Short answers, pulled from the story.
Dmitri Mendeleev published the periodic table in 1869. He arranged the elements by increasing atomic mass and left blank spaces for elements he predicted must exist to complete the pattern. His predictions were validated when gallium was discovered two years later with properties matching his calculations for eka-aluminum.
The structure of the periodic table is determined by atomic number, which represents the number of protons in the nucleus. This arrangement follows the Aufbau principle where electron shells fill up in specific regions called orbitals. Each period begins when a new electron shell starts to fill, and each group contains elements with the same number of valence electrons.
The final row of the periodic table was completed in 2010 when the first 118 elements were confirmed. The last four elements, nihonium, moscovium, tennessine, and oganesson, received their official names in 2016. These superheavy elements exist for only fractions of a second before decaying.
The placement of hydrogen remains a subject of intense debate because it shares properties with both the alkali metals of group 1 and the halogens of group 17. It fits perfectly into neither category, creating tension between electronic theory and chemical behavior. The International Union of Pure and Applied Chemistry has issued reports on these matters, yet the debate continues in textbooks and laboratories.
Relativistic effects cause the orbitals of heavy elements to contract and expand in ways that defy standard quantum mechanics. These effects explain why gold appears yellow and mercury remains liquid at room temperature. The properties of superheavy elements may diverge significantly from their lighter homologues as electrons move at speeds approaching the speed of light.