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An illustration of the helium atom
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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David Farragut

David 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 Handy

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