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

[Author © Dr Phil Brown PhD: Doc Brown's chemistry exam revision notes on the ionic bonds of sodium sulfide & potassium sulfide, suitable for students of UK GCSE Science level AQA, Edexcel, OCR, WJEC and CCEA GCSE chemistry courses, ~US grades 9-10 chemistry, also useful for more advanced pre-university A level chemistry courses [ionic bonding page updated April 11th 2026 *]

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Part 2 Ionic Bonding: compounds and properties

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

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

Pd metals Part of the modern Periodic Table

Pd = period, Gp = group

 metals => non–metals
Gp1 Gp2 Gp3 Gp4 Gp5 Gp6 Gp7 Gp0
1

1H  Note that H does not readily fit into any group

 2He
2 3Li 4Be atomic number Chemical Symbol eg 4Be 5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba Transition Metals 81Tl 82Pb 83Bi 84Po 85At 86Rn
The ionic compounds SODIUM SULFIDE and POTASSIUM SULFIDE

Where the elements sodium, potassium and sulfur are in the Periodic Table

 

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 1176oC

 

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

unit cell of potassium sulfide K2S gcse chemistry 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 (S2-) of sulfide ions at the eight corners of the cube = 1 S2-

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

making a total of 4 S2- ions in the repeating unit

Therefore the ratio is 2 K+ : 1 S2- giving the empirical formula K2S for potassium sulfide

Melting point of potassium sulfide is 840oC

Apart from caesium, all the group 1 oxides and sulfides e.g. Na2O can reuse diagram

The electronic similarities between the two examples are very obvious.

Pd metals Part of the modern Periodic Table

Pd = period, Gp = group

 metals => non–metals
Gp1 Gp2 Gp3 Gp4 Gp5 Gp6 Gp7 Gp0
1

1H  Note that H does not readily fit into any group

 2He
2 3Li 4Be atomic number Chemical Symbol eg 4Be 5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba Transition Metals 81Tl 82Pb 83Bi 84Po 85At 86Rn
A Group 1 Alkali Metal combining with a Group 6 non–metallic element

Predicted formulae and names

Gp1\6 O S
Li lithium oxide

Li2O

 lithium sulfide

Li2S
Na sodium oxide

Na2O

 sodium sulfide

Na2S
K potassium oxide

K2O

 potassium sulfide

K2S
Rb rubidium oxide

Rb2O

 rubidium sulfide

Rb2S
Cs caesium oxide

Cs2O

 caesium sulfide

Cs2S

All the formula highlighted in yellow can be described in the same way as sodium oxide, potassium oxide, sodium sulfide or calcium sulfide

The Group 1 Alkali Metal atom loses one electron to form a singly charged positive ion

The Group 6 non–metal atom gains two electrons to form a doubly charged negative ion

Note that these formulae are empirical formula - the simplest whole number ratio of atoms in the formulae

The structures of these solid oxides and sulfides consists of a giant ionic lattice.

See also 2g. Details of ionic bonding in sodium oxide & potassium oxide (Group 1 + Group 6)

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Ionic Bonding: compounds and properties

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