body-centered cubic
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Picture a cube with a ball at each of its eight corners, and then drop one more ball right into the middle of the box, floating dead-center. That central ball is the body of the cube, sitting in its core. A crystal built by repeating this little cube in all directions has the body-centered cubic structure.
Body-centered cubic, or bcc, is a cubic structure with atoms at the eight corners plus one extra atom at the very center of the cube. Counting carefully — corner atoms are each shared among eight neighbouring cubes — a single cube effectively contains two atoms. It is not a close-packed structure: each atom touches eight nearest neighbours rather than twelve, and the atoms fill only about 68 percent of space, a bit less crowded than the close-packed structures. The center atom and the corner atoms are in identical surroundings, so the points still form a proper Bravais lattice.
Bcc matters because iron at room temperature is bcc, and so are the structural metals chromium, tungsten, and the alkali metals like sodium. The slightly looser, less slippery packing tends to make bcc metals harder and less easily shaped than close-packed ones. A common confusion is to think the central atom is somehow a different kind of point — it is not; in a pure bcc element it is an identical atom in an identical environment, just located at the cube's heart.
Ordinary iron is bcc, but heat it past about 912 degrees Celsius and it rearranges into fcc. Steelmakers exploit this very switch: heating, holding, and quenching iron between these structures is how they tune hardness.
Iron switches between bcc and fcc with temperature — the basis of steelmaking.
Body-centered cubic packs atoms less densely than face-centered cubic (about 68 versus 74 percent), but denser is not always better — bcc tungsten is among the hardest, highest-melting metals we have.