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Welcome to Wikipedia,the free encyclopedia that anyone can edit.2,448,185 articles in EnglishArtsBiographyGeographyHistoryMathematicsScienceSocietyTechnologyAll portalsOverview · Editing · Questions · HelpContents · Categories · Featured content · A–Z indexToday’s featured articleAn atom is the smallest particle that composes a chemical element. An atom consists of an electron cloud that surrounds a dense nucleus. This nucleus contains positively charged protons and electrically neutral neutrons, whereas the surrounding cloud is made up of negatively charged electrons. When the number of protons in the nucleus equals the number of electrons, the atom is electrically neutral; otherwise it is an ion and has a net positive or negative charge. An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element. The concept of the atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and the early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the ‘atom’ was not indivisible. The principles of quantum mechanics were used to successfully model the atom. More than 99.9% of an atom’s mass is concentrated in the nucleus, with protons and neutrons having about equal mass. In atoms with too many or too few neutrons relative to the number of protons, the nucleus is unstable and subject to radioactive decay. (more…)Recently featured: Edward VIII – New York State Route 32 – Russian-Circassian WarArchive – By email – More featured articles…Did you know…From Wikipedia’s newest articles:… that when the Gevingåsen Tunnel (pictured) opens in 2012 it will cut rail travel time north of Trondheim, Norway, by five minutes?… that Fort Concho, a United States Army outpost in San Angelo, Texas from 1867 to 1889, was the headquarters for the Buffalo Soldiers of the American West?… that the city of Strasbourg, France is the sole or main seat of over 20 international institutions?… that Dr Nigel Cox is the only doctor ever to have been convicted in Britain for attempted euthanasia?… that Olvir Hnufa, a famous 9th-century Norwegian skald quoted in the Prose Edda, was the great-uncle of the Viking Egil Skallagrimsson?… that children’s author Aleksandra Ishimova was the last correspondent of Alexander Pushkin before his death in a duel?… that according to a letter written in 1428 by President Felip de Malla to Alfonso the Magnanimous, Catalonia was devastated by an earthquake on Candlemas that year?Archive – Start a new article…In the newsIran test-fires the Shahab-3, a missile with an operating range of 2,100 km (1,300 mi).Leaders of the nations meeting in Japan for the 34th G8 summit agree to a target of cutting world greenhouse gas emissions by at least 50 percent by 2050.A suicide bomber rams a car bomb into the Indian embassy in Kabul, Afghanistan, killing 41 people.An explosion near the Lal Masjid in Islamabad, Pakistan, kills at least 18 people on the first anniversary of the deadly siege and storming of the mosque.In tennis, Rafael Nadal (pictured) of Spain and Venus Williams of the United States win the men’s and women’s singles title, respectively, at the 2008 Wimbledon Championships.Clashes are reported between Georgian and South Ossetian forces near Tskhinvali.Wikinews – Recent deaths – More current events…On this day…July 9: Independence Day in Argentina (1816)1789 – French Revolution: The National Constituent Assembly was formed from the National Assembly, and began to function as a governing body and a drafter for a new constitution.1868 – The Fourteenth Amendment to the United States Constitution, including the Citizenship Clause, the Equal Protection Clause and the Privileges or Immunities Clause among others, was ratified by the minimum required twenty-eight U.S. states.1955 – The Russell-Einstein Manifesto, signed by Albert Einstein, Bertrand Russell (pictured) and nine other preeminent intellectuals and scientists, was issued during a press conference in London in the midst of the Cold War, calling for a conference where scientists would assess the dangers posed to the survival of humanity by weapons of mass destruction.1986 – The Parliament of New Zealand passed the Homosexual Law Reform Act, legalising consensual homosexual sex.2002 – The African Union was formed as a successor to the amalgamated African Economic Community and the Organization of African Unity, with President of South Africa Thabo Mbeki as its first chairman.More events: July 8 – July 9 – July 10Archive – By email – More anniversaries…It is now 21:32, July 9, 2008 (UTC) – Refresh this pageToday’s featured pictureDavid Farragut was the first senior officer of the United States Navy during the American Civil War. He was the first rear admiral, vice admiral, and full admiral of the Navy. He is remembered in popular culture for his possibly apocryphal order at the Battle of Mobile Bay, usually paraphrased: “Damn the torpedoes, full speed ahead!” (”torpedo” was the contemporary term for what is now known as a naval mine).Photo credit: Mathew Brady or Levin Corbin HandyRecently featured: Intercession of Charles Borromeo supported by the Virgin Mary – LAPD helicopter – Scene composed using BlenderArchive – More featured pictures…