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2h. The ionic bonding of the compounds sodium sulfide and potassium sulfide
Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes
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
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
The outer electrons of the potassium atoms (188.8.131.52) 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).
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
(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.
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
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