What makes it the preferred means of measuring time? And I guess, really, how do atomic clocks work?
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This is very hard to explain briefly and simply, but the *real* answer is that **cesium only has one electron in its outermost shell.**
This means that it can easily and controllably transition between two hyperfine states. Because all you need to do is shoot radiation with the correct wavelength and the transition will occur. The corresponding wavelength also has a good length (we don’t want e.g. visible light, since it interacts with other matter).
If there are more electrons in that shell, there are too many states in the hyperfine band so that we don’t get a controlled deexcitation. Rather you could get multiple deexcitations of lesser energy (longer wavelengths).
When we emit radiation to the cesium, we need to get the same energy out again (for how the principle of atomic clocks work).
We *could* use other elements with only one electron in the outermost shell, and we *do*, but for practical reasons, cesium is best (it’s readily available and relatively cheap, it’s stable, it’s a solid).
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