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Atomic Mass Unit Atomic Mass Unit The unified atomic mass unit (symbol: u) or dalton (symbol: Da) is the standard unit that is used for indicating mass on an atomic or molecular scale (atomic mass). It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660538921(73)Ă—10−27 kg. One dalton is approximately equal to the mass of one proton or one neutron; an equivalence of saying 1 g mol−1. The CIPM have categorised it as a "non-SI unit" because units values in SI units must be obtained experimentally. The amu without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, some technically sloppy sources still use the "amu" but now define it in the same way as u (based on carbon-12). In this sense, most use of "amu" units today actually refers to unified atomic mass units or u. Atomic Mass Number :- We know that atom is made up of neutrons, protons and electrons. The sum of the mass of these constituent particles of an element is called its atomic mass. Thus,

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Atomic mass of an element = Mass of neutrons + Mass of protons + Mass of electrons Since mass of an electron is almost nil (0.0005449), the atomic mass of an element is taken as: Atomic mass of an element = Mass of neutrons + Mass of protons. Thus atomic mass number is defined as the sum of the number of protons and neutrons present in the nucleus of an atom. For example, 1. The atomic mass of carbon = 12 The total mass of protons + neutrons = 12 2. The atomic mass of oxygen = 16 The total mass of protons + neutrons = 16 Element Symbol The symbol of the element is taken from the name of element. And it also includes the mass number as superscript and atomic number as subscript. For example, 1. Element symbol of carbon is given as 24Cr52 2. Element symbol of oxygen is given as 8O1 Atomic Mass Unit :-Three different atomic mass scales are used to calculate the relative atomic mass

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Hydrogen mass scale: The standard element selected here is Hydrogen. Its mass is unity and the atomic masses of other elements are compared with this. Oxygen atomic scale: Earlier, it was seen that if oxygen is taken as a standard and other elements are compared with it by taking atomic mass of oxygen as 16, most elements had atomic masses as whole numbers. On this scale, hydrogen had an atomic mass of 1.008. This scale was adopted before the discovery of isotopes. Carbon atomic mass scale: It was decided to select carbon – 12 as the standard element to take care of all discrepancies.

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