In the present article we are going to discuss about the fascinating structure of Aluminium Hydride . AlH3 is among those molecules which do not follow the octet rule.
The AlH3 Lewis structure consist of Aluminium atom as the central atom and three Hydrogen atoms are present around the aluminium atom. The Lewis structure of AlH3 molecule contains 6 bonding electrons and no non-bonding electrons are present on it.
AlH3 Lewis StructureTable: Details of the AlH3 Structure
| Molecular NameAluminium HydrideChemical FormulaAlH3Central atom of the lewis structureAluminiumNumber of lone pairs on central atom 0Molecular Geometry of AlH3Trigonal PlanarElectron Geometry of AlH3Trigonal PlanarBond Angle H-Al-H120 degreesNo. of valence electron for AlH3 | 6 |
| The formal charge of AlH3 molecule | 0 |
| Hybridization of AlH3 molecule | Sp2 |
How to draw Lewis dot structure for AlH3?
Following are the steps to follow to draw the Lewis structure of AlH3 molecule –
Step 1: Count total valence electrons present in AlH3 molecule
First step is to find the no. of valence electrons present in AlH3 molecule.
Aluminium belongs to group 13th and Hydrogen belongs to group 1st of the periodic table.
So,
Valence electron of Aluminium =3
Valence electron of Hydrogen = 1
| Valence Electrons | Atom In AlH3 | Total Valence Electrons | |
| Al | 3 | 1 | 3*1 =3 |
| H | 1 | 3 | 1*3 =3 |
| 6 |
Step 2: Find central atom in AlH3
Always keep the least electronegative atom in the center. When comparing aluminium and hydrogen,hydrogen is less electronegative atom than aluminium But in accordance with the rule, we will have to hydrogen outside. Now draw the skeletal structure of AlH3 molecule
skeletal structure of AlH3Step 3: Put two electrons or a bond between Al and H atom
Bonds between Al and H atomsStep 4: Complete octet of the Outer atoms (or duplet) in case of hydrogen atom.
Main group elements have tendency to attain octet configuration of their nearest noble gas element present in the 18 group. This is known as Octet Rule. Except hydrogen which completes duplet configuration like He
Complete the Duplet on outside hydrogen atoms then move on to central atom.
Now, all the six electrons of AlH3 are used in the structure and no more electron pairs remained.
Hence no change in the above Lewis Dot structure of AlH3
Lewis dot structure Of AlH3 Step 5: Check formal charge value
Before we confirm that this is a perfect Lewis structure, we need to check the formal charge values
Formal charge = Valence electrons – (1/2) * bonding electrons – non-bonding electrons
Valence electrons of Aluminium = 3
Non-bonding electrons of Aluminium = 0
Bonding electrons of Oxygen = 6
Formal charge for Aluminium atom = 3 – 1/2*6 –0 = 0
Valence electrons of hydrogen = 1
Non-bonding electrons of hydrogen = 0
Bonding electrons of hydrogen = 2
Formal charge for hydrogen atom = 1 – 1/2*2 – 0 = 0
As the Al and H elements have least possible formal charges, we have obtained suitable Lewis structure
AlH3 Lewis structure
What is the formal charge in AlH3 and how it is calculated?
Formal charge is the charge present on an atom in a molecule when all the bonding electrons are shared equally or when the electronegativity difference of atoms is ignored.
Formal charge is calculated by using the formula –
Formal charge = Valence electrons – (1/2) * bonding electrons – non-bonding electrons
Valence electrons of Aluminium = 3
Non-bonding electrons of Aluminium = 0
Bonding electrons of Aluminium =6
Formal charge for Aluminium atom = 3 – 1/2*6 – 0 = 0
Valence electrons of hydrogen = 1
Non-bonding electrons of Hydrogen = 0
Bonding electrons of hydrogen = 2
Formal charge for hydrogen atom =1 – 1/2*2 – 0 = 0
Thus, formal charge on Aluminium and hydrogen atom in AlH3 Lewis dot structure is zero.
Does AlH3 molecule follow the octet rule?
AlH3 has only 6 valence electrons as can be seen from the Lewis structure of AlH3 molecule. This proves that octet rule is not followed by the AlH3 molecule.
What is the molecular geometry and electron geometry of AlH3 molecule?
In order to find the geometry (molecular/electron) of AlH3 molecule, the three steps have to be followed. They are –
1)Find the number of lone pairs present on the central atom in the Lewis structure of AlH3 molecule
Lone Pair =1/2*(V.E. –N.A.)
Where –
V.E.= valence electron on the central atom
N.A.= Number of atoms attached to that central atom
In AlH3 molecule, the valence electrons of central Aluminium atom is 3 and three outer atoms are attached to it.
Lone Pairs = 1/2* (3 – 3) = 0
2) Find the hybridization number of AlH3 molecule
Hybridization of central atom can be found out by using the formula.
Hybridization number = N.A. + L.P
Where N.A.= number of atoms attached to the central atom
L.P. = number of lone pairs on the central atom
There are 3 atoms attached to the central atom Aluminium and there are no lone pairs present on it.
Hybridization number = 3 + 0 = 3
Hybridization number of AlH3 molecule is 3
hence hybridization of AlH3 molecule is Sp2
3) Use VSEPR Theory to determine molecular/electron geometry of AlH3
We have determined hybridization of AlH3 as Sp3 and lone pairs 0
Now, according to AXnEx notation of the VSEPR theory we are going to find VSEPR notation for AlH3 molecule
AXnEx notation
where,
A: central atom
X: number of atoms attached to the central atom
E: number of lone pairs of electrons on the central atom
As per Lewis dot structure of Oxygen difluoride, Oxygen is the central atom which has 2 lone pairs of electrons and 2 fluorine atoms are connected to it.
So AlH3 formula becomes AX3
According to VSEPR chart, the molecule with AX3 formula has molecular shape as Trigonal planar and electron geometry Trigonal planar.
Trigonal Planar geometry of AlH3| Total Domains | General Formula | Bonded atoms | Lone Pairs | Molecular Shape | Electron Geometry |
| 1 | AX | 1 | 0 | Linear | Linear |
| 2 | AX2 | 2 | 0 | Linear | Linear |
| AXE | 1 | 1 | Linear | Linear | |
| 3 | AX3 | 3 | 0 | Trigonal Planar | Trigonal Planar |
| AX2E | 2 | 1 | Bent | Trigonal Planar | |
| AXE2 | 1 | 2 | Linear | Trigonal Planar | |
| 4 | AX4 | 4 | 0 | Tetrahedral | Tetrahedral |
| AX3E | 3 | 1 | Trigonal Pyramid | Tetrahedral | |
| AX2E2 | 2 | 2 | Bent | Tetrahedral | |
| AXE3 | 1 | 3 | Linear | Tetrahedral |
VSEPR CHART
What is the Bond Angle of AlH3 molecule?
The Bond Angle of AlH3 is 120 degrees as there are no lone pairs present on the
central atom Aluminium. The bonds are equally placed from each other.
Bond angle of AlH3
What is the Hybridization for AlH3 molecule?
The central atom Aluminium atom shares 3 electrons with 3 hydrogen atoms.
To share the electrons with the hydrogen atoms, the orbitals of Aluminium atom undergo hybridization to accommodate the electrons.
The electronic configuration of Aluminium is –
Aluminium in ground state: [Ne] 3s2 3p1
Aluminium in excited state: [Ne] 3s1 3px1 3py1
Steric Number = Number of atoms bonded to central atom + Number of lone pairs of electrons attached to central atom
Steric Number = 3 + 0 = 3
So, hybridization for the central atom oxygen in AlH3 is Sp2
Does AlH3 molecule shows resonance?
A molecule can have resonance if –
The molecule has alternate single and double bond.The molecule has presence of lone pair in conjugation with the double bond.
Aluminium Hydride does not have a double bond so it does not show resonance.
Solubility of Aluminium Hydride:
Aluminium hydride reacts with water and form hydrogen gas. The hydrogen gas produced may catch fire due to the heat generated in the reaction.
AlH3 is soluble in ether and reacts with C2H5OH.
Conclusion:
The hybridisation of AlH3 molecule is Sp2.Both molecular geometry and electron geometry of AlH3 molecule is Trigonal Planar. The bond angle of AlH3 molecule is 120 degrees.
