5s2

5s2 refers to a specific electron configuration found in atomic physics. It denotes that the outermost electron shell, designated by the principal quantum number 5, contains two electrons. These electrons occupy the second subshell, which is the s subshell. The s subshell can hold a maximum of two electrons, meaning it is completely filled in this configuration. Elements with this outermost electron configuration are located in Group 2 of the periodic table, known as the alkaline earth metals. These elements include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). Their chemical properties are largely determined by these two valence electrons, which they tend to lose easily to form +2 ions, contributing to their reactivity. The 5s2 configuration specifically applies to the fifth period of the periodic table for elements within Group 2. For instance, Strontium (Sr) has an electron configuration that ends with 5s2.