2h. Describing and explaining the ionic bonding of the ionic compounds sodium sulfide Na₂S and potassium sulfide K₂S

Example 2h. A Group 1 Alkali Metal combining with a Group 6 non–metallic element

* metals \ non-metals (zig-zag line)

Li is 2.1, Na is 2.8.1, S is 2.8.6 (for group 1 sulfide compound), rest of dot and cross diagrams are up to you.

e.g. electronic structure diagrams for sodium sulfide Na₂S and potassium sulfide K₂S

sodium sulfide

The electronic dot & cross diagram for the ionic bonding in the ionic compound sodium sulfide

The outer electrons of the sodium atoms (2.8.1) are transferred to the outer shell of the sulfur atom (2.8.6) until it has a complete octet shell of outer electrons, just like a noble gas (2.8.8). At the same time, the sodium ion also attains a stable noble gas electron structure (2.8).

is the Lewis diagram for the formation of sodium sulfide

potassium sulfide

The outer electrons of the potassium atoms (2.8.8.1) are transferred to the outer shell of the sulfur atom (2.8.6) until it has a complete octet shell of outer electrons, just like a noble gas (2.8.8). At the same time, the potassium ion also attains a stable noble gas electron structure (2.8.8).

The structures of solid sodium sulfide potassium sulfide consists of a giant ionic lattice.

Melting point of sodium sulfide is 1176^oC

The electronic dot & cross diagram for the ionic bonding in the ionic compound potassium sulfide

is the Lewis diagram for the formation of potassium sulfide

potassium sulfide

(Diagram adapted from "Structural Inorganic Chemistry" by A F Wells (Oxford University 1962).

8 of (K⁺) potassium ions in repeating cube

8 x 1/8 (S^2-) of sulfide ions at the eight corners of the cube = 1 S^2-

6 x 1/2 (S^2-) of sulfide ions six faces of the cube = 3 S2-

making a total of 4 S^2- ions in the repeating unit

Therefore the ratio is 2 K⁺ : 1 S^2- giving the empirical formula K₂S for potassium sulfide

Melting point of potassium sulfide is 840^oC

Extra comments on sodium sulfide and potassium sulfide

Solid sodium sulfide and potassium sulfide consist of a giant ionic lattice of an alternating arrangement of positive sodium/potassium ions and negative sulfide ions, which strongly attract each other in the solid crystal.

For both of sodium sulfide and potassium sulfide the melting points are high because of strong force of attraction between the alternating positive metal (Na⁺/K⁺) ions and negative sulfide (S^2-) ions in the giant ionic crystal lattice.

So the vibration kinetic energies of the particles of the giant ionic lattices of Na₂S and K₂S must be very high to overcome the lattice bonding forces to melt the sulfide (remember higher temperatures increase the KE of all particles).

(Note Group 6 = Group 16 at advanced A-level theoretical-inorganic chemistry)

Why would you expect lithium sulfide, rubidium sulfide and caesium sulfide to be ionic compounds with a similar formula to sodium sulfide and potassium sulfide?

Oxygen will behave electronically in the same way, and, since lithium, rubidium and caesium are also in group 1, they will all have one outer valance electron, which is easily lost to give the Li⁺, Rb⁺ and Cs⁺ ions giving Li₂S, Rb₂S and Cs₂S (see table below).

The electronic similarities between the two examples are very obvious.

Predicted formulae and names