2f. Describing and explaining the ionic bonding of the compounds magnesium sulfide MgS and calcium sulfide CaS

Example 2f. A Group 2 Alkaline Earth Metal combining with a Group 6 non–metallic element

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

Magnesium oxide MgO, magnesium sulfide MgS and calcium sulfide CaS will be similar electronically and give identical giant ionic lattice structures.

Group 2 metals lose the two outer electrons to give the stable 2+ positive ion (cation) and S and O, both non–metals in Group 6, have 6 outer electrons and gain 2 electrons to form 2– negative ion (anion).

For magnesium sulfide: Mg (2.8.2) + S (2.8.6) ==> Mg²⁺ (2.8) S^2– (2.8.8)

For calcium sulfide: Ca (2.8.8.2) + S (2.8.6) ==> Ca²⁺ (2.8.8) S^2– (2.8.8)

so both the magnesium/calcium and sulfide ions have a full outer shell like a noble gas

The dot and cross (ox) diagrams will be identical to that for calcium oxide above, except Mg instead of Ca (same group) and S instead of O (same group of Periodic Table). e.g.

The electronic dot & cross diagrams for the ionic bonding in magnesium sulfide and calcium sulfide

electronic structure of magnesium sulfide MgS

Lewis dot and cross diagram for magnesium sulfide

Melting point of magnesium sulfide is 2000^oC

electronic structure of calcium sulfide CaS

Lewis dot and cross diagram for calcium sulfide

Extra comments on magnesium sulfide and calcium sulfide

Solid magnesium sulfide and calcium sulfide consist of a 3D giant ionic lattice of alternating M²⁺ ions and O^2- ions..

For both of magnesium sulfide and calcium sulfide, the melting points are much higher then for e.g. sodium chloride 801^oC, because of the greater charges on the ions i.e. (2+) <=> (2-) versus the weaker (+) <=> (-) attraction.

So the vibration kinetic energies for the giant ionic lattices of MgS and CaS must be greater to overcome the lattice bonding forces to melt the oxide (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 beryllium sulfide, strontium sulfide and barium sulfide to be ionic compounds with a similar formula to magnesium sulfide and calcium sulfide?

Sulfur will behave electronically in the same way, and, since beryllium, strontium and barium are also in group 2, they will all have two outer valance electrons, which are lost to give the Be²⁺, Sr²⁺ and Ba²⁺ ions giving BeS, SrS and BaS (see table below)

See also Using the ionic bonding model to describe and explain the properties of ionic compounds

and for Advanced A-level students ONLY see thermodynamics sections

2.2l Data table of lattice enthalpies, ionic radii (nm) for group 1/2 metal halides, oxides and sulfides and more discussion of trends and comparing theoretical/experiment lattice enthalpies

2.2m Comparing melting points (K) and ionic radii (nm) for group 1 and group 2 halides, oxides and sulfides and discussion of melting point trends

 

Predicting formulae and names

Gp2\6	O	S
Mg	magnesium oxide MgO	magnesium sulfide MgS
Ca	calcium oxide CaO	calcium sulfide CaS
Sr	strontium oxide SrO	strontium sulfide SrS
Ba	barium oxide BaO	barium sulfide BaS

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

The Group 2 Alkaline Earth Metal atom loses two electrons to form a doubly 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

All of these compounds consist of a giant ionic lattice like sodium chloride.

See also 2e. Details of ionic bonding in magnesium oxide & calcium oxide (Group 2 + Group 6)