Atoms at corners and one in the center, coordination number 8 and packing efficiency 68 percent.
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The body-centred cubic structure has atoms at the eight corners of a cube and one atom at the centre, giving two atoms per unit cell. The atoms touch along the body diagonal of the cube, so 4r = √3 a, and each atom has a coordination number of eight. Its packing efficiency is √3π/8, about 68 percent. Many metals, including the alkali metals and iron at room temperature, adopt this arrangement. The structure is less densely packed than face-centred cubic and hexagonal close packing.
Key ideas: Relationships: body diagonal = sqrt(3) a = 4r, so a = 4r / sqrt(3) = 2.309 r. Z = 2, packing = 0.680, coordination 8 with 6 more second neighbours at slightly greater distance. Density = 2M / (NA a^3). Iron converts from bcc to fcc near 912 C.
Common mistakes and connections: Students apply the face-diagonal relationship to bcc, which is the fcc rule. Another error is thinking the corner atoms touch each other; they touch the central atom instead. Metals with bcc are generally harder but less ductile than fcc metals. Links to allotropes of iron, steel and the unit cell counting method.
BCC: two atoms per unit cell, coordination number 8 and 68% packing efficiency, found in Fe (alpha), Na, K and Cr. The interactive lesson shows this as a live animation or chart.
Sodium, potassium, chromium and tungsten all crystallize in the body-centred cubic structure.
Atoms at corners and one in the center, coordination number 8 and packing efficiency 68 percent.
Body-Centered Cubic is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in States of Matter & Solutions first: Amorphous Solids, Unit Cell, Simple Cubic Lattice.
The next topics in States of Matter & Solutions are Face-Centered Cubic and Hexagonal Close Packing.
Yes. The interactive lesson includes a self-check question on Body-Centered Cubic, and the States of Matter & Solutions quiz draws on every topic in this area.
Body-Centered Cubic is topic 59 of 109 in States of Matter & Solutions.