Atoms are the basic building blocks of matter, categorized by the composition of their subatomic particles, which determines their identity and chemical properties. The simplest classification is based on the number of unique elements, currently 118, each representing a distinct type of atom. However, the total count of distinct atomic structures, or nuclides, is much greater due to variations beyond the basic element definition.
The Defining Factor: Atomic Number
The primary classification of any atom is defined by the number of protons contained within its nucleus, known as the atomic number (\(Z\)). This number is the absolute identifier of an element. For example, an atom with six protons is classified as Carbon, and every atom with eight protons is Oxygen, regardless of other particles it may possess.
The atomic number organizes the Periodic Table, arranging elements in ascending order of their proton count. Since the proton count dictates the number of electrons in a neutral atom, the atomic number also determines the element’s chemical behavior. The International Union of Pure and Applied Chemistry (IUPAC) currently recognizes 118 total elements, meaning there are 118 distinct atomic types defined solely by their unique number of protons.
Expanding the Count: The Role of Isotopes
While the atomic number defines the element, it does not account for all atomic variations because atoms of the same element can have differing numbers of neutrons. These structural variants are known as isotopes, representing technically distinct types of atoms. The number of neutrons influences the mass number (\(A\)), which is the sum of an atom’s protons (\(Z\)) and neutrons (\(N\)).
For example, Carbon (atomic number six) exists primarily as Carbon-12 (six neutrons) and Carbon-13 (seven neutrons), and also as the radioactive isotope Carbon-14 (eight neutrons). All three are Carbon because they share six protons, but they are separate nuclides due to their different masses and nuclear stability. Most naturally occurring elements possess more than one stable isotope, significantly expanding the count of unique atomic structures beyond the 118 elements. Considering both stable and unstable forms, scientists have identified over 3,000 distinct nuclides.
Where We Find Them: Natural Versus Synthetic Atoms
The 118 recognized elements are divided into naturally occurring and laboratory-synthesized types. Elements up to atomic number 92 (Uranium) are generally considered natural, though some, like Francium and Astatine, exist only in trace amounts from radioactive decay. The remaining elements, starting with Neptunium (\(Z=93\)), are transuranic elements created artificially by colliding atomic nuclei in particle accelerators.
These synthetic atoms are inherently unstable, often decaying almost instantly after their creation, meaning they do not exist for long outside of a controlled environment. Their existence is measured by their half-life, which can range from fractions of a second to several million years. The total count of 118 is continuously refined as scientists synthesize new, heavier elements, such as Nihonium (113) and Oganesson (118), which were formally named and added to the Periodic Table.