As you move down the periodic table, you may observe that the atomic weight of heavier elements is not exactly twice the atomic number. This discrepancy occurs due to the presence of isotopes.
Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Since the atomic weight takes into account the mass of all naturally occurring isotopes of an element and their relative abundance, it can deviate from twice the atomic number.
As you go down the periodic table, the number of protons (atomic number) increases, resulting in an increase in the number of neutrons to maintain stability in the nucleus. This leads to the existence of multiple isotopes for heavier elements.
Isotopes have different masses, and their abundance varies. Therefore, when calculating the atomic weight, the contribution of each isotope is considered, taking into account its mass and the percentage of its occurrence in nature. The atomic weight is determined by averaging the masses of the isotopes, with the more abundant isotopes having a greater influence on the overall value.
This variation in isotopic composition causes the atomic weight to deviate from a simple ratio of twice the atomic number for heavier elements. It's important to note that the atomic number (which defines the element) remains the same for all isotopes of a given element, but the atomic weight can vary due to the presence of different isotopes.