What is the coordination number of Na⁺ and Cl⁻ ions in NaCl lattice?
- (a)6, 1
- (b)1, 6
- (c)6, 6
- (d)5, 5
Correct — C, 6, 6. Sodium chloride crystallises in the rock-salt structure: the Cl⁻ ions form a face-centred cubic array and the Na⁺ ions occupy all the octahedral holes. Each Na⁺ ion is therefore surrounded by 6 nearest-neighbour Cl⁻ ions, and each Cl⁻ ion is likewise surrounded by 6 Na⁺ ions. The coordination number of both ions is 6, i.e. 6 : 6.
- (a)6, 1 — This wrongly makes the two ions unequal. In a 1:1 ionic solid like NaCl the two ions must share the same coordination number, so a 6 paired with a 1 is impossible.
- (b)1, 6 — The same error reversed — the ions cannot have coordination numbers of 1 and 6; NaCl is symmetric, with both ions six-coordinate.
- (d)5, 5 — Five is not the geometry of the rock-salt lattice; the octahedral arrangement around each ion gives six nearest neighbours, not five.
The coordination number of an ion in a crystal is the number of oppositely charged ions immediately touching it — its nearest neighbours. It is fixed by how the ions pack, which depends mainly on the ratio of the ionic radii. NaCl adopts the rock-salt structure, an octahedral 6:6 arrangement.
First use the 1:1 formula of NaCl: because there is one Na⁺ for every Cl⁻, the two ions must share the same coordination number, which eliminates the unequal options. Then recall the rock-salt geometry — each ion sits at the centre of an octahedron of six opposite ions — giving 6:6.
- In the rock-salt (NaCl) structure the Cl⁻ ions form a face-centred cubic lattice with Na⁺ in every octahedral void.
- Both Na⁺ and Cl⁻ are six-coordinate, so the structure is described as 6:6.
- The radius ratio of Na⁺ to Cl⁻ lies in the range that favours octahedral, six-fold coordination.
- By contrast CsCl is 8:8 and zinc blende (ZnS) is 4:4 — coordination changes with the radius ratio.
A 1:1 solid forces equal coordination; the octahedral rock-salt packing makes NaCl 6:6.
- Thinking the two ions can have different coordination numbers in a 1:1 salt.
- Confusing NaCl (6:6) with CsCl (8:8).
- Mixing up the coordination number with the number of ions in the unit cell.
Asked directly for the coordination number of NaCl (or CsCl/ZnS), or to match a structure with its coordination using the radius-ratio rule.
No directly related past PYQ was found.
- practice — not a real PYQ
In the CsCl structure, the coordination number of each ion is
- (a)4
- (b)6
- (c)8
- (d)12
Answer(c) 8 — CsCl has a body-centred-type 8:8 arrangement, each ion touching eight of the other kind.
- practice — not a real PYQ
The property that mainly decides the coordination number in an ionic crystal is
- (a)atomic mass
- (b)the radius ratio of the ions
- (c)colour
- (d)magnetic moment
Answer(b) the radius ratio of the ions — it fixes how many opposite ions can pack around a given ion.