Period (periodic table)

Arranged this way, elements in the same group (column) have similar chemical and physical properties, reflecting the periodic law.

Modern quantum mechanics explains these periodic trends in properties in terms of electron shells.

The elements are colour-coded below by their block: red for the s-block, yellow for the p-block, blue for the d-block, and green for the f-block.

These include iron, the heaviest element forged in main-sequence stars and a principal component of the Earth, as well as other important metals such as cobalt, nickel, and copper.

Completing the fourth period are six p-block elements: gallium, germanium, arsenic, selenium, bromine, and krypton.

Whilst five of these (from americium to einsteinium) are now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less.

Whilst francium and radium do show typical properties of groups 1 and 2, respectively, the actinides display a much greater variety of behaviour and oxidation states than the lanthanides.

These peculiarities of period 7 may be due to a variety of factors, including a large degree of spin–orbit coupling and relativistic effects, ultimately caused by the very high positive electrical charge from their massive atomic nuclei.

In the periodic table of the elements, each numbered row is a period.
The Madelung energy ordering rule describes the order in which orbitals are arranged by increasing energy according to the Madelung rule. Each diagonal corresponds to a different value of n + l.
From left to right, aqueous solutions of: Co(NO 3 ) 2 (red); K 2 Cr 2 O 7 (orange); K 2 CrO 4 (yellow); NiCl 2 (green); CuSO 4 (blue); KMnO 4 (purple).