There are currently seven complete periods in the periodic table, comprising the 118 known elements. However, in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods. 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 (vertical trends down groups are more significant).

The filling of each shell corresponds to a row in the table. As atomic number increases, shells fill with electrons in approximately the order shown in the ordering rule diagram. Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. Each diagonal corresponds to a different value of n + l. The Madelung energy ordering rule describes the order in which orbitals are arranged by increasing energy according to the Madelung rule. As of 2022, a total of 118 elements have been discovered and confirmed. For example, the halogens lie in the second-to-last group ( group 17) and share similar properties, such as high reactivity and the tendency to gain one electron to arrive at a noble-gas electronic configuration.

Arranged this way, elements in the same group (column) have similar chemical and physical properties, reflecting the periodic law. Each next element in a period has one more proton and is less metallic than its predecessor. All elements in a row have the same number of electron shells. Method of visualizing the relationship between elements In the periodic table of the elements, each numbered row is a period.Ī period on the periodic table is a row of chemical elements.

Chemistry definition of periodic table