The relative atomic masses of elements listed on the periodic table are rounded off values because they are determined based on the average mass of the naturally occurring isotopes of an element.
An element can have different isotopes, which are variants of the element with the same number of protons but different numbers of neutrons. These isotopes have slightly different masses due to the differing number of neutrons. Since isotopes occur naturally in different abundances, the relative atomic mass is a weighted average of the masses of all the isotopes present in a given sample.
The relative atomic mass listed on the periodic table is usually rounded off to the nearest whole number or a decimal place for simplicity and ease of use. This rounding is done to provide a convenient representation of the atomic mass that is easy to read and work with. The rounded values still provide a good approximation of the actual average atomic mass of the element.
It's important to note that more precise and accurate atomic mass values can be obtained through advanced scientific techniques, such as mass spectrometry, which can measure the exact masses of individual isotopes. However, for most practical purposes, the rounded values on the periodic table are sufficient.