The average atomic masses of elements on the periodic table are written without units because they are relative values. They represent the average mass of an atom of a particular element compared to a standard reference value.
The unit commonly used to express atomic masses is the atomic mass unit (u) or the unified atomic mass unit (u). However, when listing the average atomic masses on the periodic table, the units are typically omitted for convenience and clarity.
The average atomic mass of an element takes into account the different isotopes of that element and their relative abundance. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Since different isotopes have slightly different masses, the average atomic mass considers the masses of all naturally occurring isotopes and their abundance in nature.
To calculate the average atomic mass, the masses of the isotopes are multiplied by their respective abundances and then summed. The resulting value is an average, which is dimensionless and expressed relative to the standard reference value of 1/12th the mass of a carbon-12 atom. Therefore, the average atomic mass of an element is a relative quantity and is reported without units on the periodic table.