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Nuclear reactions are associated with changes in both mass and energy. In this module, the relationship between these two concepts are examined on a nuclear level.
Albert Einstein’s mass-energy equivalence relates energy and mass in nuclear reactions:
\[ E=mc^2 \]
Each time an energy change occurs, there is also a mass change that is related by the constant c2 (the speed of light squared). Compared to the amount of energy due to the nuclear reaction, energy changes in chemical reactions are small, making the mass change insignificant for chemical reactions. However, on a nuclear level, there is a significant amount of energy change in comparison and therefore a discernible mass change. In Albert Einstein’s mass-energy equivalence, “m” is the net change in mass in kilograms and “c” is a constant (the speed of light) in meters per second. Two units to express nuclear energy are joules (J) and megaelectronvolts (MeV).
1.6022 x 10-13 J = 1 MeV
The energy of 1 atomic mass unit is:
1 atomic mass unit (u) = 1.4924 x 10-10 J = 931.5 MeV
By knowing the mass change in amu, the energy released can be directly calculated using these conversion factors, which have already taken into account mass conversions and the value of \(c^2\). Keep in mind that the mass-energy sums of a nuclear reaction must equal each other (by the Laws of Conservation of Mass & Energy). The sum of the mass and energy of the reactants are equivalent to the sum of the mass and energy of the products.
Figure 1 provides an example of a mass defect. That is, the sum of the mass of the individual protons and neutrons differs from the overall mass of the nucleus. The nucleus weighs less than the masses of the individual subatomic particles. Where does this mass go when a nucleus is formed? Recall Einstein's mass-energy equivalence and how matter and energy are essentially different configurations of one another. Mass is lost and as a result, energy is released as the nucleons come together to form the nucleus. This energy is known as the nuclear binding energy. Einstein's mass-energy equivalence can be rewritten in the following terms:
Nuclear Binding Energy = Mass Defect * c2
or E = Δm * c2
*Note that Δm can be either positive or negative (it depends on where you set your zero point) but for our purposes, it won't really matter, and we should simply always use a positive mass deficit and say that is the energy released.
The mass is converted into the energy required to bind the protons and neutrons together to make a nuclei. Nuclear binding energy is also the defined as the energy required to break apart a nucleus.
One additional thing to keep in mind is what a nucleus is and what a nucleon is. A nucleus (plural nuclei) is the center of the atom, and is composed of nucleon (which are both the protons and neutrons: a nucleon is what makes up the nuclei). This means the nucleus of an atom of Carbon-14 (14C)would contain 14 nucleons a nucleus of Uranium-238 (238U) would contain 238 nucleons etc.
The mass number 60 is the maximum binding energy for each nucleon. (In other words, nuclei of mass number of approximately 60 require the most energy to dismantle). This means that the binding energy increases when small nuclei join together to form larger nuclei in a process known as nuclear fusion. For nuclei with mass numbers greater than 60, the heavier nuclei will break down into smaller nuclei in a process known as nuclear fission.
For fusion processes, the binding energy per nucleon will increase and some of the mass will be converted and released as energy (recall Figure 1). Fission processes also release energy when heavy nuclei decompose into lighter nuclei. The driving force behind fission and fusion is for an atomic nuclei to become more stable. So nuclei with a mass number of approximately 60 will be the most stable, which explains why iron is the most stable element in the universe. Elements with mass numbers around 60 will also be stable elements, while elements with extremely large atomic masses will be unstable. Something to keep in mind is that nuclear fusion releases more energy than nuclear fission. This is due to the fact that the binding energy in a larger atom (the ones which we perform fission on) is generally lower than the binding energy of the atoms we perform fusion on. The difference in binding energy is due to their size differences, and how closely these protons and neutrons can get to one another as a result.