Reality → Matter → Elements → Periodic table
The creation of the periodic table of elements by Mendeleev in the 19th century, well before the structure of the atom was known, is one of science's great milestones. Today, the table is based on the atomic model of quantum physics [1] . The elements are arranged by their atomic number, the number of protons in the nucleus. There are 7 rows (periods, each with its own principal quantum number) [2] , and 18 columns ( groups, each with an assigned number of valence electrons [3] ). The first period comprises only 2 elements, Hydrogen and Helium, which are gases and nonmetals. The last (7th) period comprises 28 radioactive metals, of which only 6 are natural. The first group comprises the alkali metals and the last (18th) group the noble gases. Overall, the table comprises 91 metals (78%), 19 nonmetals (16%), and 7 semiconductors (6%). The arrangement of elements in the table shows trends of chemical/physical properties such as valence, metal/nonmetal and acid/base characteristics, electronegativity, ionization energy, and atomic radii. Based on the order reflected in the table, some elements were predicted before they were actually discovered, a striking achievement of science.
The principal quantum number designates a main energy level or 'shell' (a term still used because of its descriptiveness based on historic atomic models). The number 1 is assigned to the lowest energy level (or innermost shell), and the number 7 to the highest energy level (or outermost shell).
Normally, the valence electrons occupy the outermost 'shell', which accommodates up to 8 electrons; only some transition metals accommodate up to 4 additional valence electrons in 'subshells' designated s, p, d, and f (the letters originate from spectroscopy, describing sharp, principal, diffuse, and fundamental spectral lines, see Sheet). The 18 groups of elements can be separated into 4 blocks: 2 main blocks (s- and p-block) with main group elements, 1 block (d-block) with transition metals, and 1 block (f-block) with lanthanides ( rare earth elements) and actinides (radioactive metals, natural and artificial).). S-block elements (groups 1 and 2) have 1 or 2 valence electrons, d- and f-block elements (groups 3 through 12) have 3 to 12 valence electrons, and p-block elements (groups 13 through 18) have 3 to 8 valence electrons.