An element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is defined by the number of protons in its atomic nucleus, known as the atomic number. Elements are the building blocks of matter and are organized in the periodic table, which displays their properties and relationships. There are currently 118 known elements, ranging from common ones like oxygen and carbon to rare ones like francium. Elements combine in various ways to form compounds, which are essential for life and the universe's structure. Understanding elements is fundamental to the study of chemistry and materials science.