The smallest unit of an element that retains all the properties of that element is an atom. Atoms consist of a nucleus, containing positively charged protons and uncharged neutrons, surrounded by negatively charged electrons in orbitals. Each element on the periodic table is defined by its atomic number, which represents the number of protons in its nucleus. Atoms can bond with one another to form molecules, participating in various chemical reactions. Understanding atomic structure is fundamental to the fields of chemistry and physics, providing insights into the behavior of matter.