Question

Question #704b6

Answer 1

The formula unit for boron carbide is `B_4C` because it better describes the crystal structure formed by the compound. As it turns out, `B_4C` is an idealized view of what the structure actually looks like.

Boron carbide forms a very complex crystal structure that can best be described as having a rhombohedral lattice unit that consists of `B_12` icosahedra and a three-atom chain that runs along the diagonal of each unit cell.

Depending on what combination of boron and carbon atoms make up that three-atom chain, you can have different formula units for boron carbide. This three-atom chain is usually believed to consists of two carbon atoms that flank a boron atom.

As a consequesnce of this arrangement of atoms, the formula unit could be given as `B_13C_2`, which implies that the compound is carbon-deficient.

If you get two boron atoms flanking a carbon atom in the three-atom chain, you'll see the formula unit given as `B_14C`, which implies that the compound is boron-rich.

A more conservative approach is to actually use `B_12C_3` as a sort of "average" formula unit for boron carbide, which is equivalent to the more common `B_4C` formula unit.

So, as a conclusion, the formula unit for boron carbide is usually given as `B_4C` because it's an attempt at describing how the actual crystal structure looks like. `B_4C_3` is not even close to matching experimental data on how boron carbide's lattice unit looks like.