2g. Describing and explaining the ionic bonding of the ionic compounds sodium oxide Na₂O and potassium oxide K₂O

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

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

e.g. sodium/potassium + oxygen ==> sodium/potassium oxide Na₂O/K₂O

or ionic formula (Na⁺)₂O^2–/(K⁺)₂O^2–

In terms of electron arrangement in the formation of the ionic compound sodium oxide, the two sodium/potassium atoms donate their outer electron to one oxygen atom.

This results in two single positive potassium ions 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).

Valencies, sodium/potassium 1, oxygen/sulfur 2. giving the following formulae:

Lithium oxide, Li₂O, sodium oxide Na₂O, sodium sulfide Na₂S and potassium K₂S etc.

2Na (2.8.1) + O (2.6) ==> 2Na⁺ (2.8.8) O^2– (2.8)

can be summarised electronically as 2[2,8,1] + [2,6] ==> [2,8]⁺₂ [2,8]^2–

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

TWO atoms combine withONE atom

 to form or

+ + ==>

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

The outer electrons of the sodium atoms (2.8.1) 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 sodium ion also attains a stable noble gas electron structure (2.8).

full electronic structure diagram of sodium oxide, the blue circle represents the nucleus.

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

is the Lewis dot and cross electron diagram for the formation of sodium oxide

Melting point of sodium oxide is 1132^oC

2K (2.8.8.1) + O (2.6) ==> 2K⁺ (2.8.8) O^2– (2.8)

can be summarised electronically as 2[2,8,8,1] + [2,6] ==> [2,8,8]⁺₂ [2,8]^2–

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

TWO atoms combine withONE atom to form

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

The outer electrons of the potassium atoms (2.8.8.1) 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 potassium ion also attains a stable noble gas electron structure (2.8.8).

full electronic structure diagram of potassium oxide,  the blue circle represents the nucleus.

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

is the Lewis dot and cross electron diagram for the formation of potassium oxide

Melting point of potassium oxide is 740^oC

Extra comments on sodium oxide and potassium oxide

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

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

So the vibration kinetic energies of the particles of the giant ionic lattices of Na₂O and K₂O must be very high 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 lithium oxide, rubidium oxide and caesium oxide to be ionic compounds with a similar formula to sodium oxide and potassium oxide?

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 Li₂O, Rb₂O and Cs₂O (see table below)

The electronic similarities between the two examples are very obvious.

Predicted formulae and names

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

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 2h. Details of ionic bonding in sodium sulfide & potassium sulfide (Group 1 + Group 6)