Element 118, Oganesson (Og), and element 117, Tennessine (Ts), having the same atomic mass is not a coincidence but a consequence of the underlying atomic structure and the synthesis methods used to create these superheavy elements.
Oganesson and Tennessine are both transactinide elements, which are highly unstable and only exist artificially. They are synthesized through nuclear reactions involving the collision of heavy nuclei. Due to their instability and short half-lives, the accurate determination of their atomic masses is challenging, and experimental measurements are limited.
The atomic mass of these superheavy elements is determined by the number of protons and neutrons in their nuclei. While the atomic number (number of protons) uniquely identifies an element, the number of neutrons can vary, resulting in different isotopes of the same element. Isotopes of an element have the same atomic number but different atomic masses.
In the case of Oganesson and Tennessine, the isotopes synthesized and detected thus far have similar numbers of neutrons, leading to similar atomic masses. The most stable isotopes of Oganesson and Tennessine that have been synthesized are Oganesson-294 (with 176 neutrons) and Tennessine-294 (with 177 neutrons), respectively. These isotopes have almost identical atomic masses, contributing to the observed similarity.
However, it's worth noting that as further research and experimentation are conducted on these superheavy elements, additional isotopes may be discovered, leading to variations in their atomic masses. Thus, the current similarity in atomic masses between Oganesson and Tennessine is a result of the isotopes synthesized and detected so far, rather than an inherent property of the elements themselves.