How many lattice points are there in one unit cell of each of the following lattice?
(a) face centred cubic
(b) face centred tetragonal
(c) body centred cubic
A face-centred cubic (fcc) unit cell contains atoms at all the corners and at the centre of all the faces of the cube.each atom located at the face-centre is shared between two adjacent unit cells and only 1/2 of each atom belongs to a unit cell
Thus, in a face-centred cubic
(fcc) unit cell:
(i) 8 corners atoms × 1/8 atom per unit cell = 8×1/8 = 1 atom
(ii) 6 face-centred atoms ×1/2 atom per unit cell = 6 ×1/2 = 3 atoms
∴ Total number of atoms per unit cell = 4 atoms
body centre wholly belongs to the unit cell in which it is present. Thus
in a body-centered cubic (bcc) unit cell:
(i) 8 corners × 1/8 per corner atom= 8×1/8 = 1 atom
(ii) 1 body centre atom = 1 × 1 = 1 atom
∴ Total number of atoms per unit cell =1+1 = 2
(c) z = 2.
It changes into paramagnetic at hight temperature due to randomization of spins.
(b) The ions in MgO carry two unit charges. In NaCl only one unit charge. Hence electrostatic forces of attraction in MgO are stronger.
Paramagnetic substances are attracted by the magnetic field. This is due to the presence of unpaired electrons in the atoms/ions/molecules of the paramagnetic substance. Paramagnetism is a temporary effect.
Ferromagnetic substances are strongly attracted by the magnetic field. They show magnetism even when the magnetic field is removed. This is due to a spontaneous alignment of the magnetic dipole in the same direction.
Transition metal compounds contains one or more unpaired electrons. So, these compounds show para magnetism. The paramagnetism of the bivalent ions of the first transition metal series first increases in going from Ti2+ to Mn2+ (number of unpaired electrons increases) and then decreases upto Cu2+ (because the number of unpaired electrons decreases).
Transition metal compounds contains one or more unpaired electrons. So, these compounds show para magnetism. The paramagnetism of the bivalent ions of the first transition metal series first increases in going from Ti2+ to Mn2+ (number of unpaired electrons increases) and then decreases upto Cu2+ (because the number of unpaired electrons decreases).