Iron(III) oxide or ferric oxide is an inorganic compound with the molecular formula Fe2O3. Fe2O3 has a molecular mass of 159.68 g/mol and is often known as rust, having a reddish-brown color.
In this article, we will understand the bonding between the atoms of Fe2O3 molecules. We know that iron is a transition metal and therefore it has a variable (more than one) oxidation state. Iron can exhibit +2 or +3 oxidation states.
Is Fe2O3 ionic or covalent? Fe2O3 is an ionic compound. The electronegativity difference between Iron and oxygen atoms is 1.6 corresponds to that of a polar covalent bond. Thus Fe2O3 has polar covalent bonds and the compound has ionic character. Hence Fe2O3 is an ionic compound
In the case of Fe2O3, Fe exists as Fe2+. The type of bond formed between iron and oxygen is dependent on the electronegativity difference between the two atoms.
Ionic vs. Covalent Compounds
A compound with ionic bonding is one where there is a large electronegativity difference between the atoms of the compounds.
The difference in electronegativities between the atoms results in the loss of electrons by the less electronegative atom and the gain of electrons by the more electronegative atom.
This causes the formation of two oppositely charged ions.
Ionic bonding occurs between a metal, which can lose electrons forming a cation (positively charged), and a non-metal which can gain electrons and form an anion (negatively charged).
These oppositely charged ions experience electrostatic attraction and therefore constitute the ionic cloud. Examples of ionic compounds: NaCl, NH4Cl, etc.
In an ionic bond, there is the transfer of valence electrons thus the sharing of electrons between the two atoms is very unequal, hence the bond has an ionic character.
Less equal the sharing of electrons greater is the ionic character of the bond.
Covalent bonding is one where the atoms of the molecule share a pair of electrons fairly equally. This type of bonding is usually observed between two nonmetals or a nonmetal and a metalloid.
The covalent bond is formed between atoms of similar or same electronegativities, as the difference in electronegativities is very little, there is no transfer of electrons, rather the electrons are shared between the atoms unlike in ionic bond.
Examples of covalent compounds: H2, SO3, etc.
Check out the article is SO3 ionic or covalent.
The bonding in covalent compounds can be of 2 types, polar and nonpolar.
Polar covalent compounds share the pair of electrons unequally and therefore have ionic character. The compounds with polar covalent bonds have partial positive and negative charges on the atoms.
But in the case of nonpolar covalent compounds, the electron pair is shared equally and therefore does not have an ionic character.
The following table explains the type of bonding in a compound:
|Difference in electronegativity of 2 atoms
|Type of bonding
|Nonpolar Covalent Bond
|0.4 to 1.7
|Polar Covalent Bond
Ionic bonds are very strong bonds and therefore have high melting and boiling points whereas covalent bonds are weak and have low melting and boiling points.
As we know “like dissolves like”, thus ionic compounds are soluble only in polar solvents whereas covalent compounds dissolve in nonpolar solvents.
Ionic compounds have high electrical conductivity due to the presence of free ions in an aqueous or molten state, whereas covalent compounds have low conductivity as they cannot dissociate into ions.
How is Fe2O3 a Covalent Compound?
Iron(III) oxide has Fe, which belongs to the transition metals, and O which belongs to the non-metals in the periodic table.
According to the definition of ionic compounds, Fe2O3is an ionic compound as the ionic bond is formed between a metal and a nonmetal. And as there is a bond formed between Fe, a metal, and O, a nonmetal, in Fe2O3, hence an ionic compound.
But if we compare the electronegativities of the atoms of Fe2O3the compound is supposed to be covalent in nature.
Iron has an electronegativity of 1.83 and oxygen 3.44. The difference in electronegativities between the two atoms is 3.44 – 1.83 = 1.61.
As we discussed earlier, if the electronegativity difference between the atoms is less than 1.7, then the compound has polar covalent bonding. Hence Fe2O3 has polar covalent bonding.
Hence we can say iron oxide is a polar covalent compound with an ionic character.
Conditions for the Formation of Covalent Bond
The several factors that affect the formation of covalent bonds are described as follows:
High Ionization Potential
For the formation of covalent bonds, the atom should have high ionization potential.
We know that ionization potential is directly proportional to the energy required for the formation of ions.
Greater will be the ionization potential, the greater amount of energy is required for the formation of ions, and hence it is difficult to obtain ions.
Due to the unavailability of ions, the bond between the two atoms is formed by the sharing of a pair of electrons, which is a covalent bond. Hence, the higher ionization potential of the atoms favors the formation of covalent bonds.
Comparable Electron Affinities
For covalent bonding to occur, both the atoms should have an almost equal affinity for electrons.
The electron affinity should be high for both the atoms so that there is a fairly equal sharing of electrons while the formation of a bond.
The low electronegativity difference between the two atoms favors the formation of covalent bonds.
A higher difference in electronegativity indicated the presence of ionic bonds as the transfer of electrons occurs instead of sharing electron pairs.
We have discussed above that the electronegativity difference between the two atoms less than 1.7 results in the formation of a covalent bond which may be nonpolar if the difference is less than 0.4 and polar covalent if it is between 0.4 to 1.7.
No. of Valence Electrons
The covalent bond is easily formed between atoms having 5, 6, or 7 electrons in their valence by sharing 3, 2, and 1 electron pairs respectively to achieve a stable octet.
High Nuclear Charge and Small Internuclear Distance
During the formation of a covalent bond, the electron density gets concentrated between the nuclei of the two atoms that combine.
This electron charge is responsible for holding the two nuclei together. Therefore, a higher nuclear charge and a smaller internuclear distance favor the formation of a covalent bond.
Polarity of Fe2O3
Fe2O3 has 2 iron atoms bonded to 3 oxygen atoms and Fe has an oxidation state of +3 in this molecule. The structure of Fe2O3is given as below:
The three oxygen atoms have 2 different types of hybridization, the two-terminal O atoms are sp2 hybridized (one sigma bond and 2 lone pairs of electrons) whereas the central O atom is sp3 hybridized (two sigma bonds and 2 lone pairs of electrons).
The difference in hybridization results in unequal O-Fe-O and Fe-O-Fe bonds angles.
The shared pair of electrons between the Fe and O are more attracted towards the O atom as it has a higher electronegativity than Fe. Therefore the dipole vectors are towards the oxygen atoms.
Due to different bond angles, the dipole moment vectors do not cancel and have some value of net dipole moment. Therefore the molecule is polar in nature. Hence we say that Fe2O3is a polar compound.
Chemical Properties of Iron Oxide – Fe2O3
1. Iron (III) oxide reacts with water to produce iron hydroxide.
Fe2O3 + H2O → Fe(OH)3
2. Iron(III) oxide reacts with sulphuric acid to produce iron(III) sulfate and water. Therefore, we can say that iron oxide has a basic nature as it neutralizes the acid.
Fe2O3 + H2SO4 → Fe2(SO4)3 + H2O
3. Iron(III) oxide reacts with sodium hydroxide to produce a sodium salt having molecular formula NaFeO2 and water.
Therefore, the neutralization of NaOH indicates that iron oxide is acidic in nature.
Fe2O3 + NaOH → NaFeO2 + H2O
The above two reactions (reaction 2 and reaction 3) show that iron oxide is amphoteric in nature, it acts as a base in an acidic medium while in a basic or alkaline medium, it acts as an acid.
Related topics you must read
Iron (III) oxide or Fe2O3 consists of iron which is a metal and oxygen which is a non-metal, therefore the bond is ionic in nature. But on comparing the electronegativities of the two atoms we find that the compound has a polar covalent bonding as the difference in the electronegativities is less than 1.7, which corresponds to polar covalent bonds.
The ionic bond is one in which the transfer of electrons takes place resulting in the formation of cations and anions whereas a covalent bond is formed by the sharing of electron pairs between the two atoms. Covalent bonds can be polar or nonpolar depending on the difference in the electronegativities.
Fe2O3 is found to be polar in nature as the terminal and central oxygen atoms have different hybridization. Due to different hybridization, the bond angles are not equal and there is some dipole moment, making the compound polar in nature. The polar nature of bonds makes the compound ionic in nature.
Therefore at the end of this article, we can conclude that Fe2O3is a polar covalent compound having an ionic character.