Depending on the definition of group 3 adopted by authors, lawrencium may also be included to complete the 6d series.

Superheavies are all named after physicists and chemists or important locations involved in the synthesis of the elements.

IUPAC defines an element to exist if its lifetime is longer than 10−14 second, which is the time it takes for the atom to form an electron cloud.

[20] The definition by the IUPAC/IUPAP Joint Working Party (JWP) states that a chemical element can only be recognized as discovered if a nucleus of it has not decayed within 10−14 seconds.

This value was chosen as an estimate of how long it takes a nucleus to acquire electrons and thus display its chemical properties.

However, its range is very short; as nuclei become larger, its influence on the outermost nucleons (protons and neutrons) weakens.

Accordingly, it was placed in the last row of the periodic table; this fueled speculation about the possible existence of elements heavier than uranium and why A = 240 seemed to be the limit.

Following the discovery of the noble gases, beginning with argon in 1895, the possibility of heavier members of the group was considered.

Danish chemist Julius Thomsen proposed in 1895 the existence of a sixth noble gas with Z = 86, A = 212 and a seventh with Z = 118, A = 292, the last closing a 32-element period containing thorium and uranium.

[53] In 1914, German physicist Richard Swinne proposed that elements heavier than uranium, such as those around Z = 108, could be found in cosmic rays.

Swinne published these predictions in 1926, believing that such elements might exist in Earth's core, iron meteorites, or the ice caps of Greenland where they had been locked up from their supposed cosmic origin.

Due to their short half-lives (for example, the most stable known isotope of seaborgium has a half-life of 14 minutes, and half-lives decrease with increasing atomic number) and the low yield of the nuclear reactions that produce them, new methods have had to be created to determine their gas-phase and solution chemistry based on very small samples of a few atoms each.

This causes a relativistic stabilization of the 7s electrons and makes the 7p orbitals accessible in low excitation states.

Experimental evidence shows that elements 103–108 behave as expected for their position in the periodic table, as heavier homologs of lutetium through osmium.

Lower oxidation states should be stabilized here, continuing group trends, as both the 7s and 7p1/2 electrons exhibit the inert-pair effect.

These elements are expected to largely continue to follow group trends, though with relativistic effects playing an increasingly larger role.

The 8s electrons are expected to be relativistically stabilized, so that the trend toward higher reactivity down these groups will reverse and the elements will behave more like their period 5 homologs, rubidium and strontium.

The 7p3/2 orbital is still relativistically destabilized, potentially giving these elements larger ionic radii and perhaps even being able to participate chemically.

This will cause the electron shells to mix so that the block concept no longer applies very well, and will also result in novel chemical properties that will make positioning these elements in a periodic table very difficult.