Atomic Theory of Matter.

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Atomic Theory of Matter

According to modern atomic theory, all substances are made up of tiny units called atoms. Each atom consists of a massive, positively charged centre called the nucleus, around which fly one or more negatively charged electrons.  

   The nucleus itself can contain two kinds of particles: neutrons, which have no electrical charge, and positively charged protons. A neutral atom has the same number of electrons as protons, so the electrical charges cancel.  

   The identity of an atom and its atomic number is determined by the number of protons in its nucleus. For example, there is one proton in the nucleus of a hydrogen atom, so hydrogen has atomic number 1. Oxygen, with eight protons, has atomic number 8; mercury has atomic number 80; and uranium has atomic number 92.  

   Substances that are composed of only one kind of atom are called elements. Only 92 elements occur naturally on Earth. The lightest is hydrogen; the heaviest is uranium.  

   The nuclei of a given element all have the same number of protons but may have a differing number of neutrons. For example, about 99.8 percent of the oxygen nuclei in nature contain eight neutrons as well as eight protons. But a very few oxygen nuclei contain nine neutrons, and some even contain ten neutrons. Each kind of nucleus is a different isotope of oxygen. Each isotope has a different number of neutrons.  

   Most hydrogen atoms are made up of a single proton with an electron circling it. However, one isotope of hydrogen contains a neutron as well. This isotope is called deuterium. Because neutrons have approximately the same mass as protons, deuterium atoms have about twice as much mass as the ordinary isotope of hydrogen. An extremely rare form of hydrogen, called tritium, has one proton and two neutrons in its nucleus. This is an unstable arrangement, so the tritium nucleus is radioactive. It gives off a negatively charged particle and changes to a stable helium nucleus with two protons and one neutron.  

   Many other isotopes of the various elements are radioactive. They can give off radiation of different kinds, changing to other elements or to different isotopes of the same element. Many radioactive isotopes are man-made, produced in nuclear reactors and accelerators.

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   In addition to the 92 naturally occurring elements, scientists have synthesized more than a dozen others that are not found in nature. These are called the transuranium elements because they are all made of atoms that have more mass than the uranium atom. All transuranium elements, therefore, have atomic numbers greater than 92. Transuranium elements through atomic number 107 have been synthesized, though many of them live only a fraction of a second. Soviet scientists have reported evidence of the elements through number 110 but this remains unconfirmed. British scientists believe they have observed element number 112. If this ...

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