FeO Structure, Characteristics: 5 Facts You Should Know

FeO is the chemical formula for ferrous oxide. It has the synonyms like iron (ii) oxide or oxoiron. Here we are discussing about the FeO structure, its characteristics and some facts.

FeO (ferrous oxide) or iron (ii) oxide is composed of two elements i.e. iron and oxygen atoms. In FeO structure the Fe ion is in +2 oxidation state. It is totally different from Fe2O3 ion in structure and characteristics. FeO is a mineral form which is known as Wustite. This compound is black in colour. It resembles the structure of NaCl.

The symmetry of the structure changes to rhombohedral and the FeO compound becomes antiferromagnetic. Hence the antiferromagnetic arrangement is in [1, 1, 1] direction necessitating the selection of cell which contains at least two different Fe atoms in this direction.

The rhombhohedral centred hexagonal site or cell is the lowest or smallest cell which fulfils this condition. FeO has the lattice in this condition has parameters a = b = c = 5. 260 and angles of α = β = γ = 33. 557. Thus, the iron atoms (Fe) are located at fractional coordinates of (0, 0, 0) and (0.5, 0.5, 0.5), and oxygen atom (O) are located at (0.25, 0.25, 0.25) and (0.75, 0.75, 0.75).

FeO 1
FeO structure in general form

Some facts about FeO structure

The FeO ferrous oxide or iron (ii) oxide is prepared by the iron (ii) oxalate thermal decomposition.

FeC2O4 → FeO + CO2 + CO

When the thermal decomposition of iron (ii) oxalate (FeC2O4) is done, iron (ii) oxide (FeO) can be formed with the liberation of carbon dioxide (CO2) and carbon mono- oxide (CO). The thermal decomposition reaction is done with inert condition to avoid the formation of ferric oxide (Fe2O3).

Here, Ferrous oxide or iron (ii) oxide cones under the group of non- stoichiometric compounds, which are especially deficient of iron compositions from the range of Fe0.84O to Fe0.95O. FeO (iron (ii) oxide) have the similar structure like NaCl (rock salt). In FeO crystalline structure, the FeO crystals are iron deficient and always show non- stoichiometric.

There are some vacant cationic sites and some site contains Fe3+ ions in it, but its combination makes the structure quite electrically neutral. The approximate formula of FeO is Fe0.95O. FeO molecule has the structure like rock salt and also has the density of 5.073 g/cm3.

What is FeO structure?

The FeO structure is like the rock salt (NaCl) structure. It is a non- stoichiometric structure as there are some vacant sites or holes in the structure or some of the Fe3+ ions are present in the structure in place of Fe2+ ions. Here, each iron (Fe2+) ion gets surrounded by six neighbouring oxygen (O2-) ions or each oxygen (O2-) ion gets surrounded by six neighbouring iron (Fe2+) ion i.e. vice versa.

FeO (iron (ii) oxide) has the cubic structure or face centred cubic (FCC) rock salt structure and each edge of FeO unit cell is 5.0 A0. Therefore there are four Fe2+ ions and four O2- ions are present in each edge of the unit cell of the crystal structure. In this structure, the iron atom is coordinated to oxygen atoms octahedrally and in similar way, the oxygen atoms get coordinated to iron atoms octahedrally (vice- versa).

The structure is non- stoichiometric as there is the comfort of oxidation of Fe2 to become Fe3 ion get disturbed by replacing small section of Fe2+ ions  with two 3rd number of Fe3+ ions which takes the tetrahedral position in closed packed lattice of FeO structure. In compare to the crystalline solid, in the melted state iron atoms are coordinated by four or five oxygen atoms predominantly. There is a quite small transformation in the structure below 200 kelvin temperature.

feo structure
FeO structure FCC (faced centred cubic structure)

Is FeO ionic or covalent?

Yes, FeO is polar covalent compound. In FeO structure, the iron atom has an electronegativity of 1.83 and the oxygen atom has electronegativity of 3.44. The difference between both electronegativities are 3.44 (O) – 1.83 (Fe) = 1.61. The nature of bond in FeO is polar covalent thus the difference is very near to that of ionic bonds. So iron (ii) oxide is a polar covalent compound with ionic characterizations.

Why FeO is ionic?

In FeO molecule, the iron (Fe) and oxygen (O) atoms are present and having 1.61 electronegativity difference. This electronegativity difference is below the given standard value 1.71. Due to low value as compared to 1.7 the FeO molecule is being polar covalent molecule.

If the electronegativity value is above 1.7 then any compound will be an ionic compound. But still there is a predominant ionic character in FeO molecule due to the presence or two oppositely charged ions i.e. Fe2+ and O2-.

How FeO is ionic?

According to Fuzan Rule, the compound with bigger cation in size and smaller anion in size favours the ionic characteristic while bond formation within cation and anion. Thus, FeO is quite more ionic but can show covalent character too.

Generally ionic bonds can formed between the two oppositely charged ions i.e. positive charge and negatively charged ions. Hence, ionic bonds are formed by the electrostatic attraction within oppositely charged ions. The anion is which gains electrons and the cation is which donates electrons.

Is FeO soluble in water?

FeO (iron (ii) oxide) or ferrous oxide is not soluble in water. The iron oxides which occurs naturally are mostly insoluble in water. Thus FeO ferrous oxide also insoluble in water as it is a naturally occurring compound. Also it is ionic in nature thus it cannot form ions in water when it reacts with water.

Why FeO is insoluble in water?

FeO is an ionic compound as it consists of two opposite charged ions. FeO molecule has Fe2+ ion (cation) and O2- ion (anion). Hence when FeO (ferrous oxide) when mixed or reacts with water it cannot form ions into water. Thus, FeO cannot dissociates or completely not dissociates in water. Therefore, FeO (iron (ii) oxide) is not soluble or insoluble in water.

How FeO is insoluble in water?

Generally iron in elemental form (elementary iron) is soluble in water at normal room condition. But according to solubility rule the oxides of iron is not soluble in water. The solubility of iron containing molecules or compounds are increases with the lower pH value.

The solubility of iron compounds can be increased with lower positive charge compounds. As the Fe2+ ion has lower charge than Fe3+ ion hence the Fe2+ containing compounds are quite soluble than other compounds but cannot dissociated in water thus they considered as insoluble compounds. Hence, FeO is insoluble in water.

Is FeO a solid?

Yes, FeO is a solid compound as it has a crystalline rock salt structure like NaCl. Even the FeO powder is a black solid powder form of compound. The crystal structure of FeO ion is non – stoichiometric.

Why FeO is solid?

FeO (ferrous oxide) is a solid compound as it has crystalline rock salt structure like NaCl. Also the crystalline structure of FeO has face centred cubic structure (FCC) in which each Fe2+ atom is surrounded by neighbouring O2- ions. Thus the ions of the structure are closely packed in a cube and hence it is a solid compound.

How FeO is solid?

In FeO crystal structure, all the Fe2+ and O2- ions are closely packed in face centred cubic structure. In any solid compound the atoms or ions are closely packed and have a particular shape. Thus FeO ion has face centred cubic shape and ions are closely packed. Thus it is a solid compound.


FeO is known as ferrous oxide or iron (ii) oxide. In FeO structure, the Fe ion is in +2 oxidation state and O atom is in -2 oxidation state. It is a non – stoichiometric structure as the FeO structure has some vacant places or some Fe2+ ions get replaced with Fe3+ ions. The FeO structure is face centred cubic structure. It has polar covalent bond but shows ionic characters. It is ionic, insoluble in water and a solid compound.

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