Periodic law

The principle that elemental properties recur periodically when elements are ordered by atomic number. It explains the repeating patterns represented by the periodic table.

Atomic number: The number of protons in an atom's nucleus, which identifies the element. Ordering elements by proton count gives the modern periodic law its organizing variable.
Dmitri Mendeleev: A Russian chemist who developed a periodic table and predicted properties of undiscovered elements. His 1869 table made periodic recurrence a powerful predictive principle.
Gallium: A chemical element with atomic number 31, discovered in 1875 and predicted by Mendeleev as eka-aluminium. Its measured properties closely matched Mendeleev's prediction from a vacant table position.
Henry Moseley: An English physicist whose X-ray measurements established atomic number as a measure of nuclear charge. His measurements supplied the correct ordering variable for the modern periodic law.
Electron configuration: The distribution of an atom's electrons among shells, subshells, and orbitals. Repeating outer-electron patterns produce recurring chemical behavior across the table.
Julius Lothar Meyer: A German chemist who independently developed an early periodic classification of the elements. His work documented recurring physical properties alongside Mendeleev's chemical system.
Germanium: A chemical element with atomic number 32, discovered in 1886 and predicted by Mendeleev as eka-silicon. Its discovery provided another striking test of periodic prediction.
Moseley's law: The relationship between an element's atomic number and the frequency of its characteristic X-ray spectra. It showed that atomic number, rather than atomic weight, tracks elemental identity.
Valence electron: An electron in an atom's outer shell that can participate in chemical bonding. Valence electrons account for many recurring similarities among elements in a group.
John Newlands: An English chemist who proposed the Law of Octaves, arranging elements so properties repeated every eighth element. His early recurrence rule anticipated periodicity but failed for heavier elements.

Linked from 18 pages