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A simplistic way of understanding why energy is released when an ionic bond is formed is to think of how the electric potential energy changes as two oppositely charged ions get near each other.
Lets say you have two ions, one which is positively charged (for example Na+), and one which is negatively charged (for example, Cl-). Opposite charges attract each other. The electric fields generated by the two charges will pull the ions together. As they get closer together, the potential energy of the system decreases. The closer the ions get (up until a point**, which I will explain at the end), the stronger they are attracted together,
F = k * q1 * q2 / r^2
As r gets smaller, F gets bigger. So ionic bonds will hold ions together very tightly.
The potential energy of one of the particles is:
PE = k * q1 * q2 / r
since the particle's are oppositely charged, q1 = -q2, so
PE = -k * |q1 * q2| / r
So the particles loose energy as they get close together.
The potential energy that is lost by the particles, at first, is converted into kinetic energy, but will eventually be converted into heat as the particles bounce around and collide with other objects in their surroundings. So that potential energy lost ultimately ends us as heat in the surroundings.
** The particles will continue to attract each other only up until a point when they reach a special separation distance. They particles will not want to get any closer than this distance (which will will call its ?bond length?). This is because, although the ions do possess a net charge, they still have electron clouds surrounding the nucleus. As the ions get close, the electron clouds start to repel each other. When the ions get VERY close, the electron repulsion can become stronger than the attraction caused by the net charge on the ions. This is why the ions do not get infinitely close together, as one might naively assume from extending the above electrostatic equations to the limit as r goes to zero.
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