It is typical for the quantum energy state of such a valence electron to be split by fine structure arising from the magnetic interaction of the electron spin with the orbital angular momentum of the inner electrons. The next available energy level is the 6s electron, so the chemistry of cesium is determined by that lone 6s electron. Following the order of filling of the electron shells, the xenon structure fills all the levels up through the 5p electrons. The electron states are described by four quantum numbers, and those of cesium fill all the electron states that are part of the noble gas xenon (54 electrons) and then there is just one additional electron outside that symmetric electron distribution. Having 55 protons in its nucleus, it must have 55 electrons in orbit around that nucleus to be a neutral atom. To understand how remarkable the cesium atom is as a basis for the cesium atomic clock, it is necessary to examine the details of the structure of this atom. Time signals based on it are available by short wave radio (WWV and WWVH). Cesium clocks have demonstrated stability to 2 parts in 10 14, or one second in 1,400,000 years according to the Naval Observatory source cited below. The frequency of this atomic clock is in the microwave region of the electromagnetic spectrum and is a convenient one for locking a microwave oscillator. Prior to 1964 the international standard second had been based upon the orbital period of the Earth, but the cesium clock period was found to be much more stable than the Earth's orbit! The SI unit of time, the second, is now defined by this transition in cesium. In 1967 a standard second was adopted based on the frequency of a transition in the Cs-133 atom:ฤก second = 9,192, 631,770 cycles of the standard Cs-133 transition The current time standard for the United States is a cesium atomic frequency standard at the National Institute of Standards and Technology in Boulder, Colorado. Atomic clocks are integral parts of the Global Positioning System since extreme accuracy in timing is necessary for the triangulation involved. Such clocks have provided the accuracy necessary to test general relativity and to track variations in the frequencies of pulsars. The two most widely used atomic clocks in recent years have been the cesium beam atomic clock and the rubidium clock. The frequencies associated with such transitions are so reproducible that the definition of the second is now tied to the frequency associated with a transition in cesium-133: 1 second = 9,192, 631,770 cycles of the standard Cs-133 transition Very accurate clocks can be constructed by locking an electronic oscillator to the frequency of an atomic transition.
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