Boron reacts with halogens to give the corresponding trihalides. Boron trichloride is, however, produced industrially by direct chlorination of boron oxide and carbon at 500 °C.
B2O3 + 3C + 3Cl2 → 2BCl3 + 3CO.
The carbothermic reaction is analogous to the Kroll process for the conversion of titanium dioxide to titanium tetrachloride. In the laboratory BF3 reacted with AlCl3 gives BCl3 via halogen exchange. BCl3 is a trigonal planar molecule like the other boron trihalides, and has a bond length of 175pm. A degree of π-bonding has been proposed to explain the short B− Cl distance although there is some debate as to its extent. It does not dimerize, although NMR studies of mixtures of boron trihalides shows the presence of mixed halides. The absences of dimerisation contrasts with the tendencies of AlCl3 and GaCl3, which form dimers or polymers with 4 or 6 coordinate metal centres.
Nitrogen and hydrogen are both non-metals. A nitrogen atom has 5 electrons in its outer shell. Nitrogen is in group 5 of the periodic table. A hydrogen atom has 1 electron in its outer shell. Hydrogen can only form 1 bond. Three hydrogen atoms each share their 1 electron with nitrogen to form three covalent bonds and make an ammonia molecule (NH3). By sharing the two electrons where the shells touch, each hydrogen atom can count 2 electrons in its outer shell and the nitrogen atom can count 8 electrons in its outer shell. These full outer shells with their shared electrons are now stable, and the NH3 molecule will not react further with other hydrogen or nitrogen atoms. Note the 3 pairs (6 electrons) shared between the atoms. Each electron pair is one bond. This is called a single covalent bond. Ammonia has three single covalent bonds. The structural formula of an ammonia molecule is written. There are no ions present (no + or - charges) in ammonia gas because the electrons are shared, not transferred from one atom to another.