“Chlorine is a deadly poison gas employed on European battlefields in World War I. Sodium is a corrosive metal which burns upon contact with water. Together they make a placid and unpoisonous material, table salt. Why each of these substances has the properties it does is a subject called chemistry.” -Carl Sagan
Every beginning chemistry student learns what happens when you put a chunk of sodium metal into water: you get an extremely violent reaction out. The sodium and water bubble and fizz, and sometimes even a flame or an outright combustion reaction is produced. This isn’t exclusive to sodium, either, but occurs for lithium, potassium, rubidium, cesium and more.
We can describe these chemical reactions using basic chemistry, of course, but there’s a more fundamental reason it occurs: the laws of quantum mechanics. By combining the laws of electromagnetism with the Pauli exclusion principle and the shapes of electron orbitals, we can understand the full step-by-step process by which this occurs. Thanks to these laws, a reaction that had been described for hundreds of years can now finally be fully explained.