Answer:
Step-by-step explanation:
Modern periodic tables arrange elements in order of their atomic number, rather than their atomic mass, due to the discovery of the fundamental principle known as the "Periodic Law" and the realization that atomic number is a more accurate indicator of an element's properties than atomic mass.
Here's why modern periodic tables are organized by atomic number:
1. **Periodic Law:** Dmitri Mendeleev's original periodic table was based on arranging elements in order of increasing atomic mass, which allowed for elements with similar properties to be grouped together. However, there were cases where the order of atomic mass did not accurately predict the periodic repetition of properties.
2. **Discovery of Atomic Number:** The discovery of the concept of atomic number by Henry Moseley around 1913 revolutionized the understanding of the periodic table. Atomic number represents the number of protons in an atom's nucleus, which is a fundamental property unique to each element.
3. **Consistent with Properties:** Moseley's work demonstrated that when elements were arranged by atomic number, their properties aligned more consistently with periodic trends. Elements with similar properties fell into the same vertical columns (groups) on the periodic table.
4. **Mendeleev's Table Modified:** As a result of Moseley's findings, the periodic table was modified to reflect the order of atomic number rather than atomic mass. This new arrangement provided a more accurate representation of the periodicity of element properties.
5. **Mendeleev's Predictions Validated:** Additionally, the discovery of new elements after Mendeleev's time showed that the predicted properties of these elements based on their atomic number were more accurate than those predicted by atomic mass.
In summary, the decision to arrange elements by atomic number in the modern periodic table was made because atomic number is a fundamental property that directly relates to the number of protons in an atom's nucleus. This arrangement better explains and predicts the periodic trends and properties of elements, making it a more accurate and logical way to organize the elements.