<P> If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values can be emitted as a type of usable energy (such as a gamma ray, or the kinetic energy of a beta particle), as described by Albert Einstein's mass--energy equivalence formula, E = mc, where m is the mass loss and c is the speed of light . This deficit is part of the binding energy of the new nucleus, and it is the non-recoverable loss of the energy that causes the fused particles to remain together in a state that requires this energy to separate . </P> <P> The fusion of two nuclei that create larger nuclei with lower atomic numbers than iron and nickel--a total nucleon number of about 60--is usually an exothermic process that releases more energy than is required to bring them together . It is this energy - releasing process that makes nuclear fusion in stars a self - sustaining reaction . For heavier nuclei, the binding energy per nucleon in the nucleus begins to decrease . That means fusion processes producing nuclei that have atomic numbers higher than about 26, and atomic masses higher than about 60, is an endothermic process . These more massive nuclei cannot undergo an energy - producing fusion reaction that can sustain the hydrostatic equilibrium of a star . </P> <P> The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force . This force binds the electrons inside an electrostatic potential well surrounding the smaller nucleus, which means that an external source of energy is needed for the electron to escape . The closer an electron is to the nucleus, the greater the attractive force . Hence electrons bound near the center of the potential well require more energy to escape than those at greater separations . </P> <P> Electrons, like other particles, have properties of both a particle and a wave . The electron cloud is a region inside the potential well where each electron forms a type of three - dimensional standing wave--a wave form that does not move relative to the nucleus . This behavior is defined by an atomic orbital, a mathematical function that characterises the probability that an electron appears to be at a particular location when its position is measured . Only a discrete (or quantized) set of these orbitals exist around the nucleus, as other possible wave patterns rapidly decay into a more stable form . Orbitals can have one or more ring or node structures, and differ from each other in size, shape and orientation . </P>

What makes the center of an atom stable
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