Boron (B) is an exception to the octet rule, which states that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration with eight valence electrons. However, boron only needs six valence electrons to achieve a stable electron configuration.
The reason for this exception lies in the atomic structure of boron. Boron is an element in Group 13 (or 3A) of the periodic table, and it has three valence electrons. Its electron configuration is 1s² 2s² 2p¹, with one electron in the 2p orbital. In order to achieve a stable configuration, boron can undergo electron loss or sharing to complete its valence shell.
When boron forms compounds, it commonly adopts an electron-deficient structure. It can achieve a stable configuration by sharing three electrons with other atoms or by losing three electrons to form a B³⁺ cation. In either case, boron ends up with a total of six valence electrons, rather than the standard eight.
The exception for boron arises due to its small atomic size and the relatively high energy required to fill its 2p orbital completely. By achieving a stable configuration with six electrons, boron attains a relatively stable arrangement that minimizes its energy.