Q.An element with molar mass 27 g/mol forms cubic unit cell with edge length of 405 pm. If density of the element is 2.7 g/cm³. What is the nature of cubic unit cell? (fcc or ccp)

A bcc cell packs 2 atoms with a packing fraction of about 0.68 (68%).

For a bcc cell the body diagonal = 4r, and body diagonal = a√3, so a = 4r/√3.

Atoms per cell = 2 (1 body centre + 8 corners x 1/8).

Packing fraction = (volume of 2 atoms)/(volume of cell) = [2 x (4/3)πr³] / a³. …

bcc packing efficiency = 68%, hence free space = 100 − 68 = 32%.

For a body-centred cubic lattice, atoms touch along the body diagonal, giving the relation 4r = √3 a (r = radius, a = edge). The unit cell has 2 atoms.

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A bcc unit cell has 2 atoms per cell and packing efficiency 68% (=√3π/8), so the remaining 32% of the cell's volume is empty space.

In a body-centred cubic (bcc) arrangement, atoms touch along the body diagonal, giving edge length a related to atomic radius r by a = 4r/√3. There are 2 atoms per unit cell (1 corner-contribution + 1 body-centre atom). Packing efficiency = (volume occupi …

In simple cubic (SC) packing the atoms touch along the cell edge and occupy only about half the available volume, giving 52.4% packing efficiency - the least efficient of the common cubic packings.

In a simple cubic lattice, atoms are present only at the corners of the cube, and each corner atom touches the atoms in the adjacent cells along the edge. So the edge length a = 2r, where r is the radius of the atom.

Each unit cell contains 8 corner atoms x 1/8 = 1 atom.

Volume of the atom = (4/3)pi r^3

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In a bcc lattice atoms touch along the body diagonal, and working through the geometry gives a packing efficiency of (√3π)/8 ≈ 68%.

In a body-centred cubic unit cell, atoms touch each other along the body diagonal (not along the edge).

Step 1 — relate radius r to edge length a:

Body diagonal = √3·a = 4r ⟹ r = (√3/4)·a

Step 2 — atoms per unit cell (Z):

Corner atoms: 8 × 1/8 = 1

Body-centre atom: 1 × 1 = 1

Z = 2

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Face centred cubic (fcc) has the highest packing efficiency (74%) of the three cubic lattices.

Packing efficiency is the fraction of total volume of a unit cell occupied by the constituent spheres (atoms), assuming they touch each other along the relevant direction.

• Simple cubic (sc): atoms touch along the cell edge; packing efficiency = 52.4%
• Body centred cubic (bcc): atoms touch along the body diagonal; packing efficiency = 68% …

In a bcc unit cell, atoms occupy 68% of the total volume.

In a body-centred cubic (bcc) arrangement, atoms touch each other along the body diagonal of the cube (not along the edge).

Step 1 — Relation between radius and edge length.

For a cube of edge $a$, the body diagonal length is $\sqrt{3}\,a$. Along this diagonal, one corner atom, the body-centre atom, and the opposite corner atom touch, so the diagonal spans $4r$ (radius $r$ of two half-atoms at the ends plus the full diameter of the centre atom):

$\sqrt{3}\,a = 4r \implies r = \frac{\sqrt{3}}{4}a$

Step 2 — Number of atoms per unit cell.

Z = 8 corners × 1/8 + 1 body-centre × 1 = 1 + 1 = 2 atoms.

Step 3 — Packing fraction. …

bcc packing efficiency = 68% → free space = 32%.

In a bcc unit cell there are 2 atoms and the body diagonal = 4r = √3·a. Packing efficiency = (2 × ⁴⁄₃πr³)/a³ …