Electronic Configurations
The ground state (the state of lowest energy) of H has a single electron is a 1s orbital, and we use the notation 1s¹ to describe the electronic configuration. With 2 electrons, He has the ground state configuration 1s². When we come to Li with 3 electrons, we must populate the 2s orbital and the ground state configuration is 1s²2s¹. Table 1.1 shows the ground state configurations for the first 30 elements. In the table, we have used the notation [He], [Ne], and [Ar] to represent the configurations of He, Ne, and Ar; e.g., [Ne] represents the configuration 1s²2s²2p⁶. This notation focuses our attention on the outermost electrons, the so-called valence electrons, which are important in chemical bonding.

Table 1.1. Electronic Configurations of Elements 1–30.