States of equilibrium
Neutral equilibrium means that, with a small deviation, the body remains in equilibrium. An example is a wheel rolling on a horizontal surface. If you stop it at any point, the wheel will be in a state of equilibrium. A ball lying on a flat horizontal surface is in a state of neutral equilibrium (Fig. 1).
Unstable equilibrium means that, with a small deviation of the body from the equilibrium state, forces emerge which tend to increase this deviation. A ball located at the top of a spherical projection is an example of unstable equilibrium.
Stable equilibrium means that, with small deviations of the body from this state, forces or moments of forces emerge which tend to return the body to the state of equilibrium. A ball located at the bottom of a spherical deepening is in a state of stable equilibrium.
Figure 1 Different types of equilibrium of the ball on a support.
(1) - neutral equilibrium, (2) - unstable equilibrium, (3) - stable equilibrium
In unstable systems, with a small deviation of the body from the state of equilibrium, forces emerge which tend to increase this deviation. An atom is an unstable system; at the slightest deviation in an electron's rotation around a proton (an increase or decrease of the distance between the proton and electron), centripetal forces increase this deviation (see Wikipedia).