Introduction to Quantum Theory

Chemistry began as a discipline to document materials that restore health, as pharmacy is today. During the 16th to 18th centuries, we have learned that material consists of compounds that are combinations and variation of only about 90 chemical elements, each with a unique atomic weight. The variation of their macroscopic properties as a function of the atomic weight is very interesting. For example, lithium, sodium, potassium and cesium react with water vigorously, and their reactivity increases as their atomic weights increase. This discovery led to their arrangement in a periodic table in the 19th century.