The smallest unit of an element is called an atom. An atom is the fundamental building block of matter and consists of a nucleus surrounded by electrons. The nucleus, located at the center of the atom, contains positively charged protons and uncharged neutrons, while the negatively charged electrons orbit the nucleus in various energy levels or shells.
Atoms are incredibly small; their size is measured in picometers (one trillionth of a meter), making them unattainable to the naked eye. Each element on the periodic table is made up of atoms unique to that element, distinguished primarily by the number of protons in their nucleus, a characteristic referred to as the atomic number. For example, a hydrogen atom has one proton, while a carbon atom has six protons.
Atoms can combine to form molecules through chemical bonds. A molecule is created when two or more atoms, bonded together, share electrons either through covalent bonds or ionic bonds. The structure and behavior of molecules depend on the arrangement of their constituent atoms, which plays a major role in the properties of materials.
The concept of atoms dates back to ancient Greek philosophers, who theorized about indivisible particles that make up all matter. However, the modern understanding of atoms evolved primarily through the work of scientists in the 19th and 20th centuries, like John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr. Dalton first proposed the atomic theory in the early 1800s, while Thomson's discovery of the electron and Rutherford's gold foil experiment further advanced our understanding of atomic structure.
Atoms are also responsible for various phenomena in chemistry and physics, such as radioactivity, electron configurations, and the formation of chemical compounds. The study of atoms and their interactions has led to significant advancements in technology and medicine, from the development of nuclear energy to breakthroughs in materials science and drug formulation.