The elements are the basic building blocks of matter, consisting of pure substances 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. There are currently 118 known elements, organized in the Periodic Table based on their properties and atomic structure. Elements can exist as solids, liquids, or gases at room temperature. They combine in various ways to form compounds, which are substances made up of two or more different elements. Examples of common elements include oxygen, which is essential for respiration, and carbon, a key component of all known life forms.