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In the late 1800s, scientists did not have the full periodic table that we are familiar with today. Dmitri Mendeleev, a visionary Russian chemist, proposed an ingenious idea. He acknowledged that not all elements had been discovered, which is why he intentionally left gaps in his periodic table.

Mendeleev’s approach was revolutionary because it allowed for the prediction of the existence and properties of elements that had not yet been found. This gave other scientists a roadmap to locate these missing pieces of the elemental puzzle. He even predicted certain properties: how heavy they might be, how they might react with other elements, and more.

Eventually, these predictions went on to guide researchers in discovering elements like Gallium and Germanium, which neatly filled the placeholders on his table. By anticipating undiscovered elements, Mendeleev showed that science moves forward not just by observing the world as it is, but by daring to predict what it can become.

Central to Mendeleev’s organization of the periodic table was the concept of atomic weight, known today as atomic mass. At the time, atomic weights were one of the few quantitative measures available to chemists, describing how heavy an atom of each element was.

Mendeleev ordered the elements by increasing atomic weight while considering similarities in their chemical properties. This was a significant leap because it established a logical and systematic way of categorizing elements that could also predict chemical behavior.

However, this method wasn't without its challenges. Atomic weight doesn’t perfectly reflect all periodic properties due to the existence of isotopes and modern understanding of atomic number. Despite this, Mendeleev's focus on atomic weight laid the foundation for our current understanding, where elements are organized by atomic number, which corrected some of the discrepancies he observed in his time.

The periodic law is a fundamental principle in chemistry derived from Mendeleev’s work. It states that the properties of elements are a periodic function of their atomic weights—or as modern chemistry understands it, their atomic numbers. This principle suggested that when elements are arranged according to increasing atomic weight, their chemical and physical properties repeat in a predictable pattern.

Mendeleev's realization of periodicity led to a more profound understanding of elemental behavior and reactivity, providing insight into how elements could form compounds with one another.

By trusting in the periodic law, Mendeleev could predict the properties of yet unknown elements simply based on their position in the table. When new elements like Scandium, Gallium, and Germanium were discovered and perfectly matched his predictions, it confirmed his revolutionary insights and the predictive power of the periodic law.