The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configurations, and recurring chemical properties. The structure of the table shows periodic trends. Elements with similar behaviors are placed in the same column. The columns are numbered from left to right starting with 1; groups are numbered from top to bottom starting with 1. The first row contains only one element: hydrogen.
The modern periodic table is based on the original 1869 version proposed by Dmitri Mendeleev and expanded through discovery and research over more than a century since then. As of 2016, 118 elements have been identified and confirmed by experiment, including the recently synthesized 120th element oganesson (element 118). All but one of these elements (hydrogen) occur naturally on Earth; about 82% of atoms in the Universe are hydrogen atoms. Of the remaining 94%, most (~75%) are helium; almost all (~99%) of everything else consists primarily of oxygen, carbon, nitrogen, and iron. About 0.01% consists of other heavier metals, such as copper. A very small fraction (<0.001%) consists of radioactive isotopes that will decay over time into different elements. The Lanthanide and Actinide series at the bottom were added in 1949 and 1951 respectively when it was realized that they had similar properties to those immediately above them in respective groups 3–12 (the transition metals) and groups 13–18 (the post-transition metals). In 1962 chemists finally realized that there could be undiscovered or yet-to-be synthesized heavier analogues to lanthanum or actinium, so they added placeholders below yttrium (group 3) and between thorium (group 4) and protactinium (group 5);:p596 these placeholder names eventually became "lanthanoids" for all 15 elements below yttrium in group 3,:p170 while actinoid refers only to those 14 – along with plutonium – directly below thorium in group 4.-:pp196–197 All others beyond uranium are referred to as "transactinides".-:pp1127–1128 Mendeleev's idea was simple but powerful: if one could arrange all known chemicals by increasing atomic weight—a feat possible now that accurate weights had been determined for most—and if one could further subdivide this list into subgroups based on similarities in behavior or appearance—as he himself did using his intuitive knowledge—then perhaps some underlying order would reveal itself."