![]() ![]() In order to make the result dimensionless we divide it by 1 u.ġ u is also defined as the one-twelfth atomic mass of carbon-12, 1 u = 1∕ 12 m ( 12C). The numerator in the above formula gives the average atomic mass, which has a unit of the dalton or the unified atomic mass (u). Where p i is the percentage or relative abundance of an isotope i of atomic mass m i. The formula of atomic weight can be formulated based on the above definition. Unlike the atomic mass, the atomic weight does not have any unit. The atomic weight is denoted by the symbol A r. The atomic weight is also known as the relative atomic mass. It is the weighted average atomic mass calculated using the relative abundance of isotopes of an element. The Atomic weight is defined as the average atomic mass of isotopes of an element in a given sample. The weighted average of their atomic masses gives the atomic weight of chlorine. Figure 1: Chlorine has two naturally-occurring isotopes: 35Cl, 75.76 % 37Cl, 24.24 %. This average atomic mass is called the atomic weight. To solve this problem and have better accuracy in calculations, we average out atomic masses. The dilemma here is which atomic mass to consider. The atomic mass of these isotopes is 34.969 u and 36.966 u respectively. Because of this variance in the neutron number, an atom of the same element can have a different atomic mass.Ĭonsider an example of chlorine, which has two naturally-occurring isotopes: 35Cl and 37Cl. For a given element, the proton number (more commonly known as the atomic number) is fixed, but the neutron number can vary. Protons and neutrons are mainly responsible for the mass of an atom. ![]() Atoms consist of electrons, protons, and neutrons. The atomic weight (also known as relative atomic mass) is a quantity used to express the average weight of an atom. ![]()
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