Why did Mendeleeff arrange the elements in the increasing order of their atomic weight in his periodic table?

When Dmitri Mendeleev set out to organize the chemical elements, he noticed that by listing them based on their atomic weight, he could see a recurring trend in their properties. This significant observation became known as the Periodic Law, a cornerstone of chemistry that implies the regular and repeating nature of element characteristics. It states that the properties of the elements are not random but follow a predictable pattern when the elements are arranged according to increasing atomic weight.

The Periodic Law is crucial because it provides a systematic framework through which we can understand chemical behavior. For students learning chemistry, this means that once you grasp the pattern in the periodic table, you can predict a lot about an element's properties without having to memorize separate facts for each one.

The atomic weight, or atomic mass, is the average mass of atoms of an element, measured in atomic mass units (amu). These weights take into account the various isotopes of an element that occur in nature and their relative abundance. Mendeleev realized that atomic weight was a key characteristic in understanding how the elements relate to one another.

The idea of organizing elements by atomic weight is akin to categorizing books by their number of pages. Just as we can discern certain characteristics about books with a similar number of pages, elements with close atomic weights exhibit similar chemical and physical behaviors. In Mendeleev's table, this approach allowed him to create a coherent and practical layout that mirrored how elements actually behave.

Mendeleev's approach led to a logical grouping of elements with shared traits. Elements that displayed similar properties were grouped together in columns, known as groups or families. For example, noble gases, which are largely unreactive, were clustered in one group, while alkali metals, all reactive and with one electron in their outermost shell, formed another group.

This grouping is a crucial aid for students as it simplifies learning. You can focus on the commonalities within a group, and these traits can often be extrapolated to understand the behavior of the elements when they react with others. It's as if you're learning the personality of a family of elements rather than individual characteristics for each separate element.

One of the most impressive feats of Mendeleev's periodic table was the prediction of elements that had not yet been discovered. There were gaps in his table, and based on the patterns he observed, he predicted the existence of new elements and their possible properties. This is not unlike predicting a missing piece in a jigsaw puzzle by analyzing the surrounding pieces. His correct predictions, including elements like gallium and germanium, underscored the table's accuracy and tinted it with an aura of prophecy.

For students struggling to see the practicality of scientific theories, these predictions are evidence of the power of scientific inquiry and methodology. They also offer a historical context of how the periodic table has evolved, which adds another layer to understanding how and why elements are organized as they are today.