Defining a Chemical Element
A chemical element is a pure substance that consists only of atoms having the same numbers of protons in their atomic nuclei. Elements cannot be broken down into simpler substances by ordinary chemical means. Each element is defined by its atomic number (the number of protons in its atoms), which is unique to that element.
Defining a Pure Substance
A pure substance is a form of matter that has a constant chemical composition and characteristic properties. It cannot be separated into simpler components by physical methods without breaking chemical bonds. Pure substances include both elements and compounds.
Key Differences and Relationships
The crucial difference is that while all elements are pure substances, not all pure substances are elements. Pure substances can be either elements (like oxygen or gold) or compounds (like water or carbon dioxide). Compounds are formed when two or more different elements are chemically bonded together in a fixed ratio, and they also have a uniform composition and distinct properties, making them pure substances.
Practical Examples
For example, elemental gold (Au) is a pure substance and an element. Water (H₂O) is also a pure substance because it has a consistent composition and properties, but it is a compound made of hydrogen and oxygen elements. Salt (NaCl) is another pure substance (a compound), while a mixture like salt water is not a pure substance, as its composition can vary.