One example in the periodic table where the elements are not in order of their relative atomic weight is the positioning of isotopes. Isotopes are variants of an element with different numbers of neutrons in their nuclei, resulting in different atomic weights.
For example, one of the common isotopes of hydrogen is deuterium, which has an atomic weight of approximately 2.014. Deuterium has a higher atomic weight than some of the other elements in the periodic table that appear after it, such as helium (atomic weight of about 4.0026) and lithium (atomic weight of about 6.94).
In the periodic table, hydrogen is usually listed first due to its atomic number of 1. However, if elements were arranged strictly in order of increasing atomic weight, deuterium (with a higher atomic weight) would appear after helium and lithium.
This example illustrates how the periodic table is primarily organized based on atomic number (number of protons) rather than atomic weight, leading to certain anomalies where isotopes can disrupt the order of elements based on atomic weight.