Forces and energy are intimately connected, so we may view gravity as a disguised form of energy. Scientists have found it useful to classify matter in several different ways, two of which are discussed here. Elements are often referred to as the building blocks. The first letter of an element symbol is capitalized. Names and symbols for the known elements are presented on the inside front cover of the book.
Compounds are combinations of two or more elements in definite proportions by mass.
Any pure sample of sodium chloride, no matter where or how obtained, contains these two elements in this mass ratio. Sodium chloride is only one of more than six million known compounds. Together, elements and compounds are called pure substances . A physical property of a pure substance is a characteristic of that substance alone. It can be determined and described without changing the substance into other substances.