Two elements almost as far apart as you can get on the Periodic Table – caesium in Group 1 and krypton right over in Group 18 – and both have been used for accurate measurement.
Using the colourless gas krypton to define the metre may at first sight seem odd, and I’m sure it would have been to the Parisians who made the first standard metre in the 1799 from a platinum bar. This bar represented one ten-millionth of the shortest distance from the North Pole to the equator, passing through Paris. This was only useful if you happened to be in Paris so in 1889 it was replaced by thirty platinum-iridium bars (which also corrected some inaccuracies in the original calculation) held in various places across the world. Then, in 1960 the definition moved away from being a physical object related to the circumference of the Earth, and instead was related to something less changeable: the metre was defined as exactly 1,650,763.73 wavelengths of a line in the atomic spectrum of the isotope krypton-86. Krypton was chosen because of the sharpness of this emission line, but it still was not perfect. This definition of the metre lasted for 23 years, at which point krypton was sidelined in favour of a definition in terms of the speed of light.
Caesium’s involvement in accurate measurement began at a similar time to krypton’s, in 1955, with the world’s first atomic clock. This clock, developed by the UK’s National Physical Laboratory, measured time by means of the natural resonance frequency of caesium-133 atoms. It was thirty times more accurate than quartz clocks and 300 times more accurate than the best pendulum clocks. A second is defined as 9,192,631,770 periods of the radiation corresponding to the transition between the hyperfine levels of the unperturbed ground state of the 133Cs atom. Caesium is still used in many of today’s atomic clocks which are accurate to around 1 second in about 100 million years.
Many of us now have alarm clocks which take their signal from a local atomic clock. The first of these that I had was sent to me by my German pen friend in the late 1990s and was quite a novelty. I once knocked it off my bedside table in the middle of the night, it clattered to the floor and the batteries fell out. I hastily shoved them back in and fell back to sleep. The next morning I woke in horror – I had forgotten to reset the alarm, it was 8.50 and I was due at a staff away-day in ten minutes! Fortunately I lived close to the venue and so leapt into some clothes and onto my bike, arriving just five minutes late. As I dashed into the building a couple of cleaners gave me odd looks, and on rounding the corner to the conference room I found it empty. The clock read 8.05. It was then that I remembered that my clock picked up its signal from the Frankfurt atomic clock and had reset to German time!