Aluminum Oxide or Aluminium (III) Oxide is commonly known as alumina. In nature, it occurs as mineral corundum which is crystalline polymorphic phase α- Al2O3.
It is an amphoteric substance i.e. it can behave as an acid as well as a base. It is an ionic substance found as a diatomic molecule of aluminum.
In this article, we will study this molecule and its chemical bonding among its atoms with the help of lewis structure.
Apart from it, we will also check its geometry, polarity, and hybridization of Al2O3. So, let’s get started.
Lewis Structure of Al2O3
The concept of Lewis structure was first introduced by Gilbert N. Lewis in 1916. It is also known as the Lewis dot diagram or electron dot structure.
It is the structural illustration of the position of the valence electrons, involved in the formation of a chemical bond, around the atoms inside a molecule.
The dots in the proximity of the atoms represent their valence electrons. The Lewis structure depicts the best possible placement or arrangement of electrons.
Aluminum Oxide is an ionic compound i.e. the bonds between aluminum and oxygen atoms are formed through the transfer of electrons from one atom to another atom.
The Lewis structure for Aluminium Oxide (Al2O3) is represented as:
In the above structure, it can be seen that aluminum atoms become stable by donating three electrons each while oxygen atoms complete their octet by accepting two electrons each.
Steps to Draw Lewis Structure of Al2O3
Step 1: To draw the Lewis structure of the Aluminium Oxide molecule we first have to understand that the bond between the Oxygen atom and the Aluminium atom.
As aluminum is metal while oxygen is a non-metal therefore the bond between these two atoms is ionic and thus the Lewis structure for this compound, unlike covalent compounds, is illustrated with the help of both dots (i.e. electrons) as well as the charge on different atoms.
Step 2: Now let’s look have a look at the valence shell electrons present in this molecule.
Aluminum is a group 13 element and has three electrons in its valence shell while oxygen is a group 16 element and has six electrons in its valence shell.
Step 3: As aluminum oxide is an ionic compound the chemical bond is formed due to electron transfer.
The aluminum atoms are lesser electronegative and therefore, they donate the electrons to the more electronegative oxygen atoms.
Step 4: The oxygen atoms have six valence electrons and require two more electrons to complete their octet while the aluminum atoms have three electrons in their valence shell and tend to become stable by donating them.
Step 5: Therefore, the two aluminum atoms overall donate six electrons to the three oxygen atoms.
Step 6: After the electron transfer, the aluminum atoms acquire a +3 charge each (as they have donated three electrons) while the oxygen atoms acquire a -2 charge each (as they have accepted two electrons).
Step 7: The final Lewis structure of Al2O3 looks like this:
Al2O3 Molecular Geometry
The Valence Shell Electron Pair Repulsion (VSEPR) Theory, given by Sidgwick and Powell in 1940, predicts the shape of the molecules based on the number and arrangement of electrons around the central atom of the molecule.
This theory assumes that inside a molecule the electrons tend to remain as distant as possible from each other in order to avoid inter-electronic repulsion because if the electrons are kept closer the repulsion forces increase the overall energy of the molecule making it unstable.
The inter-electronic repulsion exists amongst the lone pairs as well as the electrons involved in chemical bond formation in a molecule however, the strength of repulsion is strongest between the two lone pairs and weakest between the two bond pairs.
The following steps shall be followed to predict the structure of a molecule based on the VSEPR theory:
• The least electronegative atom is chosen as the central atom. In the case of Al2O3, aluminum is the least electronegative, therefore, one Al atom is chosen as the central atom.
• The total number of valence electrons is counted for the central atom. In the case of Al2O3, aluminum has three valence electrons.
• The electrons from the other atoms involved in the formation of a bond with the central atom are also counted.
In the case of Al2O3, all the three valence electrons from the ionic bond with the three valence electrons of the oxygen atom.
Hence, the total number of valence electrons in the Al2O3 molecule becomes six.
• The Valence Shell Electron Pair (VSEP) number for Al2O3 = 3 (6 electrons)
• The VSEP number is used to predict the geometry of the molecule based on the following table:
Therefore, it is clear from the above table that as the VSEP number for Al2O3 is 3, the molecule should acquire a Trigonal Planar shape.
However, this shall be mentioned here that originally the aluminum oxide molecules occur in a 3-dimensional array.
Therefore, the general concepts related to molecular geometry are not applicable in this case.
Hybridization of Al2O3
The process of mixing two or more orbital of similar energy is known as hybridization.
The new hybrid orbital formed is different from the atomic orbital involved in their formation, both in shape as well as energy.
The theory of hybridization was given by Linus Pauling in 1931.
The names of the hybrid orbital are evolved from their elemental atomic orbital viz. dsp3 indicate that one d, one s, and 3 p-orbital are involved in the formation of this hybrid orbital.
The formula for calculating hybridization is given as:
Hybridization = Number of sigma (σ) bond on central atom + lone pair on the central atom
Determining the hybridization for Al2O3:
Number of sigma bond on central atom = 3
Number of lone pair on central atom = 0
Therefore, Hybridization = 3 + 0 = 3 i.e. sp2 hybridization
Therefore, it is clear that Al2O3 molecules have sp2 hybridization.
The sp2 hybridization is also known as trigonal hybridization.
The hybrid orbital formed by mixing of one s and two p-orbital forms trigonal symmetry and is maintained at an angle of 120°.
Polarity of Al2O3
A condition in which two opposite charges viz. positive and negative are located on the same molecule or atom is known as polarity.
In a molecule, polarity arises due to the difference in electronegativity of the two bonding atoms.
In the case of Al2O3, we have already discussed that it is an ionic molecule i.e. it is formed by the transfer of an electron from the aluminum atom to the oxygen atom.
The oxygen atom is more electronegative due to which it attracts the electron pair involved in the bond towards itself and hence, develops a negative charge.
On the other hand, aluminum being electropositive gives away its valence electrons to oxygen and develops a positive charge.
Thus, Al2O3 is a polar ionic molecule.
One such molecule is ClO3. Check out the article I wrote on the ClO3 Lewis Structure.
However, it should be noted here that as Al2O3 is a 3-dimensional array compound the general principle of polar and non-polar molecules is not applicable.
Aluminum Oxide is a white solid which is also known as aloxide, or alundum, or aloxite, as per its form or application.
Various properties of Aluminium Oxide are given in the table below:
|Molecular weight||101.96 g/mol|
|Melting Point||2.072 °C|
|Boiling Point||2.977 °C|
|Solubility in water||Insoluble|
|Thermal conductivity||30 Wm−1K−1|
• A few important uses of Aluminium Oxide are:
• It is used in refractories, polishing, ceramics, etc.
• As Al2O3 is an inert substance it is used as a filler for plastics.
• It is used in cosmetics such as sunscreen, blush, nail paints, etc.
• It is used in the manufacturing of aluminosilicate glasses.
• It is used as a catalyst in various industrial reactions.
• It is used in the manufacturing of paints, body armors, commercial fiber material, etc.
• It is used as an electrical insulator for internal circuits.
• Also, it is used for abrasion protection by anodizing.
• The Lewis structure for Aluminium Oxide (Al2O3) is represented as:
• The VSEP number for Al2O3 is 6, the molecule should therefore acquire a Trigonal Planar shape.
• The Al2O3 molecules have sp2 hybridization.
• The Al2O3 is a polar ionic molecule.
So, this was all about the lewis structure of Al2O3. Feel free to ask your doubts through comments.