Periodic table
A table of the elements in order of atomic number, folded so that elements with similar properties line up in columns.
In 1869 the Russian chemist Dmitri Mendeleev noticed that when the elements are arranged by atomic weight, similar properties recur at regular intervals, and gathered them into a table[1]. He is known for leaving gaps for elements not yet discovered and predicting their properties; gallium and germanium were later found and bore his predictions out.
In 1913 the English physicist Henry Moseley measured the wavelengths of the X-rays given off by the elements and showed that the true order is set not by weight but by the charge of the nucleus: the atomic number[2].
Why similar properties recur periodically was answered by quantum mechanics. Under the Pauli exclusion principle of 1925, electrons fill up an atom's seats from the inside out, a fixed number at a time[3]. Elements with the same number of outermost electrons have similar chemistry.
Related terms
- Pauli exclusion principleThe rule that no two electrons in the same atom can be in exactly the same state. Set out by Wolfgang Pauli in 1925.
- SpinA property that electrons and other particles are born with, making them behave like tiny magnets. For an electron it takes only two values: "up" or "down".
- Atomic nucleusThe tiny, heavy core at the centre of an atom, made of protons and neutrons and carrying a positive charge.
Sources
- D. Mendeleev, “Соотношение свойств с атомным весом элементов (The relation between the properties and atomic weights of the elements),” Zhurnal Russkago Khimicheskago Obshchestva 1, 60–77 (1869).
- H. G. J. Moseley, “The high-frequency spectra of the elements,” Philos. Mag. 26, 1024–1034 (1913).
- W. Pauli, “Über den Zusammenhang des Abschlusses der Elektronengruppen im Atom mit der Komplexstruktur der Spektren,” Z. Phys. 31, 765–783 (1925).