The masses in which we are interested are those of the proton, neutron, deuteron and deuterium: A convenient unit used for atomic masses is called simply the atomic mass unit (symbol u), which is one twelfth of the mass of an atom of the most prevalent isotope of carbon, 12C. (The effect is grossly exaggerated here.) For nuclei, the mass defect is a very small proportion for chemical reactions, it is an extremely small proportion. In each case, the binding energy E gives rise to a mass defect Δm = E/c 2. A stable atom or molecule has a lower mass than the sum of its components. Conversely, when two particles that attract each other come together, they lose energy and their mass decreases.Ĭartoon: a stable nucleus has a lower mass than the sum of its components. When we supply this energy, the mass is increased by Δm = E D/c 2. We call this the binding energy of deuterium. If we 'pull apart' a deuterium nucleus, we require energy E D. When we add an electron (e) to the deuterium nucleus, we get an atom of deuterium (D or 2H). For example, a deuterium nucleus (which is called a deuteron, symbol d) has just two components, a neutron (n) and a proton (p). Happily for us, most of them are stable - they don't disintegrate spontaneously. Conversely, if it is standing vertically on one pointed end it is unstable: it can lose energy by falling over. To lift up one end or the other, we have to give it energy, to do work on it. For example, a stick is stable lying on the ground. For macroscopic objects, a system is stable if it requires energy to put it into another state. (The symbol Δ, pronounced 'delta', means 'the change in'.)įirst, something about stability. Conversely, when we do work on something, we provide energy, so we can increase its mass by an amount Δm = E/c 2. Measurement of mass defect in atomic and molecular reactionsĮinstein's famous equation describes how mass m can be turned into energy E.Mass defect in atomic and molecular reactions.Apart from their strength, there is nothing special about the nuclear forces with regard to the mass defect. The difference is quantitative, however, not qualitative. Consequently, the mass defect in atomic and molecular calculations is much smaller and is usually neglected. The strong force in the nucleus is much larger than the electric force in atoms and molecules. The mass defect is evident in nuclear reactions when the masses are measured to several significant figures. Similarly, the mass of a stable atom or molecule is less than the sum of its parts by Δm = E/c 2, where E is the binding energy. Summary: The mass of a stable nucleus is less than the sum of its parts by Δm = E/c 2, where E is the binding energy. E = mc 2: The mass defect and binding energies in the nucleus.
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