Short answers, pulled from the story.
Robert Boyle published The Sceptical Chymist in the year 1661. This book fundamentally altered how humanity understood the building blocks of the universe by introducing the term compound to describe substances formed from different elements. Boyle used the word corpuscles to describe the tiny particles that made up these elements.
Isaac Watts provided one of the earliest definitions of a chemical element in his book Logick by 1724. He argued that an element was a substance that could not be broken down into simpler substances while a compound was a substance formed from two or more elements. This logical framework helped scientists move away from vague descriptions and toward precise, measurable definitions.
Today, more than 350,000 chemical compounds have been registered for production and use. Each registered compound is assigned a unique identifier known as a CAS number by the Chemical Abstracts Service. This registry serves as a comprehensive database of chemical substances maintained by the American Chemical Society.
Chemical compounds are held together by four major types of bonds including covalent bonds, ionic bonds, metallic bonds, and coordinate covalent bonds. Molecular compounds are held together by covalent bonds where atoms share electrons to achieve stability. Ionic compounds are held together by ionic bonds which involve the transfer of electrons from one atom to another.
Non-stoichiometric compounds are substances such as palladium hydride that have variable compositions that can change depending on the conditions of their formation. These compounds form most of the Earth's crust and mantle making them a significant part of the planet's composition. The proportions of elements in these compounds may be reproducible but they do not always result in simple whole-number ratios.