no choice but to accept naval and army bases on two Estonian islands and at the port of Paldiski; the corresponding agreement was signed on 28 September 1939. Latvia followed on 5 October 1939 and Lithuania shortly thereafter, on 10 October 1939.
The agreements permitted the Soviet Union to establish military bases on the Baltic states' territory for the duration of the European war and to station 25,000 Soviet soldiers in Estonia