CHARACTERISTICS OF ATOMSDOE-HDBK-1015/1-93Fundamentals of ChemistryCH-01Rev. 0Page 4ChemicalSymbolsAt one time chemists used various symbols, similar to shorthand, for the atoms of thedifferent elements. These symbols were very cumbersome and were replaced byabbreviations of the names of the elements. Each element has been assigned a specificone or two letter symbol based on the first letter of its chemical name. Because there areseveral elements with the same first letter, it is often necessary to add the second letterto the symbol. In some cases the symbol comes from an abbreviation for the old latinname of the element. For example, Fe stands for iron (ferrum) and Cu for copper(cuprum). The first letter of the chemical symbol is always capitalized. If the symbolhas two letters, the second letter is always lowercase.AtomicNumberThe number of protons in the nucleus plays such an important role in identifying theatom that it is given a special name, the atomic number. The symbol Z is often used foratomic number (or number of protons). Hydrogen has an atomic number of 1 andlawrencium has an atomic number of 103. The atomic number is also equal to thenumber of electrons.AtomicMassNumberThe sum of the total number of protons, Z, and the total number of neutrons, N, is calledthe atomic mass number. The symbol is A. Not all atoms of the same element have thesame atomic mass number, because, although the Z is the same, the N and thus the A aredifferent. Atoms of the same element with different atomic mass numbers are calledisotopes.AtomicWeightIn Table 1, the masses of atomic particles are given in atomic mass units (amu). Theseunits represent a relative scale in which the mass of the isotope carbon-12 is used as thestandard and all others are related to it. Specifically, 1 amu is defined as 1/12 the massof the carbon-12 atom. Since the mass of a proton or a neutron is approximately 1 amu,the mass of a particular atom will be approximately equal to its atomic mass number, Z.The atomic weight of an element is generally more useful than isotopic masses. Theatomic weightof an element is defined as the weighted average of the masses of all ofits natural occurring isotopes. The atomic weight of the elements are listed in Table 2.The elements that have their atomic weights in parentheses are unstable. For theseelements, the atomic weight of the longest living isotope is used rather than the averageof the masses of all occurring isotopes.
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