Today's featured article
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An atom is the smallest particle that comprises 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...)
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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.
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