In chemistry, a symbol is a code for a chemical element. Chemical symbols are one to three letters long, and are written with only the first letter capitalized. Chemical symbols often resemble classical Latin and Greek vocabulary. For some elements, this is because the material was known in ancient times, while for others, the name is a more recent invention. For example, "He" is the symbol for helium (New Latin name, not known in ancient Roman times), "Pb" for lead (plumbum in Latin), and "Hg" for mercury (hydrargyrum in Greek). Some symbols come from other sources, like "W" for tungsten (Wolfram in German, not known in Roman times). Temporary symbols assigned to newly or not-yet synthesized elements use 3-letter symbols based on their atomic numbers. For example, "Uno" was the temporary symbol for hassium which had the temporary name of unniloctium and "Uuo" is the symbol for ununoctium (temporary name) with the atomic number 118. Chemical symbols may be modified by the use of prepended superscripts or subscripts to specify a particular isotope of an atom. Additionally, appended superscripts may be used to indicate the ionization or oxidation state of an element. They are widely used in chemistry and they have been officially chosen by the International Union of Pure and Applied Chemistry. There are also some historical symbols that are no longer officially used. Attached subscripts or superscripts specifying a nucleotide or molecule have the following meanings and positions: \n* The nucleon number (mass number) is shown in the left superscript position (e.g., 14N). This number defines the specific isotope. Various letters, such as "m" and "f" may also be used here to indicate a nuclear isomer (e.g., 99mTc). Alternately, the number here can represent a specific spin state (e.g., 1O2). These details can be omitted if not relevant in a certain context. \n* The proton number (atomic number) may be indicated in the left subscript position (e.g., 64Gd). The atomic number is redundant to the chemical element, but is sometimes used to emphasize the change of numbers of nucleons in a nuclear reaction. \n* If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Ca2+). \n* The number of atoms of an element in a molecule or chemical compound is shown in the right subscript position (e.g., N2 or Fe2O3). Often omitted if the value is one. \n* A radical is indicated by a dot on the right side (e.g., Cl• for a neutral chlorine atom). This is often omitted unless relevant to a certain context because it is already knowable from the charge and atomic number information values. In Chinese, each chemical element has a dedicated character, usually created for the purpose (see Chemical elements in East Asian languages). However, Latin symbols are also used, especially in formulas. A list of current, dated, as well as proposed and historical signs and symbols is included here with its signification. Also given is each element's atomic number, atomic weight or the atomic mass of the most stable isotope, group and period numbers on the periodic table, and etymology of the symbol.

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