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Chemical Elements are substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different elementary particles . More than 100 chemical elements are known to exist in the universe, although several of these, the so-called transuranium elements , have not been found in nature, and can only be produced artificially. See also Chemistry . Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge on the nucleus of an atom of an element), elements of similar physical

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Actinium Symbol: Ac Atomic Number: 89 Atomic Weight: (227) Group: Actinide Date Discovered: 1899 Discovered By: André Debierne Name: Aluminum Symbol: Al Atomic Number: 13 Atomic Weight: 26.9815 Group: Other metals Date Discovered: 1824 Discovered By: Hans Oersted (also attributed to Friedrich Wöhler 1827) Name: Americium Symbol: Am Atomic Number: 95 Atomic Weight: 243 Group: Actinide Date Discovered: 1944 Discovered By: Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso Name: Antimony Symbol: Sb Atomic Number: 51 Atomic Weight: 121.760 Group: Other metals Date Discovered: prehistoric Discovered By: unknown Name: Argon Symbol: Ar Atomic Number: 18 Atomic Weight: 39.948 Group: Noble gases Date Discovered: 1894 Discovered By: John Rayleigh and William Ramsay Name: Arsenic Symbol: As Atomic Number: 33 Atomic Weight: 74.9216 Group: Nonmetals Date Discovered: prehistoric Discovered By: unknown Name: Astatin

Elements, Chemical, substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different elementary particles . More than 100 chemical elements are known to exist in the universe, although several of these, the so-called transuranium elements , have not been found in nature, and can only be produced artificially. See also Chemistry . Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge on the nucleus of an atom of an element), elements of similar physical a