Elements are arranged in the periodic table based on their atomic number and not atomic mass because the atomic number is a fundamental property of an element, representing the number of protons in the nucleus of an atom. The atomic number determines an element's unique identity and its position in the periodic table.
The decision to prioritize atomic number was made because it provides a more consistent and meaningful way to organize elements. The concept of atomic number was introduced by Henry Moseley in the early 20th century, and it was found to correlate with various properties of elements, such as their chemical behavior and electron configurations.
Arranging elements by atomic number helps to group elements with similar properties together in the same columns, known as groups or families. This arrangement reflects the periodicity of properties observed across the elements, with similar electronic structures and chemical behaviors recurring at regular intervals. This arrangement is the basis for the periodic law and the foundation of modern chemistry.
While atomic mass does provide information about the mass of an atom, it is subject to variations due to the presence of isotopes. Isotopes are atoms of the same element with different numbers of neutrons, resulting in different atomic masses. If elements were arranged strictly by atomic mass, the resulting periodic table would not exhibit the clear patterns and trends observed with the current arrangement based on atomic number.
Therefore, the use of atomic number as the organizing principle allows for a more consistent and predictive understanding of the properties and behavior of elements.