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Period (periodic table)
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<p>A period on the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a> is a row of <a href="/facts/Chemical_element/0jducgWD">chemical elements</a>. All <a href="/facts/Chemical_element/0jducgWD">elements</a> in a row have the same number of <a href="/facts/Electron_shells/QxI30eh9">electron shells</a>. Each next element in a period has one more <a href="/facts/Proton/clCjepXP">proton</a> and is less <a href="/facts/Metal/PT8hNX6R">metallic</a> than its predecessor. Arranged this way, elements in the same <a href="/facts/Group_(periodic_table)/YGBcUabv">group</a> (column) have similar <a href="/facts/Chemical_property/uMVxKspS">chemical</a> and <a href="/facts/Physical_property/ujcMUceW">physical properties</a>, reflecting the <a href="/facts/Periodic_law/dI0mkSju">periodic law</a>. For example, the halogens lie in the second-to-last group (<a href="/facts/Halogen/YeFgWHeA">group 17</a>) 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 2022<a href="https://en.wikipedia.org/w/index.php?title=Period_(periodic_table)&action=edit">[update]</a>, a total of 118 elements have been discovered and confirmed. </p> <p>Modern <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum mechanics</a> explains these <a href="/facts/Periodic_trends/dI0mkSju">periodic trends</a> in properties in terms of <a href="/facts/Electron_shell/QxI30eh9">electron shells</a>. 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. </p><p>In the <a href="/facts/F-block/BbzD5jUU">f-block</a> and <a href="/facts/P-block/BbzD5jUU">p-block</a> of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However, in the <a href="/facts/D-block/BbzD5jUU">d-block</a>, trends across periods become significant, and in the <a href="/facts/F-block/BbzD5jUU">f-block</a> elements show a high degree of similarity across periods. </p>