Inert Pair Effect
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In chemistry, the inert pair effect occurs when electrons are pulled closer to the nucleus, making them stabler and more difficult to ionise. This is a relativistic effect.
An electron around the nucleus requires sufficient kinetic energy in order not to be pulled towards the nucleus. This results in it having higher speeds, with a higher force acting on it by the nucleus. The effects for the heavier elements are appreciable, as electrons travel closer to the speed of light, c. The s-orbital electrons are more affected in this way since they have a greater penetrating power.
The mass of the electron depends on its speed, given by its rest mass multiplied by the Lorenz factor. Electrons in heavier elements thus have greater increases in their relativistic masses.
The consequence of this is that the Bohr radius is decreased, as seen by: <math>r=n \cdot h/(2 \cdot m \cdot v \cdot pi)<math> which can be obtained through Planck's relationship, Einstein's equation for energy, and the concept that electrons in a Bohr atom would be a standing wave in a stationary state.
As the electrons are pulled closer to the nucleus by this effect, they are stabilised and harder to ionise. This is called the inert pair effect. The contraction of p-orbitals and particularly d and f-orbitals is somewhat less as the time spent near the nucleus decreases as the orbital angular momentum increases.
The inert pair effect is apparent from the chemistry of the Group III and Group IV elements and beyond. The lighter elements in Group IV tend to have a valency of +4, whereas the heavier elements form 2+ ions that are more stable than 4+ ions. For instance, PbO is much more stable than PbO2 which decomposes readily to PbO.