page_title: Period (periodic table)
section_title: Period 4
hierarchical_section_title: Period (periodic table) / Periods / Period 4
caption_attribution_description: English: From left to right, aqueous solutions of: cobalt(II) nitrate, Co(NO3)2 (red); potassium dichromate, K2Cr2O7 (orange); potassium chromate, K2CrO4 (yellow); nickel(II) chloride, NiCl2 (green); copper(II) sulfate, CuSO4 (blue); potassium permanganate, KMnO4 (violet). Français&#160;: De gauche à droite, des solutions aqueuses de&#160;: nitrate de cobalt(II), Co(NO3)2 (rouge); dichromate de potassium, K2Cr2O7 (orange); chromate de potassium, K2CrO4 (jaune); chlorure de nickel(II), NiCl2 (vert); sulfate de cuivre(II), CuSO4 (bleu); permanganate de potassium, KMnO4 (violet).
context_page_description: A period in the periodic table is a row of chemical elements. All elements in a row have the same number of electron shells. Each next element in a period has one more proton and is less metallic than its predecessor. Arranged this way, groups of elements in the same column have similar chemical and physical properties, reflecting the periodic law. For example, the halogens lie in the second-last column and share similar properties, such as high reactivity and the tendency to gain one electron to arrive at a noble-gas electronic configuration. As of 2020, a total of 118 elements have been discovered and confirmed.
  Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown in the ordering rule diagram. The filling of each shell corresponds to a row in the table.
  In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties. However, in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods.
context_section_description: Period 4 includes the biologically essential elements potassium and calcium, and is the first period in the d-block with the lighter transition metals. 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. Almost all have biological roles.
  Completing the fourth period are the post-transition metals zinc and gallium, the metalloids germanium and arsenic, and the nonmetals selenium, bromine, and krypton.
caption: From left to right, aqueous solutions of: Co(NO3)2 (red); K2Cr2O7 (orange); K2CrO4 (yellow); NiCl2 (green); CuSO4 (blue); KMnO4 (purple).
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