<P> The number of electrons in an electrically neutral atom increases with the atomic number . The electrons in the outermost shell, or valence electrons, tend to be responsible for an element's chemical behavior . Elements that contain the same number of valence electrons can be grouped together and display similar chemical properties . </P> <P> For elements with high atomic number Z, the effects of relativity become more pronounced, and especially so for s electrons, which move at relativistic velocities as they penetrate the screening electrons near the core of high - Z atoms . This relativistic increase in momentum for high speed electrons causes a corresponding decrease in wavelength and contraction of 6s orbitals relative to 5d orbitals (by comparison to corresponding s and d electrons in lighter elements in the same column of the periodic table); this results in 6s valence electrons becoming lowered in energy . </P> <P> Examples of significant physical outcomes of this effect include the lowered melting temperature of mercury (which results from 6s electrons not being available for metal bonding) and the golden color of gold and caesium . </P> <P> In the Bohr Model, an n = 1 electron has a velocity given by v = Z α c (\ displaystyle v = Z \ alpha c), where Z is the atomic number, α (\ displaystyle \ alpha) is the fine - structure constant, and c is the speed of light . In non-relativistic quantum mechanics, therefore, any atom with an atomic number greater than 137 would require its 1s electrons to be traveling faster than the speed of light . Even in the Dirac equation, which accounts for relativistic effects, the wave function of the electron for atoms with Z> 137 is oscillatory and unbounded . The significance of element 137, also known as untriseptium, was first pointed out by the physicist Richard Feynman . Element 137 is sometimes informally called feynmanium (symbol Fy). However, Feynman's approximation fails to predict the exact critical value of Z due to the non-point - charge nature of the nucleus and very small orbital radius of inner electrons, resulting in a potential seen by inner electrons which is effectively less than Z . The critical Z value which makes the atom unstable with regard to high - field breakdown of the vacuum and production of electron - positron pairs, does not occur until Z is about 173 . These conditions are not seen except transiently in collisions of very heavy nuclei such as lead or uranium in accelerators, where such electron - positron production from these effects has been claimed to be observed . See Extension of the periodic table beyond the seventh period . </P>

What do you understand by the term orbital