What is the couse of abnormal behavior of boron

Boron has only three valence electrons and a vacant 2p orbital. This makes it electron-deficient and prone to forming compounds where it acts as a Lewis acid, readily accepting electron pairs.

Its small size and high charge density allow it to form strong covalent bonds, but it struggles to accommodate more than four electron pairs around itself, limiting its coordination number.

Unlike other Group 13 elements that typically form three covalent bonds and have a +3 oxidation state, boron often forms compounds with bridging hydrogen atoms (boranes) or exhibits unusual bonding in clusters.

This electron deficiency and tendency to form diverse structures are key to its "abnormal" behavior compared to its heavier congeners.