2e. Describing and explaining the ionic bonding of the ionic compounds magnesium oxide MgO and calcium oxide CaO

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

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

e.g. magnesium/calcium + oxygen ==> magnesium/calcium oxide MgO/CaO

or ionic formula Mg²⁺O^2–/Ca²⁺O^2–

In terms of electron arrangement in the formation of the ionic compound magnesium oxide, one magnesium/calcium atom donates its two outer electrons to one oxygen atom.

This results in a double positive calcium ion to one double negative oxide ion via electron transfer.

All the ions have the stable electronic structures 2.8.8 (argon like) or 2.8 (neon like). the valency of both calcium and oxygen is 2.

ONE atom combines withONE atom

 to form

Note in this electron diagram, only the original outer electrons are shown above.

The two outer electrons of the magnesium atoms (2.8.2) are transferred to the outer shell of the oxygen atom (2.6) until it has a complete octet shell of outer electrons, just like a noble gas (2.8). At the same time, the magnesium ion also attains a stable noble gas electron structure (2.8).

full electronic structure of magnesium oxide

For magnesium oxide: Mg (2.8.2) + O (2.6) ==> Mg²⁺ (2.8) O^2– (2.8)

the stable 'noble gas' structures can be summarised electronically as [2,8] + [2,6] ==> [2,8]²⁺ [2,8]^2–

so both the magnesium and oxide ions have a full outer shell like a noble gas

The electronic dot & cross diagram for the ionic bonding in the ionic compound magnesium oxide

is the Lewis diagram for the formation of magnesium oxide

Melting point of magnesium oxide is 2852^oC

Ca (2.8.8.2) + O (2.6) ==> Ca²⁺ (2.8.8) O^2– (2.8)

can be summarised electronically as [2,8,8,2] + [2,6] ==> [2,8,8]²⁺ [2,8]^2–

ONE atom combines withONE atom

to form

Note in this electron diagram, only the original outer electrons are shown above.

The two outer electrons of the calcium atoms (2.8.8.2) are transferred to the outer shell of the oxygen atom (2.6) until it has a complete octet shell of outer electrons, just like a noble gas (2.8). At the same time, the calcium ion also attains a stable noble gas electron structure (2.8.8).

full electronic structure of calcium oxide

The electronic dot & cross for the ionic bonding in the ionic compound calcium oxide

 is the Lewis diagram for the formation of calcium oxide

Melting point of calcium oxide is 2572^oC

All of these solid oxides and sulfides consist of a giant ionic lattice.

Extra comments on magnesium oxide and calcium oxide

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 oxide and calcium oxide, 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 MgO and CaO 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 oxide, strontium oxide and barium oxide to be ionic compounds with a similar formula to magnesium oxide and calcium oxide?

Oxygen 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 BeO, SrO and BaO (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

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

KCl, MgO, CaO, have the same NaCl crystal structure

All of these solid oxides and sulfides consist of a giant ionic lattice.

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