Extended periodic table

The periodic table is a map of all known chemical elements. Currently, the table ends at element 118, which is named oganesson. This element completes the seventh period, or row, of the table. However, scientists theorize about an extended periodic table. This would include elements with atomic numbers higher than 118. These elements are currently hypothetical because they have not been discovered or synthesized. An extended table helps researchers predict how new matter might behave.

To build an extended table, scientists must understand how electrons fill orbitals. Orbitals are the regions around a nucleus where electrons reside. In the seventh period, orbitals like 7s, 5f, 6d, and 7p are filled. A simple extension uses the Aufbau principle to predict the next steps. This principle suggests the eighth period would fill 8s, 5g, 6f, 7d, and 8p orbitals. The introduction of the 5g orbital would create a new section called a g-block. This block is expected to contain at least 18 elements in each period. The first element in this g-block might be element 121, known systematically as unbiunium.

Predicting these elements is difficult due to relativistic effects. These effects occur because the massive positive charge of a heavy nucleus affects electron movement. In very heavy atoms, electrons move at speeds that change their behavior. These changes can break the standard patterns seen in lighter elements. For example, Burkhard Fricke and Pekka Pyykkö used computer models to study elements up to atomic number 172. They found that many elements are displaced from the Madelung rule. This rule usually dictates the order in which orbitals fill. Because of these shifts, there is no scientific consensus on the exact layout of the extended table.

History shows that scientists have long suspected these elements exist. In 1895, Danish chemist Hans Peter Jørgen Julius Thomsen predicted a 32-element period. In 1913, Swedish physicist Johannes Rydberg predicted element 118 would be a noble gas. Niels Bohr suggested in 1922 that elements beyond uranium were simply too unstable to find. Later, in 1969, Glenn T. Seaborg proposed an eighth period. His model included the g-block and the superactinide series. By 1957, the concept of nuclear shells led to the theory of an "island of stability." This theory suggests certain heavy elements might last longer than others.

Stability is a major concern for superheavy elements. Most elements in this region are likely to be highly unstable. They undergo radioactive decay, such as alpha decay or spontaneous fission. These processes happen with extremely short half-lives. However, element 126 is hypothesized to be within an island of stability. This means it might resist fission, though it would still undergo alpha decay. Some researchers even theorize another island of stability around element 164. The exact limits of these effects remain uncertain.

Finding these elements requires immense effort in laboratories. No elements beyond 118 have been found in nature. Scientists attempt to synthesize them using particle accelerators. The heaviest element successfully synthesized is oganesson, completed in 2002. The most recent discovery was tennessine in 2010. The International Union of Pure and Applied Chemistry (IUPAC) defines an element as existing if its lifetime exceeds 10⁻¹⁴ seconds. This is the time needed for a nucleus to form an electron cloud. Without this minimum lifetime, a substance cannot be called an element.

There are theoretical limits to how large an atom can be. Some calculations suggest that neutral atoms cannot exist beyond element 137. This is based on the reciprocal of the fine-structure constant. Other, more rigorous analyses suggest a different limit near atomic number 168 to 172. At this point, the 1s subshell dives into the Dirac sea. Beyond this limit, scientists discuss "supercritical atoms," which may consist of bare nuclei. It remains unknown if a ninth period or more periods are physically possible. The search for the end of the periodic table continues to challenge modern physics.