Which one of the following elements is used as a timekeeper in atomic clocks?

Atomic clocks are high-precision timekeeping devices that utilize the microwave transitions of specific atoms to measure time with exceptional accuracy [1]. Since 1967, the international definition of the 'second' has been based on the caesium-133 atom [3]. Specifically, one second is defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. While other elements like rubidium, hydrogen, and mercury are used in various types of atomic clocks for specific applications like GPS satellites or deep space navigation, caesium remains the primary standard for the SI unit of time [5]. Modern caesium clocks are so precise that they may lose only one second in millions of years [1].

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > THINK LIKE A SCIENTIST! > p. 111
2. [3] https://www.scientificamerican.com/article/how-does-one-arrive-at-th/
3. [4] https://en.wikipedia.org/wiki/Atomic_clock
4. [5] https://www.science.gov/topicpages/i/ion+atomic+clock.html