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The noble gases are a group of chemical elements that share a high degree of similarity in their properties: under standard conditions, they are all odorless, colorless, monatomic gases, with a very low chemical reactivity. They are placed in group 18 of the periodic table. The six noble gases that occur naturally are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). So far, three atoms of the next member of the group, ununoctium (Uuo) have been synthesized in a supercollider, but very little is known of its properties due to the tiny amount produced and its short half-life. The properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons is considered to be "full", giving them little tendency to participate in chemical reactions, and only a few hundred noble gas compounds have been prepared as of 2008. Noble gases have several important applications in industries such as lighting, welding, and space exploration. Helium is often used in scuba diving to replace part of the breathing mixture. After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons. (more...)

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August 18: Long Tan Day in Australia

• 1572 – French Wars of Religion: Marguerite de Valois (pictured) was married to Huguenot King Henry of Navarre, in a supposed attempt to reconcile Protestants and Catholics.
• 1590 – John White, the governor of the Colony of Roanoke, returned from a supply trip to England and found his settlement deserted.
• 1868 – Astronomer Pierre Janssen discovered helium while analyzing the chromosphere of the sun during a total solar eclipse in Guntur, India.
• 1966 – Vietnam War: A company of the Royal Australian Regiment fought a much larger North Vietnamese unit at the Battle of Long Tan, defeating them the next day.
• 1989 – Leading Colombian presidential hopeful Luis Carlos Galán was assassinated during a public demonstration in the town of Soacha, Cundinamarca.

• More events: August 17 – August 18 – August 19

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