Someone told me that there is a limit to the number of trans uranium elements that can be in the periodic table. His reasoning was that when the number of electrons for an element gets big enough, the electrons in the outer shell will be so far out that they will have to travel faster than the speed of light to orbit the nucleus. Could he be right?
In: 22
It is the inner electron shell where they move the fastest, not the outer one. Using the Bohr model, you can work out the “speed” of the electron in the inner shell with the equation (Z/137)*100 where Z is the atomic number
The answer you get is as a % of c (speed of light)
So for Uranium, the inner electrons “move” at 67% the speed of light. For hydrogen it is only 0.7% c
Using this model you can see that no atom can have an atomic number greater than 136 with a full set of electrons (ie not an ion). So far the largest we’ve found is oganesson with an atomic number of 118.
The Bohr model is useful for showing why the properties of atoms change as you move down the group. After a certain point, electrons start feeling the relativistic effects of moving so fast, their rest mass increases so the orbital radius must decrease as a result. This changes the electron shielding for the outer electrons, moving them further away from the nucleus and making the atom slightly more reactive than otherwise expected in groups where reactivity decreases down the group.
However, the Bohr model is a bit outdated. More recent models don’t really have electrons orbiting the nucleus like stars, but simply existing as a wavefunction.
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