MHT-CET Chemistry · Solid State

Types of Solids, Crystal Systems and Properties

A crystalline solid has long-range order, a sharp melting point and direction-dependent properties; an amorphous one has none of these. Crystalline solids sort into four bonding classes and their lattices into seven crystal systems with fourteen Bravais lattices.

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Why this matters

17 PYQs, all EASY or MODERATE — pure recall. The exam asks which statement about crystalline solids is NOT true (isotropy is the planted error), which listed solid is amorphous or isotropic, the class of ice or silica, the number of Bravais lattices or crystal systems, and which unit cell every system has. Learn the tables below and every one of these is a ten-second question.

Concept 1 of 3

Crystalline Versus Amorphous Solids

Intuition

Order is the whole difference. Particles arranged in a repeating pattern give a sharp melting point and properties that depend on direction (anisotropy); a random arrangement melts over a range and looks the same from every direction (isotropy). The exam plants 'crystalline solids are isotropic' as the false statement.

Definition

Crystalline: long-range order, sharp melting point, anisotropic (refractive index, conductivity differ with direction), definite heat of fusion, cleaves along planes. Diamond, NaCl, ice, graphite, ceramics, metals.
Amorphous (pseudo-solid, supercooled liquid): short-range order only, softens over a range, isotropic, irregular fracture. Glass, plastic, rubber, metallic glass.
Polymorphism: one substance in more than one crystalline form, made under different conditions and with DIFFERENT crystal shapes; in an element it is called allotropy (diamond and graphite).
Isomorphism: different substances with the same crystal structure and the same atomic ratio — $\text{NaNO}_3$ and $\text{CaCO}_3$ (1:1:3), $\text{NaF}$ and $\text{MgO}$ .

Property	Crystalline	Amorphous
Arrangement	Long-range order, regular and periodic	Short-range order only
Melting	Sharp, definite temperature	Softens over a range of temperature
Directional properties	Anisotropic — refractive index, conductivity vary with direction	Isotropic — the same in every direction 'Crystalline solids are isotropic' is the planted false statement.
Heat of fusion	Definite	Not definite
Examples	Diamond, NaCl, ice, graphite, ceramics, sodium	Glass, plastic, rubber, metallic glass Ice and ceramics are crystalline; only glass-like solids are amorphous.

Isotropy belongs to amorphous solids; anisotropy to crystalline ones.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Solid StateEASY

Which from the following statements is NOT true regarding crystalline solid?

[Q72 · 3rd May Shift 2 · 2023]

Reading 'NOT true' as 'true'

Three of the four statements are correct properties of crystalline solids; the one that is wrong is always the isotropy line. Mark the odd one, not the first true one.

Drill 6 more on crystalline versus amorphous solids

Concept 2 of 3

Ionic, Covalent Network, Molecular and Metallic Solids

Intuition

Ask what holds the particles together. Ions — ionic solid; a continuous web of covalent bonds — covalent network; whole molecules held by weak forces — molecular; cations in a sea of electrons — metallic. Ice is molecular (hydrogen-bonded water molecules), silica is covalent network.

Definition

Ionic: cations and anions, electrostatic attraction. Hard, brittle, high m.p., conduct only when molten or dissolved. NaCl, CaF $_2$ .
Covalent network: atoms joined by covalent bonds throughout. Very hard, very high m.p., insulators (graphite the exception). Diamond, silica $\text{SiO}_2$ , SiC, graphite.
Molecular: molecules held by dispersion forces (Ar, CH $_4$ ), dipole–dipole (HCl, SO $_2$ ) or hydrogen bonds (ice, solid NH $_3$ ). Soft, low m.p., insulators.
Metallic: metal cations in a sea of mobile electrons — the attraction is the metallic bond. Malleable, ductile, conductors. Cu, Fe, Na.

Class	Particles	Binding force	Examples
Ionic	Ions	Electrostatic	NaCl, KCl, CaF $_2$
Covalent network	Atoms	Covalent bonds in a network	Diamond, silica ( $\text{SiO}_2$ ), SiC, graphite Silica is covalent, not ionic — Si–O bonds run through the whole crystal.
Molecular	Molecules	Dispersion, dipole–dipole or hydrogen bonds	Ice, dry ice, solid Ar, I $_2$ , naphthalene Ice is a MOLECULAR solid: hydrogen bonds between H₂O molecules.
Metallic	Cations + mobile electrons	Metallic bond	Cu, Fe, Na, Ag

Classify by the force between the particles, not by the element's name.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Solid StateEASY

What type of solid is the silica?

[Q92 · 16th May Shift 1 · 2023]

Ice as an ionic or covalent solid

Water has covalent O–H bonds inside each molecule, but the SOLID is held by hydrogen bonds between molecules — molecular. The bond inside the particle never decides the class.

Drill 3 more on ionic, covalent network, molecular and metallic solids

Concept 3 of 3

Seven Crystal Systems, Fourteen Bravais Lattices

Intuition

A lattice is the repeating grid of points; the unit cell is its smallest repeating box. Seven cell shapes (by edge lengths and angles) combined with where the extra points sit (none, body, face, base) give exactly fourteen lattices — the simple (primitive) cell is the only one every system has, and triclinic has nothing else.

Definition

Seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, rhombohedral (trigonal).
Fourteen Bravais lattices: cubic 3 (simple, body-centred, face-centred) · tetragonal 2 (simple, body-centred) · orthorhombic 4 (simple, body-, face-, base-centred) · monoclinic 2 (simple, base-centred) · triclinic 1 · hexagonal 1 · rhombohedral 1.
The simple (primitive) cell, points at corners only, is common to all seven systems; triclinic has only that one.
Cubic cell: $a = b = c$ , $\alpha = \beta = \gamma = 90^\circ$ . Tetragonal: $a = b \neq c$ , all $90^\circ$ . Orthorhombic: $a \neq b \neq c$ , all $90^\circ$ .
Structure is found by X-ray diffraction (Bragg's law, an X-ray diffractometer); a scanning electron microscope images the SURFACE; a transmission electron microscope images the interior.

Bravais count

3 + 2 + 4 + 2 + 1 + 1 + 1 = 14 \text{ lattices in } 7 \text{ systems}

Worked example

How many Bravais lattices does the orthorhombic system have, and which one type of unit cell occurs in every crystal system?

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Solid StateEASY

What is the total number of Bravais lattices present for different crystal systems?

[Q86 · 10th May Shift 1 · 2024]

Swapping 7 and 14

Both numbers are offered as options in the same question. Systems are the SHAPES (7); Bravais lattices are shapes × centring (14).

Drill 5 more on seven crystal systems, fourteen bravais lattices

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Seven Crystal Systems, Fourteen Bravais Lattices
Bravais count
$3 + 2 + 4 + 2 + 1 + 1 + 1 = 14 \text{ lattices in } 7 \text{ systems}$

Reference tables (2)

Crystalline Versus Amorphous Solids5 rows

Property	Crystalline	Amorphous
Arrangement	Long-range order, regular and periodic	Short-range order only
Melting	Sharp, definite temperature	Softens over a range of temperature
Directional properties	Anisotropic — refractive index, conductivity vary with direction	Isotropic — the same in every direction 'Crystalline solids are isotropic' is the planted false statement.
Heat of fusion	Definite	Not definite
Examples	Diamond, NaCl, ice, graphite, ceramics, sodium	Glass, plastic, rubber, metallic glass Ice and ceramics are crystalline; only glass-like solids are amorphous.

Isotropy belongs to amorphous solids; anisotropy to crystalline ones.

Ionic, Covalent Network, Molecular and Metallic Solids4 rows

Class	Particles	Binding force	Examples
Ionic	Ions	Electrostatic	NaCl, KCl, CaF $_2$
Covalent network	Atoms	Covalent bonds in a network	Diamond, silica ( $\text{SiO}_2$ ), SiC, graphite Silica is covalent, not ionic — Si–O bonds run through the whole crystal.
Molecular	Molecules	Dispersion, dipole–dipole or hydrogen bonds	Ice, dry ice, solid Ar, I $_2$ , naphthalene Ice is a MOLECULAR solid: hydrogen bonds between H₂O molecules.
Metallic	Cations + mobile electrons	Metallic bond	Cu, Fe, Na, Ag

Classify by the force between the particles, not by the element's name.

Watch out for (3)

Reading 'NOT true' as 'true'
→ Crystalline Versus Amorphous Solids
Ice as an ionic or covalent solid
→ Ionic, Covalent Network, Molecular and Metallic Solids
Swapping 7 and 14
→ Seven Crystal Systems, Fourteen Bravais Lattices

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