Elements are made of small particles known as atoms. Atoms are the basic building blocks of matter and are the smallest unit of an element that retains all of the element's chemical properties. Each atom consists of a nucleus, containing protons and neutrons, surrounded by a cloud of electrons. The identity of an element is defined by the number of protons in its nucleus, which determines its atomic number.
In contrast, molecules are formed when two or more atoms bond together, which can consist of the same or different elements. Ions are atoms or molecules that have gained or lost one or more electrons, resulting in a net electrical charge. Crystals refer to a solid material whose constituents, such as atoms or molecules, are arranged in a highly ordered microscopic structure, but they are made up of atoms at the fundamental level. Thus, atoms are the fundamental particles that constitute elements.