Boron is an exception to the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight valence electrons. However, boron has only three valence electrons, and it can achieve a stable configuration by having a total of six valence electrons. This behavior can be understood by considering the electron configuration and bonding patterns of boron.
In its ground state, the electron configuration of boron is 1s²2s²2p¹. Boron has an incomplete 2p orbital, and it is energetically favorable for it to form compounds where it can share or accept electrons to complete its valence shell. Boron commonly forms three covalent bonds in compounds, allowing it to achieve a stable electron configuration with six valence electrons.
One example is boron trifluoride (BF₃), where boron shares electrons with three fluorine atoms. In this molecule, boron achieves a stable electron configuration by forming three covalent bonds, with each bond sharing one electron from boron and one from a fluorine atom. The resulting molecule has a trigonal planar shape.
While boron can accept additional electrons to complete its octet, it is less common for it to do so. The reason for this is that accepting additional electrons would require a significant amount of energy due to the repulsion between the extra electrons and the electrons already present in the 2p orbital. As a result, boron typically forms compounds where it has fewer than eight valence electrons, exhibiting an exception to the octet rule.