Boron is the 5th element on the periodic table because it has 5 protons in its nucleus. With five electrons, boron is the first atom to contain electrons that are in a p-orbital. This is not a sphere-shaped harmonic, and so a single electron in a p-orbital cannot find a symmetrical arrangement around a sphere by itself. Boron therefore cannot simply add its p-electron on top of the same (2s2) configuration that beryllium has, as shown here, because it would not be stable:
The asymmetry therefore causes the two electrons in the 2s orbital to uncouple from their di-electron state and form a tri-electron state with the single p-electron. This is called hybridization. Three equal-energy (degenerate) electrons can now achieve maximum stability by assuming a trigonal planar (sp2-hybridized) arrangement around the core electron shell, because this minimizes their mutual repulsion by having them as far from one another as they can get. (The wireframe simply indicates the boundary of the n=2 shell, since there are no electrons defining the boundary of its sphere.)
The outer spheres above simply indicate the directions of maximum electron density. The orbitals themselves will be more like three equal longitudinal sections that can only occupy volume within their shell. The entire shell will be filled with electron density. It will be highest at the center of the face of each orbital, and will decrease toward the nodal regions that divide the orbitals, where electron density will be lowest.
Each of these three hybrid orbitals contains one electron. This arrangement is symmetrical in the equatorial plane (in 2 dimensions) but does not have equivalent symmetry in all directions. This makes boron keen to connect with more electrons, because that would give the atom a 4th direction and tetrahedral symmetry (like carbon). Four directions is more stable around a sphere than three directions. Boron will therefore either bond ionically by losing its 3 valence electrons and forming the spherically symmetrical B3+ ion, or it will seek to gain electrons through covalent bonding. This duality underscores boron’s metalloid (semi-metal) character.
Covalent bonding, where another atom donates both electrons for the bond, allows boron to achieve tetrahedral geometry, which is why it forms adducts — combination molecules like B2H6 — and molecular structures like BH3NH3. It is also why boron can participate in (and be doped into) both trigonal and tetrahedral carbon crystals.
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